Thursday, March 21, 2019

1.5V Power Supply For Quartz Clock

Are you a clock collector? Got a lot of clocks on your wall? Tired of buying AA cells? Disturbed by the amount of leaked Duracell you have to clean? Here is a solution for you, make a simple circuit that will power multiple clocks just fine.


Components required:

1. IC – LM317 with Heat sink and thermal compound.

2. Capacitor – 0.1μF, 10μF, 100μF and 1000μF, one each.

3. Resistor - 39Ω and 220Ω, one each.

4. Diode – 2 1N4001.

5. 12V power supply or 12V Center tapped transformer along with two 4007 diodes.


How does it work?

If we recall the voltage output equation for LM317 and put the values in there we will get -

Output Voltage = 1.25 * (1+ 39/220) V = 1.47V

Which is very close to the 1.5V of a standard AA cell.

To increase the output voltage slightly a 47Ω resistor can be used instead of the 39Ω one and to decrease it a 33Ω can be used.


1. Do keep in mind this circuit won’t work without a input power, so use an uninterruptible power source if you plan to run clocks.

2. Two 4001 diodes are used as bleeder diode that keep the circuit protected. D1 and D2 protect against charge stored in C3 and C2 respectively.

3. 0.1μF ceramic capacitor is used to reduce noise, which is very important here as noise can interfere with clock circuitry.

4. If the power supply output stage already has capacitor, the 1000μF one can be removed.

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Sunday, March 10, 2019

High current from 78XX Regulator 1

Previously we have seen 78XX series Regulators in action and how easy and simple their usage is. However, one of the big drawbacks of such a chip is the amount of current it can deliver. For a typical 78XX the maximum safe current is about 1A. Yes, there are 78HXX available which can do up to 5A but they are a bit rare. So, let’s look at a diagram where 78XX regulator is used with a power transistor as pass element to increase the current capacity.

Components required:

Regulator - LM7805 with heat-sink and Thermal Compound.
Transistor – TIP2955C with heat-sink and Thermal Compound.
Resistor – 3.3Ω 1W
Capacitor – 2 1000μF, 1 0.33μF, 1 0.1μF. 


How does it work?

Working process is pretty simple, when the load is drawing very small amount of current 7805 provides that and transistor 2955 stays off. As soon as current draw increases voltage drops across the 3.3Ω Resistor increases as well. When there is enough voltage drop across it transistor turns on and start carrying current to the load and bulk of the load current is then supplied via the transistor. 7805 keeps on regulating though. TIP2955 can easily deliver 5-10A with adequate heat sink.

1.     You might get away without using a heat-sink on the 7805 as most of the current will be passed via the transistor.