65W Notebook Laptop Power Adapter

Using TOP269EG off-line
switcher IC, (U1), in a flyback configuration can be designed a very
simple high efficiency notebok laptop power adapter.TOP269EG IC has an
integrated 725 V MOSFET and a multi-mode controller. It regulates the output by adjusting the MOSFET duty cycle, based on the current fed into its CONTROL
pin.This laptop power adapter circuit will provide a fixed 19 volts
output voltage at a maximum current of 3.5A. input voltage range is
between 90 to 265VAC.

Common-mode inductors L3 and L4 provide filtering on the AC input. X
class capacitor C1 provides differential filtering, and resistors R1
and R2 provide safety from shock if the AC is removed, by ensuring a
path for C1 to discharge. This is required by safety agencies when the
capacitor value exceeds 100 nF. Bridge rectifier D1 rectifies the AC
input, and bulk capacitor C2 filters the DC.

Y capacitor C11, connected between the primary and secondary side
provides common mode filtering.Capacitor C7 provides the auto-restart
timing for U1. At startup this capacitor is charged through the DRAIN
(D) pin. Once it is charged U1 begins to switch. Capacitor C7 stores
enough energy to ensure the power supply starts up. After start-up the
bias winding powers the controller via the CONTROL pin. Bypass capacitor C6 is placed as physically close as possible to U1.

U1 utilizes an output overvoltage shutdown function. An increase in
output voltage causes an increase in the bias winding on the primary
side, sensed by VR1. A sufficient rise in the bias voltage causes VR1 to
conduct and bias Q1 to inject current into the Voltage Monitor (V) pin
of U1. When the current exceeds 336 uA, U1 enters the overvoltage
shutdown mode and latches off.


Switchable 6V, 9V, and 12V Linear Voltage Regulator


We can build a multiple voltage power supply 6, 9, and 12V  (AC-DC Adapter) with the circuit shown in the following schematic diagram.  Not only provide multiple voltage output with single voltage supply, this circuit add the benefit of regulating the voltage for better stability.

Switchable Linear Voltage Regulator

A transformer with rectifier diodes and filtering capacitor can be used to supply this circuit. You can use 1 A 15V transformer with 2200uF filtering capacitor for the AC to DC adapter. The transistor TIP31 should be installed with proper heatsink to avoid overheating.


VGA to BNC Adapter (Converter)

There are monitors which only have three BNC
inputs and which use composite synchronization (‘sync on green’). This
circuit has been designed with these types of monitor in mind. As can be
seen, the circuit has been kept very simple, but it still gives a
reasonable performance. The principle of operation is very
straightforward. The RGB signals from the VGA connector are fed to three BNC connectors via AC-coupling capacitors. These have been added to stop any direct current from entering the VGA card. A pull-up resistor on the green output provides a DC offset, while a transistor (a BS170 MOSFET)
can switch this output to ground. It is possible to get synchronisation
problems when the display is extremely bright, with a maximum green

In this case the value of R2 should be reduced a little, but this
has the side effect that the brightness noticeably decreases and the
load on the graphics card increases. To keep the colour balance the
same, the resistors for the other two colors (R1 en R3) have to be
changed to the same value as R2. An EXOR gate from IC1 (74HC86) combines the separate V-sync and H-sync signals into a composite sync signal. Since the sync in DOS-modes
is often inverted compared to the modes commonly used by Windows, the
output of IC1a is inverted by IC1b. JP1 can then by used to select the
correct operating mode. This jumper can be replaced by a small two-way
switch, if required.

This switch should be mounted directly onto the PCB, as any connecting wires will cause a lot of interference. The PCB
has been kept as compact as possible, so the circuit can be mounted in a
small metal (earthed!) enclosure. With a monitor connected the current
consumption will be in the region of 30 mA. A 78L05 voltage regulator
provides a stable 5 V, making it possible to use any type of mains
adapter, as long as it supplies at least 9 V. Diode D2 provides
protection against a reverse polarity. LED D1 indicates when the supply is present. The circuit should be powered up before connecting it to an active VGA
output, as otherwise the sync signals will feed the circuit via the
internal protection diodes of IC1, which can be noticed by a dimly lit LED. This is something best avoided.

