Power Supplies (Low Voltage)
Ref |
Value |
Description |
Ref |
Value |
Description |
R1 |
1K |
330mW carbon film |
C1 |
100n |
Ceramic capacitor +80- |
R2 |
10K |
330mW carbon film |
C2 |
100n |
Ceramic capacitor +80- |
R3 |
1K5 |
330mW carbon film |
C3 |
4,700uF, 25v |
Electrolytic capacitor |
D1 |
LED |
Standard 10mA LED and holder |
C4 |
1,000uF, 15v |
Electrolytic capacitor |
D2 |
50v PIV, 1Amp |
1N4001 or similar |
T1 |
|
Mains transformer, 240v primary, 15v 1A secondary |
D3 |
LED |
Standard 10mA LED and holder |
F1 |
250mA |
Slow blow 20mm fuse and holder. Terminals should be sleeved after wiring. |
BR1 |
50v PIV, 1Amp |
Bridge rectifier or four discrete diodes (1N4001 or similar) |
F2 |
30V, 1A DC |
Single pole single throw toggle switch |
IC1 |
7812 |
12v, 1A maximum regulator IC mounted on a suitable heatsink |
S1 |
250V, 1A AC |
Single pole single throw toggle switch. See note 7. |
Notes:
1. The following circuit is a simple, low cost, mains powered supply that will provide a stabilised output of 12 volts at up to 1 amp DC.
2. The following circuit shows how an emitter follower may be used as a current amplifier to provide a simple semi-
The 10R resistor provides a simple form of current limit in the event of a short circuit across the 9.3v rail.
Decoupling as shown is mainly for RF purposes but additional larger value capacitors may be used if the incoming line is very noisy.
3. A simple solution to a wide voltage range adjustable regulated power supply without excessive dissipation in the current amplifier is to use a transformer with two identical but isolated secondaries and two sets of rectifiers and smoothing. On low voltage the secondary supplies are connected in parallel and on high voltage they are connected in series. The changeover can be automated by using an op-
The op-
I developed such a power supply for bench applications many years ago with no problems and the basic block diagram is shown below. The maximum output current is a function of the current amplifier, transformer and diode ratings. A reverse protection diode (1N540n) may be connected between the two output lines if required.