Portable Solar Lantern

This portable solar
lantern circuit uses 6 volt/5 watt solar panels are now widely
available. With the help of such a photo-voltaic panel we can construct
an economical, simple but efficient and truly portable solar lantern
unit. Next important component required is a high power (1watt) white LED module.

When solar panel is well exposed to sunlight, about 9 volt dc
available from the panel can be used to recharge a 4.8 volt /600 mAh
rated Ni-Cd batterypack. Here red LED (D2)
functions as a charging process indicator with the help of resistor R1.
Resistor R2 regulates the charging current flow to near 150mA.

Solar Lantern Circuit Schematic

Solar Lantern Circuit Schematic

Assuming a 4-5 hour sunlit day, the solar panel (150mA current set
by the charge controller resistor R2) will pump about 600 – 750 mAh into
the battery pack. When power switch S1 is turned on, dc supply from the
Ni-Cd battery pack is extended to the white LED (D3). Resistor R3 determines the LED current. Capacitor C1 works as a buffer.

Note: After construction, slightly change the values of R1,R2 and R3 up/down by trial&error; method, if necessary.

diagramwirings

Garden Lighting Using Solar Cells

Completely
‘self-supporting’ garden lamps using solar cells as their energy source
are gradually becoming more and more common. How do they actually work?
We took one apart to find out. From environmental and technical
considerations, buying such a solar-cell garden lamp has a lot to
recommend it. It’s a great thing that the energy necessary for the lamp
to burn in the evening can be drawn from the sunlight that is available
for free during the day. In addition, such a lamp is enormously
practical, since you can place it in any desired location in the garden
without having to dig a trench through the lawn or flowerbeds.

You are also free to change your mind about the best location for
the lamp – something that would have unpleasant consequences with
ordinary garden lamps. What makes up a typical solar-cell garden lamp? A
certain number of elements are in any case necessary for it to
function. It’s clear that there must be a light bulb and some solar
cells. However, the bulb is naturally not powered directly from the
solar cells, so there must be a storage battery and a suitable charging
circuit to allow the battery to be charged by the solar cells. In
addition, the idea is that the lamp should only burn during the evening
and the night, and that needs a twilight switch with a light-sensitive
cell.

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It’s not necessary to do anything to switch off the lamp, since that
happens automatically as soon as the battery is fully discharged. Some
of the more luxurious models have a small fluorescent tube in place of a
normal light bulb, and in this case a small converter is also
necessary. However, the model that we examined contained a small
2.5V/75-mA halogen bulb, and thus did not need a converter. As far as
the electronics are concerned, the whole thing can thus remain very
simple.

Simplicity wins out:

Our garden lamp consists of a simple plastic structure. Eight solar
cells are mounted at the top, and inside there are a small halogen bulb,
two penlight NiCd cells and a small printed circuit board for the
electronics. As can be seen from Figure 1, there isn’t all that much
inside. This lamp costs around 15 pounds, and it can be found in several
different shops. The electronics also turn out to be extremely simple.
Figure 2 shows the complete schematic of the internal circuitry. The
twilight switch is on the left, and its output controls the lamp via
transistor T4. To the right are the on/off switch, a diode and the eight
solar cells.

Charging:

During the day, as long as there is sufficient light, the voltage
generated by the solar cells is 8 × 0.45 V under ideal conditions, with a
current that depends on the size of the cells — in this case,
approximately 140mA. With less light, less current is supplied. The
charging circuit consists simply of a single Schottky diode (D1). The
current generated by the solar cells passes through this diode, with its
typical low voltage drop of 0.3 to 0.4 V, and charges the NiCd cells.
There is no overcharge protection. It is not actually necessary, since
all NiCd cells can handle a continuous charging current equal to 1/10 of
their capacity (60mA in this case), while modern cells are so robust
that twice this amount of current does not cause any problems.

{image3}

The advantages of using a somewhat higher charging current are
naturally that the battery is already fully charged after several hours
of sunlight, and that a certain amount of charging takes place even on
rainy days and during the winter. Solar cells act as light-dependent
current sources, so the more light there is, the more current they
produce. The voltage is determined by the load, but it can never be
higher than the previously mentioned 0.45 V per cell. Approximately 2.8 V
is necessary to charge two NiCd cells. If we add the voltage drop
across D1, we arrive at a required voltage of 3.2 V. This is 0.4 V per
solar cell.

