# I need help with this amplifier power supply! (dual rail)

Hello everyone. My name is Dylon J. I'm trying to make a dual rail +/- 24 volt power supply for the tda1514a power audio amplifier. I used a 120vac to 24vac buck transformer which goes to a 50 amp full wave bridge rectifier, then that goes to two 10,000uF 35 volt caps wired in series with each other, Those caps are then wired in parallel with the bridge rectifier, and wow. Testing the output I have only -18.2 volts and +18.5. What? Where did this voltage drop come from? It should be at least 22 volts. BTW the schematic for the power supply is here: Construya una Fuente de Poder Dual con Salida Positiva Regulada | Video Rockola . So I tested some other caps and even worse, I got -30 and +2 or something close to that. How is this possible. I measured the caps on my bk precision 2709b dmm. I got 5.5-5.6mF on all of them. Off topic here, is it the multimeter's fault or caps for such a low reading? Please help me!

What did you expect ?
24V AC * sqrt(2) = 34V DC (no load) => +/- 17 (substact diode drop and you get +/- 16V)

This is just as it should be..

You need a 48Vac centertapped transformer. With a 24Vac center tapped transformer, each half is only 12Vac (for some reason, this is usually a multiple of 6.3V, so 12.6Vac), which after rectification and a double diode drop, is about 12.6x1.414 - 1.4 = 16.4V.

Without a load and with variations between transformers, +-18V isn't surprising. What you really have here is two full-wave center tapped rectifiers, one providing the positive supply, the other the negative supply.

Here is what one positive supply full wave center tapped rectifier looks like:

It acts as two half-wave single diode rectifiers, each using only half of the secondary. So you must do your calculations based on the voltage from the center tap.

If you are getting off-center voltages, I must ask if you are using a center tapped transformer. You -must- use a center tapped transformer for this power supply.

i'm using a regular transformer. Not center tapped.

Can dual rail power supplies only be made with center tapped transformers? So if I use the center tapped transformer, 24vac, will I get +/- 12 (will load on) power supply?

..or another, same type you already have..

What voltages are you measuring? With a center-tapped transformer, I'd expect you to measure voltage between the transformer's center tap and the DC outputs, to read something like +18V and -18V. If the transformer isn't center-tapped, what points are you measuring between?

I think we need a schematic, with the measurement points indicated.

Yes, it must be center tapped. Otherwise, where is your ground reference? If one rail has more power pulled out than the other, the voltage drops and appears on the other rail.

You -must- use a center tapped transformer, as specified in the link you gave.

Otherwise, you need two transformers and two bridge rectifiers.

here is what I made, after all reading all of your comments, I think I see the problem. I think, I need a center tapped transformer correct, center tab to gnd, top one to + and bottom most tab to (-) rail on the ct transformer. Thank you.

Dylon J

I'm not sure what you mean about connecting the transformer to + and -. The inputs of the bridge rectifier are usually labeled "AC" or with a little sine wave ~

You want this:

Without a center tap for ground, you might read a no-load voltage from one rail to the center of the capacitors, but once a load is applied, the voltage will vanish and no current will flow.

You need a 48Vac centertapped transformer. With a 24Vac center tapped transformer, each half is only 12Vac (for some reason, this is usually a multiple of 6.3V, so 12.6Vac), which after rectification and a double diode drop, is about 12.6x1.414 - 1.4 = 16.4V.

No you do not need a 48V centre tap.
You need a transformer that has 18Vac either side of centre tap.
18(RMS)*1.414=25.45V(PEAK) less 1.4(DIODES DROP)
25.45-1.4= 24.05Vdc

Tom....

..amplifier is built for +/- 27,5V (max +/- 30V)

Doh! You are correct, thank you. They are correct, you want a 36Vac center tapped transformer, or two 18Vac transformers.