R1,R2,R3 = 470Ω
R4 = 100Ω
R5 = 3kΩ3

C1,C3,C5 = 47µF 25V radial
C2,C4,C6,C7,C10 = 100nF ceramic
C8 = 4µF7 63V radial
C9 = 100µF 25V radial

D1 = LED, high-efficiency
D2 = 1N4002
T1 = BS170
IC1 = 74HC86
IC2 = 78L05

JP1 = 3-way pinheader with jumper
K1 = 15-way VGA socket (female), PCB mount (angled pins)
K2,K3,K4 = BNC socket (female), PCB mount, 75Ω


9V DC Adapter With Battery Backup

With just a low cost DC
adapter and the circuit described here it is possible to build a low
cost stabilized, uninterruptable 9V supply. On the grounds of safety and
economy, a simple unstabilized 12V D.C. adapter is used as the power
source, a universal adapter with its output set to 12 V will do equally
well. The output voltage of an adapter under low load conditions (up to
approximately 1/3 of the rated output current) is over 15 V, even at the
rated output current, there will be sufficient voltage to supply a 9 V
voltage regulator. The rating of the DC adapter should be chosen
according to the output current required at 9V. Common values are 300mA,
500mA and 1A.

The 9V voltage regulator used in this circuit has a built in thermal
shutdown mechanism so that if too much current is drawn from the
device, it simply turns off as it overheats and will not supply any
current until the case temperature returns to normal. If the unit is
intended to supply more than say 150-200mA then to prevent thermal
shutdown it will be necessary to fit a heatsink to the voltage
regulator. The rule of thumb used to calculate the size of heatsink is
that you should be able to touch it during operation at maximum load,
without burning you finger. When choosing the DC adapter, it is always
better to select one with a higher current rating than is needed this
will ensure that its output voltage is high enough to be able to also
charge the 12V cells.

As long as mains voltage is on the DC adapter, the voltage across C1
will be higher than the voltage of the cells. Charging current will flow
through R1 and D1 to the cells. Current also flows to the voltage
regulator and out to the load connected at the output. Diode D2 in this
situation will not conduct because the voltage at its cathode is greater
than that at its anode When the mains voltage fails or is turned off,
diode D2 conducts and current will now flow from the Nickel Cadmium
cells to the voltage regulator, thereby automatically keeping the output
voltage at 9V. The value of resistor R1 is chosen so that a charging
current to the cells is not greater than 1/10th of the cells capacity
(if the cells are rated at 1100mAh, the charging current must not exceed

From the point of view of cell longevity it is better to reduce this
charging current even further (1/20 or 1/50 C). When calculating this
resistor, the value of the no-load voltage should be used. This will
give the highest charging current. To calculate the charging current
using R1 with a value of 180 Ω. The cells measure 13.8 V when fully
charged and the no-load output voltage of the DC adapter is 17V.
Charging current is given by the formula: (17V – 13.8V – 0.7V) / 180 =
13.9mA. Substituting the actual measured values in this formula will
enable you to calculate the value of R1 to give the correct charging
current for the cells.

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Battery-Charging Indicator For Mains Adaptor

Although you may well be
the proud owner of the very latest NiCd battery charger, you may still
come across the odd ‘incompatible’ battery, for example, one having a
rare voltage or requiring a much higher charging current than can be
supplied by your off-the-shelf charger. In these cases, many of you will
resort to an adjustable mains adaptor (say, a 500-mA type) because that
is probably the cheapest way of providing the direct voltage required
to charge the battery. Not fast and not very efficient, this ‘rustic’
charging system works, although subject to the following restrictions:

Circuit diagram:

Battery Charging Indicator Circuit

Battery-Charging Indicator Circuit Diagram

  • You should have some idea of the charging current. In case you use
    an adaptor which is adjustable but of the unregulated, low output
    current type, you can adjust the current by adjusting the output
  • You have to know if the current actually flows through the
    battery. A current-detecting indicator is therefore much to be preferred
    over a voltage indicator.
  • To prevent you from forgetting all about the charging cycle, the
    indicator should be visible from wherever you pass by frequently. Using
    the circuit shown here, the LED lights when
    the baseemitter potential of the transistor exceeds about 0.2 V. Using a
    resistor of 1 ? as suggested this happens at a current of about 200 mA,
    or about 40 mA if R1 is changed to 4.7?. The voltage drop caused by
    this indicator can never exceed the base-emitter voltage (UBE) of the transistor, or about 0.7V. Even if the current through R1 continues to increase beyond the level at which UBE
    = 0.7 V, the base of the transistor will ‘absorb’ the excess current.
    The TO-220 style BU406 transistor suggested here is capable of accepting
    base currents up to 4A. Using this charging indicator you have overcome
    the restrictions 2 and 3 mentioned above.

RS232 to RS485 Converter

RS485 interface gives some more benefits compared to RS232: simplicity and the robustness for long distance transfer. RS485 is simpler because it uses only two wire (or a single pair) to carry out the data in bidirectional manner, using RS232, we need 3 wires at minimum to carry the data bidirectionally. RS485 cable length can be as long as 1.2 km, while RS232 only about 25 meters.

RS232 to RS485 Converter

To connect an RS485 device or RS485 network to any PC or controller, the PC or the controller should provide RS485 interface also. If your PC or controller only has RS232 port then you need an RS232 to RS485 converter or adapter for your PC or controller.