Charging takes place continuously, even when switch S1 is off. It is
important to make sure that both NiCd cells are fully charged the first
time. Otherwise, one cell may become fully discharged before the other
one when they are discharged. As a result, this cell may have a
reverse-polarity voltage applied to it, which will shorten its useful
life. Therefore, when first putting the lamp into service, you should
place it outside with S1 switched off for at least one day in full
sunlight, or two days if the weather is cloudy.

Burning:

When S1 is closed, voltage is applied to the part of the circuit containing the light bulb. An LDR is used to determine whether it is light or dark outside. During the day, the resistance of the LDR
is low, and the voltage on the base of T1 is also low, so that it is
cut off. T2, T3 and T4 are then also cut off, so that the bulb is not
illuminated. As soon as it becomes dark, the resistance of the LDR
increases, and the voltage on the base of T1 rises. T1 starts to
conduct when the voltage is around 0.65 V. This causes T2, T3 and T4 to
conduct as well, and the lamp starts to burn. T1 then receives a bit of
extra current via R4, so that positive switching takes place when the
circuit is sitting ‘on the edge’. This is called hysteresis. It means
that a threshold is set such that the light level has to drop a bit more
before the lamp will switch on again once it is off, and vice versa.

This means that the circuit does not react to every passing cloud or
insect that is flying around. As long as it remains dark, the lamp
continues to burn until the battery is fully discharged. A fully charged
battery has a capacity of 600 mAh, which is enough to supply the 75-mA
bulb for approximately eight hours. This is sufficient for the evening
and a large part of the night. In the winter, this is not possible,
since the battery will probably not be fully charged due to a lack of
sunlight. When the battery becomes fully discharged, its voltage drops.
If the voltage drops below 1.25 V, T2 and T3 are cut off, since their
base-emitter junctions are in series and thus need at least this amount
of voltage. The lamp is then switched off, and the battery is not
further discharged.

In the long term:

NiCd batteries usually have a lifetime of around 500 to 1000
charge/discharge cycles. After two to three years of continuous use,
therefore, the two penlight cells of the garden lamp will probably be
ready for replacement. However, these cells are presently so inexpensive
that this is not a serious disadvantage. Naturally, there is also a
limit on the life-time of the light bulb, but here again, making a
replacement is quick and inexpensive.

diagramwirings

Solar Relay

With extended periods of
bright sunshine and warm weather, even relatively large storage
batteries in solar-power systems can become rather warm. Consequently, a
circuit is usually connected in parallel with the storage battery to
either connect a high-power shunt (in order to dissipate the excess
solar power in the form of heat) or switch on a ventilation fan via a
power FET, whenever the voltage rises above
approximately 14.4 V. However, the latter option tends to oscillate,
since switching on a powerful 12-V fan motor causes the voltage to drop
below 14.4 V, causing the fan to be switched off.

In the absence of an external load, the battery voltage recovers
quickly, the terminal voltage rises above 14.4 V again and the switching
process starts once again, despite the built-in hysteresis. A solution
to this problem is provided by the circuit shown here, which switches on
the fan in response to the sweltering heat produced by the solar
irradiation instead of an excessively high voltage at the battery
terminals. Based on experience, the risk of battery overheating is only
present in the summer between 2 and 6 pm. The intensity of the sunlight
falling within the viewing angle of a suitably configured ‘sun probe’ is
especially high precisely during this interval.

This is the operating principle of the solar relay. The trick to
this apparently rather simple circuit consists of using a suitable
combination of components. Instead of a power FET,
it employs a special 12-V relay that can handle a large load in spite
of its small size. This relay must have a coil resistance of at least
600 Ω, rather than the usual value of 100-200 Ω. This requirement can be
met by several Schrack Components relays (available from, among others,
Conrad Electronics). Here we have used the least expensive model, a
type RYII 8-A printed circuit board relay. The
light probe is connected in series with the relay. It consists of two
BPW40 phototransistors wired in parallel.

Solar Relay Circuit

Solar Relay Circuit Diagram

The type number refers to the 40-degree acceptance angle for
incident light. In bright sunlight, the combined current generated by
the two phototransistors is sufficient to cause the relay to engage, in
this case without twitching. Every relay has a large hysteresis, so the
fan connected via the a/b contacts will run for many minutes, or even
until the probe no longer receives sufficient light. The NTC
thermistor connected in series performs two functions. First, it
compensates for changes in the resistance of the copper wire in the
coil, which increases by approximately 4 percent for every 10 ºC
increase in temperature, and second, it causes the relay to drop out
earlier than it otherwise would (the relay only drops out at a coil
voltage of 4 V).

