Full Featured Touch Control Programmable Power Supply

Touch control of electronic gadgets is no longer a dream. Almost all electronic hobbyists have their own power supply using the most common IC regulator LM317. They may have a selectable rotary switch to select a particular output or a potentiometer to vary the output smoothly.

Here is a circuit which will change the output of the power supply by a slight touch of the finger! Those who have a power supply can just add this feature while those who do not have one may construct it fully as presented here.

The circuit

The logic of the circuit is very simple and even a common hobbyist can easily understand it. Moreover, it uses easily available components which are also very cheap, thus suiting a common man’s wallet.

Here diodes D1 through D4 act as a rectifier bridge while capacitor Cl is used for smoothing. Finally, IC1 regulates the output voltage which is selected by transistors T1 through T10. These transistors are made to switch on and off turn by turn, thus selecting resistors R2 through Rll one by one. Thus, instead of a single-pole 10-way rotary switch, we are using transistors to select a particular output voltage at a time. Capacitor C2 is used to improve the transient response of the power supply, while C3 is used to filter the 50Hz hum.

Touch control llie touch control is achieved by a very simple principle. IC2 is used in mono-shot mode with output pulse width of slightly more than a second. Touch plate TP1 is connected at base of T11. The 50Hz hum of our body is coupled through this touch plate to the base of Til and switches the circuit ‘on’, thus grounding pin 2 of IC2. This low-going pulse applied at pin 2 triggers IC2. The output pulse at pin 3 of IC2 is in turn given to pin 14 (clock) of IC3 which is a 4-bit decade up/down counter. At the positive transition of each clock pulse at pin 14, the counter advances in forward (up mode) direction when pin 5 is kept low through T13. Thus, in this condition touching TP1 each time will change the output such that it goes on increasing. Touching TP2 makes pin 5 of IC3 switch to logic 1. Now for each clock at pin 14, the counter advances in reverse down mode direction.

In this condition, touching TP1 each time will change the output across A-B such that it goes on decreasing. IC4 is a BCD-to-decimal decoder with active low output. The output pins 1 through 11 go ‘ low ’ according to the BCD input at pins 12 through pin 15 of IC4. Thus, the outputs of IC4 are used to switch the particular transistor ‘on’, changing the voltage across A-B.

Short circuit protection Fig. 4 shows a fully solid-state short circuit protection for the given power supply. The current value over which short circuit protection is to be archived can be set by VR1.

The circuit is a simple capacitor commutated SCR flip-flop On connecting the load as shown in Fig. 4 and pressing S2 momentarily latches SCR 1, and the load gets the supply. Parallel combination of R27 and R28 senses the load current while R26 along with VR1 varies the base bias of transistor T14.

When load is operated, capacitor C8 charges through R30. When short circuit is detected, which depends on VR1 settings, T14 conducts and fires SCR2. This discharges C8 through SCR2 and SCR1 commutates, thus tripping the load. If we wish to stop the load from conducting in-between, pressing S3 momentarily will fire SCR2, thus disconnecting the load.

The only disadvantage using this short circuit protection is that minimum of 4.5V is required across A-B terminals. For the value shown in Fig. 4, VR1 can be calibrated to achieve short circuit protection from 30 mA to 1 A. Also, the load must consume more than the holding current of SCR1, which in this case is 15 mA.

Indicating output voltage

The indication of output voltage can be done using a voltmeter across A-B terminals or an analogue-to-digital converter which in turn drives the 7-segment display.

Both the above methods are expensive and bulky too. Here we are using simple indication using LEDs. Depending upon the output voltages, which in this case are in standard steps of 1.5V, 3V, 4.5V, 5V, 6V, 7.5V, 9V, 12V, 15V and 18V, a particular LED will glow giving a simple but still reliable indication of output voltage (Fig. 5).

Construction

IC1 is to be mounted on a relatively large heatsink because it will dissipate a maximum of 18W on full-load current of 1 -amp. IC5 is to be mounted on a small heatsink as very little heating is expected. IC2, IC3 and IC4 must be mounted on sockets for convenience. Touch plates can be constructed using aluminium foils. Each should measure lcm x 1cm. Copper plates can also be used. If aluminium foils are used, the surface has to be roughened up using a file, otherwise the solder will not adhere. Use shielded wire to connect the bases of transistors T11 and T12 with the touch plates. Wire lengths must be kept as small as possible.

Fault finding and testing the circuit

After assembling the circuit on a PCB and switching the power supply on, carry the following tests:

(1) Check the voltage across capacitor Cl. It must be between +18 and +2IV. If a good quality transformer is used, it will be +21V approximately.

(2) Check the output of 7805 (IC5) which must be +5 volts.

(3) Measure the collector current drawn by IC2, IC3 and IC4. It should be 30 to 40 m A. If it is more, there may be some wrong connection which should be detected.

(4) Touch TP1 with your fingertip and see the response with a multimeter across pin 3 and ground of IC2. It should give a pulse of approximately 1 second.

(5) Check the outputs of IC3 in response to each touch on TP1. This must increment the counter upwards. Now touch TP2 and then touch TP1. This must decrement the outputs of IC3 (in BCD fashion).

(6) Finally, check the output across A-B terminals repeating the procedure in steps 4 and 5.

(7) To calibrate the current limit in case of short protection, connect a wire-wound potentiometer across the load terminals. Now vary the potentiometer and for each setting measure the current drawn. Keeping multimeter across the pot, vary VR1 until the load trips.

If all the tests are satisfactory the power supply is ready for operation. The circuit costs around Rs 250.

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