RS485 Circuit

The RS232-RS485 adapter circuit diagram is show below, RS232 DB9 is used as the RS232 port, while only a terminal block is used as RS485 connector. A twisted pair is ideal for RS485 cable, since the twisting make both wires (+ and -) exposed to an identical noise figure, thus canceling the noise itself when received at the interface port.

Rs232 rs485 adapter

Electrical Insulation Using Optics

Especially when the transmission is carried over a long wire outdoor, the wires is potential to carry the dangerous signal like lightning or other high voltage induction. This can be fatal to your PC or controller. The circuit uses optical isolation (opto-coupler) to isolate your RS232 port from the RS485 cables.


Simple TTL-RS232 Level Converter Using Transistor

Using only transistors as the active components, we can build a simple and low cost TTL-to-RS232 level converter. It uses only two transistors as the active components. Here is the schematic diagram of the converter:

Simple ttl rs232 level converter

TTL-RS232 Level Converter

You can use any small signal transistor for this RS232 converter circuit. You can see the collector of Q1 transistor is connected to TxD, and the purpose of this connection is to borrow a negative voltage for Q1. This condition make the converter works only for half duplex, so the negative voltage is always present when transmitting the data from TTL level to RS232 level. You can modify the 4.7k resisor to be connected to ground, not borrowing the negative voltage from RS232 TxD, and the converter will work for full duplex. This will work for most RS232 port even the conversion from TTL transmission doesn’t swing to negative. [Source: Wichit Sirichote’s project]


TTL-RS232 Level Converter Using MAX232 IC

Using MAX232 integrated circuit, you can build a TTL-to-RS232 level converter that totally comply with RS232 electrical specification. The IC generate the higher positive and negative voltage (+10 and -10 Volts)  from TTL power supply (5V DC) using switched capacitor. The switching circuitry is provided inside the chip, so you need only few caps for external components. Here is the schematic diagram of the circuit:

Ttl rs232 converter

In applications that are sensitive to power-supply noise, VCC should be decoupled to ground with a capacitor (C5) of the same value as C1 and C2 connected as close as possible to the device. [Source: Maxim Integrated Products Application Notes]


Current to Voltage Converter

In the simplest form, current to voltage converter can be implemented as simple as one resistor,  since a voltage is developed when an electrical current flow against a resistance. The problem is that the resistance always affect the current by  decreasing the current flow, and the current to voltage conversion is not accurate. In some cases, the resistor that convert the current can be chosen as low as possible to make it much lower than the current source impedance, but it means a smaller conversion factor is also lowered, make the reading become more difficult.

Current to voltage converter

The above circuit will make the converter act like a zero impedance resistor, so any current from any source impedance would be perfectly converted. The feedback mechanism will keep the inverting input voltage to be same with the non-inverting one by adjusting its output, and this will make the inverting input virtually shorted to ground. [Source: National Semiconductor Application Notes]


USB-RS232 Converter

Many new computers now only has USB port, and this serial to USB adapter might be the only help for old devices that provides only RS232/COM Port. This serial adapter uses FT232 USB-RS232  converter IC from FTDI (Future Technology Devices International).  The main features of this USB serial adapter chip are:

  • 384 byte receive buffer / 128 byte transmit buffer for high data throughput
  • Full hardware assisted or X-On/X-Off handshaking
  • RS232 link from 300 baud to 920K baud
  • Single Chip Multi-Function Data Transfer Solution
  • RS422/RS485 Link to 2000K baud
  • Support for Event Characters and Line Break condition
  • Auto Transmit Buffer control for RS485
  • Integrated 3.3v Regulator – No External Regulator Required
  • 4.4v .. 5.25v Single Supply Operation
  • UHCI / OHCI Compliant
  • Compact 32 pin  ( 7mm x 7mm ) MQFP package
  • Integrated 6MHz – 48MHz Clock Multiplier aids  FCC and CE compliance
  • USB 1.1 Specification Compliant
  • USB VID, PID, Serial Number and Product Description Strings in external E2PROM.

Many application can be built based on FT232 chip, for example: USB Instrumentation; USB-USB data transfer cables; USB-PDA Interface Cables; USB- RS232 Converters / Cables; USB-RS422 / RS485 High Speed Industrial Links; USB Digital Cameras; USB-USB  null-modem cables; USB-Serial Bar Code Readers;  USB ISDN and ADSL Modems; USB-56k / V90 Modems; USB I/F for MP3 players; Ultra-high performance Serial Port for legacy -free PC system boards / Easy PC’s;

Usb rs232 converter circuit

The circuit is very simple, need only few components to complete a working converter. More information about datasheet and application notes can be obtained in FTDI website (www.ftdichip.com).