Depending on the intended use, the 220-Ω resistance of the
thermistor can be modified by connecting a 100-Ω resistor in series or a
470-Ω resistor in parallel. If the phototransistors are fastened with
the axes of their incident-angle cones in parallel, the 40-degree
incident angle corresponds to 2 pm with suitable solar orientation. If
they are bent at a slight angle to each other, their incident angles
overlap to cover a wider angle, such as 70 degrees. With the tested
prototype circuit, the axes were oriented nearly parallel, and this
fully met our demands. The automatic switch-off occurs quite abruptly,
just like the switch-on, with no contact jitter.

This behaviour is also promoted by the NTC
thermistor, since its temperature coefficient is opposite to that of
the ‘PTC’ relay coil and approximately five times as large. This yields
exactly the desired effect for energising and de-energising the relay: a
large relay current for engagement and a small relay current for
disengagement. Building the circuit is actually straightforward, but you
must pay attention to one thing. The phototransistors resemble
colourless LEDs, so there is a tendency to think that their ‘pinning’ is the same as that of LEDs,
with the long lead being positive and the short lead negative. However,
with the BPW40 the situation is exactly the opposite; the short lead is
the collector lead. Naturally, the back-emf diode for the relay must
also be connected with the right polarity. The residual current on
cloudy days and at night is negligibly small.

diagramwirings

Temperature Sensitive Switch For Solar Collector

This circuit can be used
to turn the pump on and off when a solar collector is used to heat a
swimming pool, for example. This way the water in the collector has a
chance to warm up significantly before it is pumped to the swimming
pool. A bonus is that the pump doesn’t need to be on continuously. The
basis of operation is as follows. When the temperature of the water in
the solar collector is at least 10 °C higher than that of the swimming
pool, the pump starts up.

The warm water will then be pumped to the swimming pool and the
temperature difference will drop rapidly. This is because fresh, cool
water from the swimming pool enters the collector. Once the difference
is less than 3 °C the pump is turned off again. R10/R1 and R9/R2 each
make up a potential divider. The output voltage will be about half the
supply voltage at a temperature around 25 °C. C7 and C8 suppress any
possible interference.

The NTCs (R9 and R10) are usually
connected via several meters of cable, which can easily pick up
interference. Both potential dividers are followed by a buffer stage
(IC1a/IC1b). IC1c and R3, R4, R5 and R6 make up a differential amplifier
(with unit gain), which measures the temperature difference (i.e.
voltage difference). When both temperatures are equal the output is 0 V.
When the temperature of the solar collector rises, the differential
amplifier outputs a positive voltage.

This signal is used to trigger a comparator, which is built round an
LM393 (IC2a). R7 and P1 are used to set the reference voltage at which
the comparator changes state. R8 and P2 provide an adjustable
hysteresis. R11 has been added to the output of IC2a because the opamp
has an open collector output. A power switch for the pump is created by
R12, T1 and Re1. D1 protects T1 against voltage spikes from the relay
coil when it is turned off.

A visual indication of the state of the controller is provided by IC4 (UAA170), a LED spot display driver with 16 LEDs. The reference voltage for the comparator is buffered by IC1d and fed to input VRMAX
of the UAA170. R20/D21 and R23/D22 limit the input voltages of IC4 to
5.1 V, since the maximum permissible input voltage to the UAA170 is 6 V.
When there is no temperature difference, LED D20 turns on.

Temperature Sensitive Switch Circuit For Solar Collector

Temperature Sensitive Switch Circuit Diagram For Solar Collector

As the temperature difference increases the next LED turns on. The full scale of the LED bar is equal to the reference voltage of the comparator. This means that when the last LED
(D5) of the UAA170 turns on, the comparator switches state. This is
also indicated by D2. The power supply has been kept fairly simple and
is built around a LM7812 regulator. The circuit is protected against a
reverse polarity at the input by D3.

You have to make sure that the input to the regulator is at least 15
V, otherwise it won’t function properly. There are a few points you
should note regarding the mounting of the NTCs. NTC
R9 should be placed near the output of the solar collector. You should
choose a point that always contains water, even when some of the water
flows back a little. NTC R10 should be mounted inside the filter compartment (where it exists), which continually pumps the swimming pool water.

