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I am making this circuit:

enter image description here

C1, C2 2200 μF 50 V Electrolytic Capacitor

C3, C4, C5, C7 2.2 μF 50 V Electrolytic Capacitor

C6, C8 100 μF 50 V Electrolytic Capacitor

R1, R4 5K Potentiometer

R2, R3 220 \$ \Omega \$ 1/4 W Resistor

D1 to D4 IN 4007 Diodes

U1 LM317 U2 LM337 T1 24 0 24 Center Tapped 2 Ampere Transformer

the goal is to create a power supply that can give a variable supply of -15V -to-0 and 0-to-15V

I have already placed all the components in the breadboard and tested the output. The problem is the readings on the positive side is from 1.6 V to 30 V when I turn the potentiometer from he left most to the right most same with the negative side( but the sign is negative)

What can I do to get the specific range I need of of -15V -to-0 and 0-to-15V

Bence Kaulics
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Edward Montilla
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  • I was going to comment that you can't get zero volts out of this, but then I realized that you actually can, because you have a negative supply (and vice versa) – pipe Jul 31 '16 at 18:39
  • Are the midpoints of C1/C2 etc. connected to ground? I don't see a dot at the connection. – markrages Aug 02 '16 at 00:58

1 Answers1

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The simple solution is to install a 4.7K in series with each of the 5K pots leads going to ground. That should cut the 30vdc to about 15 vdc for both positive and negative supplies, at the 'maximum' setting. That is two 4.7K resistors total.

Since the LM317/LM337 series have an internal 1.25vdc reference, the easy fix is to install series diodes in the outputs, after the capacitors, causing a 1.25 volt drop. Two each 1N5408 3 amp diodes plus using 2.2K 1 Watt resistors across the outputs as minimum loads will bring the outputs close to zero volts at the 'zero' setting. That is 4 1N5408 diodes total and 2 2.2K 1Watt resistors total.

Be sure to heatsink the IC's if drawing more than 200mA from them. Also, a room temperature change or regulator change of 20 degrees F will cause at least a 100mV change in the output voltage. These are general purpose voltage regulators, not precision regulators.

There are more accurate solutions that use zener diode and op-amps and trim-pots, but I was not sure what level of priority that had.

EDIT: Your probably using 5% resistors, so don't expect precise tracking or that maximum volts is = +15.00, etc. Or that zero volts is equal to 0.00. Or that a substantial change in temperature of U1 or U2 causes a change in voltage. If you want better accuracy replace the 4.7K resistors with 10K trim pots. Put their wiper positions in the center to start with, then trim for max voltage = +/-15.00vdc.

  • You're missing a trick here. See my answer to [Making LM317 output voltage adjustable down to 0 V](http://electronics.stackexchange.com/questions/244354/making-lm317-output-voltage-adjustable-down-to-0-v/244358#244358) and Russell's suggestion for further improvement. – Transistor Aug 01 '16 at 22:53
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    @transistor. I had suggested as much to the OP about more accurate solutions, including tracking regulators. I have built all of them over the decades, but the OP seems content with a simple if-not-accurate fix. You could post an answer to get the OP's attention. Worst case is a null or +1 vote. –  Aug 01 '16 at 23:22
  • @transistor. OK, I'll give you a +1 for posting that more accurate option as an answer. The OP should see the options, if he is still paying attention to this page. –  Aug 01 '16 at 23:41
  • I like to ask is it possible to remove some of the capacitors? and replace it with a different value? what benefit does having that kind of set up with capacitor have? – Edward Montilla Aug 07 '16 at 06:34
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    You could remove some of the low value capacitors, often placed close to the LM317/LM337 for stability. The large value capacitors keep the voltage study under changes in load. You have an ideal capacitor layout already. Remove them at the risk of loosing voltage stability. –  Aug 07 '16 at 18:58
  • @sparky256 thanks for the help the power supply supply worked can you help me explain how did it came out with that output coming from a 24v-0v-24v transformer to produces a voltage range of -15v to 15v... i have to make a presentation about it – Edward Montilla Aug 23 '16 at 03:30
  • I don't mean to cop out on you, but the LM317 and LM337 are doing all the work for you. They only require that the source voltage be at 3 volts above the highest expected output voltage. They have a 1.25 volt internal reference that is multiplied by the ratio of R1 and R2, R3 and R4, to get your output voltage. The extra diodes caused a 1.25 volt drop so you could have zero volts out, so the added trim pots were used to add 1.25 volts to the output only at full scale. It is cheating at its best. Read the fine details of the LM317 and LM337 series. –  Aug 25 '16 at 20:14