# Reduce power supply voltage

I'm trying to build the power supply described here.

R1: 1K8 1/4W Resistor

C1,C2: 4700µF 25V Electrolytic Capacitors

D1: Diode bridge 100 to 400V, 1.5 to 4A

D2: LED Any type and color

SW1: SPST Mains switch

T1: 230V Primary, 30V Center-tapped or 15 + 15V Secondary, about 45 to 60VA or 1.5 to 2A, Mains transformer

PL1: Male Mains plug with cord

In fact, I already did it and it's working. The problem is that the output voltage is higher than the voltage supported by the IC (ne5534), that should be 22V (maximum).

After I built it, I measured +-24V. I would like to limit it to +-20V. How do I do this?

Searching the internet I found that I could use LM317/337 (since I need +/- output). Or I could use 78xx/79xx (it seems to have a 18 or a 24 version, but not a 20V). Or I could use a resistor. Or diodes (regular or zener).

I would like to use the resistor/diodes solution, since I have some spare/salvaged ones In hand.

If I calculated (V = I*R and P = I^2 * R) correctly, to use resistors, I should use two 4R 4W (one for each output).

If I use standard silicon diodes, it will require a minimun of 6 (in series) for each (12 total).

The approach that I prefer is the use of a zener diode, but unfortunatelly (I'm a hobbyst) I dont know how to calculate it... My problem is with the current. How much power the diode/resistor should accept (considering 1A). I found many papers describing zener theory, but I didn't manage to solve it by myself... It seems that a 20V 1W zener diode would fit, but it will require the same 4R4W resistor to regulate the current...

• Your power supply is unregulated. Its output likely to fall somewhat just from loading. Try just putting a resistive load on it (drawing a current similar to your amplifier) and see what the output is then. Commented Feb 16, 2015 at 5:43

Your root problem is that either you are using the wrong transformer or your mains voltage is too high. Replacing your transformer with a lower-output unit is your best bet.

Assuming that you really are using a 30 VCT transformer, rather than the 36 VCT unit which I suspect you have, replace it with a 24 VCT unit.

None of the solutions which you are considering will work terribly well. A zener can do the job, but you need a high-power device in order to handle the surge currents which occur at the mains voltage peaks. If you use 1 ohm resistor and a 20 volt, 10 watt zener you should be OK, but those things are expensive.

Something like an LM317/LM337 will not work well, since at full amplifier output you're running beyond their current capability, and the reliability of the circuit will be very poor.

Putting a resistive load on the supply to drop the voltage is actually the worst of your options. The problem is that it won't do any good if you don't have a load connected to the amplifier. Even during test you would need to maintain a load, or risk killing your op amp.

Using diodes to drop the voltage is just as iffy as using resistors, and for the same reason.

• Actually, low-power transformers have horrible regulation that needs to be taken into account when designing a circuit. Finding one with 25% more voltage open loop than when fully (loaded) to the rated voltage and amperage is actually rather common/easy. I have on hand 24V+24V 16VA transformers that output 29-30 V (RMS) open loop [that 1V is due to main variation between measurements at various points in time], but the 5-6V "extra" are just due to low/lack of load. (Fully loaded they give just 24V.) Commented Sep 22, 2015 at 23:35
• If you're luck to find transformers that have this [de]regulation number in the datasheet, it agrees with what I said above. Here's an example from a different mfg, but similar product where you can find this stuff in the datasheet (15VA, 2x25V under load, but 2x29V open) tme.eu/en/Document/505ab7a151e6fa111ec7ce1e42277c4f/55xxx.pdf Commented Sep 22, 2015 at 23:43