This will give a good indication of the temperature of the water.
The way the circuit has to be set up depends how it has been installed
and is very much an experimental process. To start with, set hysteresis
potentiometer (P2) halfway. Then set the reference voltage to about
1.5-2 V with P1. On a sunny day you can measure the voltage difference
to get an idea as to which reference voltage needs to be adjusted. The
hysteresis setting determines how long the pump stays on for, which is
until the minimum temperature difference has been reached.

Author: Tom Henskens – Copyright: Elektor Electronics Magazine

diagramwirings

Solar Powered Reading Lamp

Introduction:

The goal of this project was to produce a self contained reading
lamp that could be used by students in developing countries for reading
at night. The circuit can be used for a wide variety of lighting
pplications.The reading lamp consists of a small solar panel, a standard
UPS style lead acid battery, and an LED circuit board. The circuit board contains a low power solar charge controller (regulator), a set of 8 white LEDs, a switch, an LED current regulator, and a low voltage disconnect circuit.

The circuitry will insure a long battery life by preventing over
charging and excessive discharging. The circuit was designed to work
with lead acid batteries, it should also work with a string of 10 NiCd
cells. Both the charge controller and LED regulator circuits can be used independently for other applications.

Specifications:

Solar charging current: 150 ma – 1 Amp
Voltage drop through charge controller: 0.5V typical
Nominal battery voltage: 12 Volts
Battery rating: 3-7 amp hours
LED lamp current: 100ma regulated, 25ma per LED pair (1.2W nominal)
LED regulation range: constant light level from 11V to >15V
Low voltage disconnect: gradual current drop from 10.8V to 9.8V
Night time battery current drain with LED off: almost zero
Light duration: approximately 70 hours with a 7AH battery
Charge time: approximately 40 hours max, several hours typical

Theory:

Charge Controller: The charge controller section consists of an
LM2941CT low dropout voltage regulator and a 1N5817 schottky diode. The
regulator determines the battery full voltage, this set-point is
adjusted by the 5K 20 turn trimmer potentiometer. The 1N5817 schottky
diode prevents the battery from discharging through the voltage
regulator during the night. It also protects the circuitry against
reverse battery connection. The V727 part is a transzorb, it absorbs
lightning induced voltage spikes above 27V. The fuse prevents short
circuits from burning up the battery wiring.

Circuit

Circuit diagram

LED Circuitry: The 8 white LEDs
are connected in series with an LM317L IC that is wired as a constant
current regulator. The 13 ohm resistor sets the regulated current to
100ma. This current is split evenly through the four pairs of LEDs. The 33 ohm resistors help to keep the current through the four LED pairs balanced evenly. The 2N3904 transistor, 1N5239 zener diode, and 470 ohm resistor form the Low Voltage Disconnect (LVD) circuit. Current through the LED
starts to drop when the battery voltage drops below 10.8V, the circuit
shuts off almost all of the current when the battery drops below 10V.
The 1N5817 schottky diode blocks current flow in the event of a reverse
battery connection.

Construction:

The lamp consists of a small wood battery box and a vertical board for holding the LED assembly and solar panel. A small carrying handle protrudes from the top of the vertical board. The LED
assembly consists of a small printed circuit board and the various
parts. It is sandwiched between a piece of hard-board and a piece of
clear plexiglass to protect the circuitry from physical damage and
short-circuiting. The battery used for this device is a standard 12V-7
Amp-hour gell cell UPS battery. The solar
panel is home-made, two or three parallel-wired GM-684 12V 60ma solar
panels (p/n 08SLC09 from Elecronix Express), would be a good substitute.

Alignment:

Connect the board’s BAT – terminal to the battery – terminal.
Connect the board’s BAT + terminal to the battery + terminal.
Connect the board’s PV – terminal to the solar panel – terminal.
Connect the board’s PV + terminal to the solar panel + terminal.
Point the solar panel toward the sun.
Measure the connected solar panel’s voltage with the meter.
Measure the battery voltage with the meter.
The solar panel must be at a higher voltage than the battery for the
circuit to charge the battery.
Turn the potentiometer, VR1 25 turns clockwise.
Monitor the battery voltage, as the battery charges, the voltage should
rise above 13V.
When the battery voltage has reached 13.8V or the desired full point,
turn VR1 counter-clockwise until the battery voltage stabilizes.
Let the battery charge for several minutes at the full setting, then
re-adjust VR1 for the final desired full voltage setting.

Use:

Daytime:

Place the solar panel in a location that gets at least a few hours of direct sunlight each day. Turn the LED switch off. If the battery is extremely discharged, it may take several days in the sun to fully recharge.

Night:

Use the lamp as you would use any other reading lamp. If the lamp
starts to dim, the battery is almost completely empty, shut the light
off. If the lamp is recharged daily, the battery should rarely reach the
Low Voltage Disconnect (LVD) point.

diagramwirings

Solar Battery Protector Prevents excessive Discharge

This circuit prevents
the battery in a solar lighting system from being excessively
discharged. It’s for small systems with less than 100W of lighting, such
as several fluorescent lights, although with a higher rated Mosfet at
the output, it could switch larger loads. The circuit has two
comparators based on an LM393 dual op amp. One monitors the ambient
light so that lamps cannot be turned on during the day. The second
monitors the battery voltage, to prevent it from being excessively
discharged. IC1b monitors the ambient light by virtue of the light
dependent resistor connected to its non-inverting input. When exposed to
light, the resistance of the LDR is low and so the output at pin 7 is low.

Circuit diagram:

Solar Battery Protector Circuit

Solar Battery Protector Circuit Diagram

IC1a monitors the battery voltage via a voltage divider connected to
its non-inverting input. Its inverting input is connected to a
reference voltage provided by ZD1. Trimpot VR1 is set so that when the
battery is charged, the output at pin 1 is high and so Mosfet Q1 turns
on to operate the lights. The two comparator outputs are connected
together in OR gate fashion, which is permissible because they are
open-collector outputs. Therefore, if either comparator output is low
(ie, the internal output transistor is on) then the Mosfet (Q1) is
prevented from turning on. In practice, VR1 would be set to turn off the
Mosfet if the battery voltage falls below 12V. The suggested LDR is a NORP12, a weather resistant type available from Farnell Electronic Components Pty Ltd.
Author: Michael Moore – Copyright: Silicon Chip Electronic

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diagramwirings

Power Diode For Solar Power Systems

Apart from the sun,
solar power systems cannot work without a reflow protection diode
between the solar panel and the energy store. When current flows into
the store, there is a potential drop across the diode which must be
written off as a loss in energy. In the case of a Schottky diode, this
is not less than 0.28 V at nominal current levels, but will rise with
higher ones. It is clear that it is advantageous to keep the energy loss
as small as possible and this may be achieved with external circuitry
as shown in the diagram. The circuit is essentially an electronic switch
consisting of a high precision operational amplifier, IC1a, a Type
OP295 from Analog Devices, and a MOSFET, T1.

This arrangement has the advantages over a Schottky diode that it
has a lower threshold voltage and the lost energy is not dissipated as
heat so that only a small heat sink is needed. When the potential at the
non-inverting input of the op amp, which is configured as a comparator,
rises above that at the inverting input, the output switches to the
operating voltage. The transistor then comes on, whereupon
light-emitting diode LD1 lights. Diode D3 clamps the inputs of IC1a so
that the peak input voltage cannot be greater than half the threshold
voltage, provided the values of R3 and R4 are equal.

Circuit

Circuit diagram

The op amp provides very high small-signal amplification, a small offset voltage, and consequent fast switching. The MOSFET changes from on to off state and vice versa at drain -source voltages in the microvolt range. In the quiescent state, when UDS
is 0 V, the transistor is on, so that LD1 lights. The operating voltage
(C–A) may be between 5 V (the minimum supply for the op amp and the
input control potential, UGS, of the transistor) and 36 V (twice the zener voltage of D1). Zener diode D1 protects the MOSFET
against excessive voltages (greater than ±20 V). Diode D3 and resistors
R3 and R4 halve the potential across the inputs of the op amp.

This ensures that operation with reversed or open terminals is harmless. The substrate diode of the MOSFET is of no consequence since it does not become forward biased as long as the forward voltage, USD, of the transistor is held very low. The on -resistance, RSD,
of the transistor is only 8 mΩ and the transistor can handle currents
of up to 75 A. When the nominal current is 10 A, the drop across the
on-resistance is 80 mV, resulting in an energy loss of 0.8W. This is low
enough for a SUB type with a TO263-SMD case to be used without heat sink. When the current is 50 A, however, it is advisable to use a SUP type with a TO220 case and a heat sink since the transistor is then dissipating 12.5 W.

Even then, the voltage drop, USD = 0.32 V
is significantly lower than that across a Schottky diode in the same
circumstances. Moreover, owing to the high precision of IC1a, a number
of transistors may be used in parallel. The circuit proper draws a
current of 150 µA when only one of the op amps in the OP295 is used. An
even lower current is drawn by the alternative Type MAX478 from Maxim.
However, the differences between these two types are only relevant in
the low current and voltage ranges. Both have rail-to-rail outputs that
set the control voltage accurately even at very low operating voltages.

This is important since the switch-on resistance of MOSFETs
is not constant: t drops significantly with increasing gate potentials
and decreasing temperature. A experimental circuit may use an LM358 op
amp and a Type BUZ10 transistors, but these components do not give the
excellent results just described.

diagramwirings

Series Commutated SCR SSS Solar Charge Control

A series commutated SCR makes a very unusual Solid State Switch in this solar charge regulator control. Advantages include simplicity and robustness. The SCR performs (3) functions: switch, latch and reverse polarity diode (reverse blocking thyristor). The SCR conducts the charging current from the solar panel to the battery while a series MOSFET performs the function of commutating (turning off) the SCR current at the end of the conduction period. All circuitry consists of readily available discrete components.

This circuit is applied in fashion similar to this previously published circuit:
http://www.electroschematics.com/10218/sss-solar-charge-control/

Parallel forced-commutation of SCRs is relatively common, but the series commutation technique described here may be new to the world. Check out the previously published forced commutation circuit: http://www.electroschematics.com/10783/scr-based-sss-solar-charge-control/

Schematic

Series Commutated SCR SSS Solar Charge Control Schematic

Block diagram

Series Commutated SCR SSS Solar Charge Control Block

Programmable Unijunction Transistor Gate Driver

The SCR has its gate driven by a PUT (Programmable Unijunction Transistor) gate driver. The advantage is high gate drive current (100mA peak), fast pulse rise-time and Schmitt Trigger type of operation. Q1 is wired as a free-running oscillator that outputs pulses every 6seconds (0.1HZ). Resistors R3 & R4 bias the gate of the PUT at mid voltage. When C1 charges to one junction drop above the gate voltage, it fires and the charge in C1 is dumped into the SCR gate. R5 limits the peak current and extends the pulse duration. When the PUT anode voltage drops below the threshold current, the PUT turns off and the cycle repeats.

Basic operation

Operation is simple –Q1 keeps firing the SCR while the voltage regulator keeps commutating the SCR via turning off the normally conducting MOSFET (Q2). When the SCR is Off, LED D3 is illuminated. When the battery is low, the LED does not light –it lights only when the battery voltage setting has been reached. When the battery is fully charged, the SCR conducts for only a short period of time in each 6sec oscillator period.

Voltage Reference

A TL431 is strapped to regulate at its minimum voltage of 2.5V. R10 biases it at about 1mA. It is referenced to the positive battery terminal. This is far superior to a low voltage zener because it has very low dynamic resistance and low thermal temperature coefficient (sharp threshold).

Voltage Comparator

Q4 is a PNP transistor that performs the function of voltage comparator. It compares the output of the feedback voltage divider from the battery with the 2.5V reference (plus the Vbe junction voltage drop). As the battery voltage increases beyond this threshold, collector current begins to flow and subsequently turns on Q3. When the collector voltage of Q3 drops to zero, the MOSFET turns Off, subsequently commutating the SCR. Positive feedback is provided by capacitor C2 –it couples the change in Q3 collector voltage back to the base of Q4. This allows for clean oscillation-free switching. Resistor R14 reduces the positive feedback in order to prevent oscillation.

While a bipolar transistor has a Vbe that is temperature dependent, it tends to track the battery voltage that is also temperature dependent. No attempt was made for accurate temperature compensation.

SCR turn-off time

The S2800A has a 50uS turn-off time specification. The minimum commutation period that I observed is about 1mS –overkill.

Misc components

D2 is a zener that prevents the gate voltage of Q2 from exceeding 10V. Otherwise, it could float up to the open circuit voltage of the solar panel and be subject to damage. D1 prevents reverse current through LED D3 when there is no solar activity.

Circuit Shortcoming

Connecting two power devices in series (SCR1 & Q2) causes additional conduction loss. In order to minimize the subsequent conduction voltage drop, I selected a MOSFET with low Rds on. As a result, both SCR1 and Q2 require heatsinks. Conduction voltage drop is about 1.4V. This is not all that bad considering that a reverse polarity rectifier is not required –this is inherent in the reverse-blocking thyristor.

Oscillographs

Oscillograph 108 3
Oscillograph 108 2
Oscillograph 108 1

Photos

OLYMPUS DIGITAL CAMERA
OLYMPUS DIGITAL CAMERA

For the future:

Self-commutated SCR resonant link voltage regulator for 12V to 5V conversion (very exciting prospect –topology is new to the world)

diagramwirings

6V Solid State Switch Solar Charge Control

This is a Solid State Switch upgrade to the 6A, 6V Relay Based Solar Charge Regulator. Solid state switching provides longer life, smaller size and higher efficiency than a relay. In addition, the clock frequency may be doubled in order to better track battery load /charging requirements. The parts cost is essentially identical. One drawback is the loss of the secondary output feature. For more information go the above link as this write-up contains mostly supplemental information.

Application

The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases.

Schematic

6V SSS Solar Charge Control Schematic

Bill of materials

6A, 6V SSS Based Charge Control BOM

Eliminate the relay life issue

While relay life is OK, it is definitely limited. I estimate the relay life to be approximately 1year, but I have no direct experience with such. This makes the solid state switch desirable.

MOSFET thermal considerations

This is an issue that I did not cover in the 6A, 12V SSS Based Solar Charge Control. With an Rdson of 0.07Ω, the FQP27P06 device dissipates appreciable power. Also, note that the Rdson is specified at a gate voltage of -10V, while in this circuit it runs at approximately -8V. At the lower gate voltage the Rdson may be higher. Fortunately, the typical Rdson specification is close to 0.055Ω, so we will use the maximum Rdson spec of 0.07Ω for our calculations.

Thermal impedances

Θj-c (junction to case) 1.25°C /W
Θc-h (case to heatsink) 0.5°C /W
Θh-a (heatsink to air) 11°C /W (natural convection)
Θj-a (total resistance) 12.75°C /W
MOSFET power dissipation @6A = I²R = 6² * 0.07 = 2.5W
MOSFET power dissipation @10A = I²R = 10² * 0.07 = 7W

Junction temperature calculation

Max Tj (junction temp) = Pmax (power) * Θj-a + Ta max (max ambient temp)
= 7 * 12.75 + 40°C = 129°C (this is well below the rated Tj max of 175°C)

Effect of parallel MOSFET devices

While the above exercise indicates an acceptable, conservative temperature rise using my favorite, inexpensive TO-220 heatsink, parallel devices have merit. Simply paralleling two FQP27P06 MOSFETs, reduces the Rdson by a factor of two to 0.035Ω. This also reduces the total conduction loss to half, but there is an additional benefit here in that the power is split between the two devices. So the worst case power dissipation of 7W @ 10A is now reduced to 1.75W per device.

According to the FQP27P06 datasheet, the thermal resistance (Θj-a) without a heatsink is 62.5°C /W. So at 1.75W, the maximum junction temperature calculates out to 149°C.

Simply put, NO HEATSINKs ARE REQUIRED in this case using parallel power devices. Also, depending upon how much you pay for the MOSFET and /or the heatsink, the cost is essentially ‘a wash.’ Now 149°C is definitely ‘toasty,’ and can scorch your finger, but laying the MOSFET power device down on a large PCB copper pad will conduct heat away and reduce the thermal resistance –while it consumes additional circuit board area, it makes it run cooler without the addition of a physical heatsink.

LED function and Schottky isolation diode

LED (D4) indicates that the battery is charging. Note that if D5 is changed to a Schottky type rectifier, a shunt resistor (2.2K or so) may be required across the LED to prevent diode leakage from turning it on. Schottky rectifiers, while offering substantially reduced forward voltage drop suffer from lower peak inverse voltage (PIV) rating as well as higher leakage current than the usual silicon rectifier.

Setup

To set the Max Voltage Adjust potentiometer (R12), start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 7.2V). At this point, turn R12 CCW until the LED extinguishes. When the LED comes on again at the next clock cycle, recheck the voltage at which it extinguishes.

Photos

For the future

  • SSS solar charge control with synchronous isolation rectifier
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge control schematics

Undocumented words and idioms (for our ESL friends)

(a) wash –idiomatic phrase –essentially equal or no difference –literally, it all comes out clean when it is washed (like laundry).

toasty –idom, slang –adjective –hot –literally the temperature of a slice of bread when it pops out of the electric toaster.

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6A, 6V Relay Solar Charge Control

This 6V solar charge regulator meets most small-scale 6V application requirements. With a current rating of 6A, it can handle up to a 50W solar panel. Its principle of operation is very simple and there is only one adjustment: cut-out voltage. Many inexpensive commercial solar charges available use, I believe, this technique. However, this particular design is superior in that it is powered via the solar panel rather than the battery –the implications of this are obvious: low battery discharge current. The switching is accomplished via an inexpensive 10A relay.

One of the interesting features is the secondary load output that can be used for other purposes within specific loading limits –in other words, there are two outputs: one for charging the battery and the other for use when the battery is not charging so that the solar power may be more fully utilized. This is a low voltage extension of the 6A, 12V relay based solar charge control. While the new schematic may appear very similar, there are subtle differences that are geared to make it function for 6V applications. Also, I have specified an alternative 10A Schottky rectifier in order to stretch the current rating.

The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases. Most of the chargers that I have posted on electroschematics to date have been of the linear type. I have plans for at least two more solar chargers, so keep watching.

Schematic of the Solar Charge Control Circuit

6V Relay Based Solar Charge Control Schematic

Bill of materials

BOM

Circuit function

U2 provides a 2.5V reference voltage. U1 is a LM339 quad comparator connected as a S-R flip flop with wired OR outputs so that it may be conveniently driven via open collector output structure. U3 is a CMOS 555 clock generator that generates 30mS pulses every 30S. Its function is to set the flip flop so that charging may commence. (Another descriptive name for this topology is “minimum on-time regulator.”) D1 effectively changes the totem pole output of the 555 to open collector. As soon as the battery terminal voltage exceeds the set point (7.2V or so), comparator U1D resets the flip flop and drops the relay. At this point, it remains dormant until the next clock pulse. When fully charged, the relay flips on and off briefly upon each clock pulse. U1C and Q1 constitute a relay driver –the NPN open collector output of the LM339 and Q1 (PNP) make up the classic composite transistor connection. This is necessary to boost the low power output of the LM339 sufficiently for driving the relay coil.

Clock timing issue

Unlike the 12V version, the 555 is powered via a variable voltage –when the relay drops or closes, the solar panel input voltage makes step voltage changes. The timing capacitor (C2) cannot follow this change in voltage and causes the clock pulse timing to be somewhat variable. Fortunately, this is of no consequence as the circuit continues to function. This info is for the observant technician.

Secondary output limitation

While this neat feature is provided, its application is limited. Note that the load resistance is not allowed to drag the solar panel output below about 5V under marginal lighting conditions or else the relay may not have sufficient voltage to pick up. This load could conceivably operate a low-power heater etc. The big challenge is to obtain the proper load resistance –a number of aluminum resistors such as the Vishay /Dale TMC50 series may be a good solution. Also, before selecting the resistors make actual measurements to see what works –even then, experimentation may be required for best results.

Alternative rectifier for 10A current rating

(Note that this upgrade is applicable for just about all previously published solar charge controls.)
The current rating is limited by D5 (6A) and the relay contacts (10A). By using a 10A rectifier such as the Micro Commercial Schottky SR1045, a 10A current rating is feasible. To go beyond this, both a high current rectifier and relay must be selected. Note that these rectifiers run hot at the rated current, so additional PCB foil area or other heat sink is be required. On one previous project, I fabricated heatsinks by hammering pieces of AWG#14 wire into flat strips and soldering them to both ends of the rectifier. Note that the Schottky rectifier is very desirable and well suited for this low voltage application due to its low voltage drop (0.48V @ 6A or 0.55V @ 10A). This means that it will function well if the solar panel generates only 8V instead of the specified 9 to 10V.

Setup

To set the Max Voltage Adjust (R12) potentiometer, start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 7.2V). At this point, turn R12 CCW until the relay drops. When the relay picks up again, recheck the voltage at which it drops.

Charging a dead battery

If the battery voltage is too low (e.g. 2 or 3V), the relay may drop out immediately due to undervoltage. To get around this problem, connect the solar panel directly to the battery until the voltage charges to at least 5V or so, or flash it by connecting it directly across a charged battery for a few seconds.

Photos

For the future

  • 6A, 6V SSS based solar charge control
  • Solar charge control with synchronous rectifier
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge controls
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