Need help with powersupply.

[Cyberzim]

Hi there,
I need a powersupply that can delivery a stable 25VDC output at 200 watt r.m.s.
I have looked on some transformer on digikey, and founded a transformer that deliver 20VAC @ 8.8 A, so that would be 28VDC peak, and I guess it would drop 1,4V after it have passed the diodes and also I need some huge capacitors, but how huge should they be?
And is it the correct transformer to use?

Thank you,

Simon H.A.

[mjf]

hello.
20v x 8,8a gives you only 176w................it is a little bit small if you need 200w.
and the transformer must have reserves for chargeing the (big) elco's.
greetings..............

[Cyberzim]

Yes you are right.
I have looked on a cheaper one that delivers 20VAC @ 6.5A, so I could buy 2 of them and then parallel them.
But what would you suggest in capitance of the capacitors?
I need very low humming.

Thanks,

Simon H.A.

[AndrewT]

Hi,
if you plan to power a single channel of 200W into 4ohms then your transformer should be between 200VA and 400VA.
<200VA will reduce performance too much.
>400VA will cost too much, weigh too much, take up too much space for the very small improvement in performance.

Most amplifiers need a dual polarity supply.

What are you building?

Do not buy two smaller transformers. They will have higher regulation than a larger one and they will cost more.

[Cyberzim]

I have made a sound system that is on Roskilde Festival, but it runs on 2 car batteries, so that is arround 25.3VDC.
I need a powersupply so it also can run on a normal powersupply.
But it's important that the supply is around 25-26volt, since some of the designs are made only for 25VDC.

Simon H.A.

[Cyberzim]

btw, it is dual polarities so it's 12.65 +/-

[AndrewT]

Hi,
a 20Vac transformer is the wrong transformer for a dual polarity +-13Vdc supply.

You need a 10+10Vac transformer of 200VA to 400VA.
This may be difficult to find. Try 9+9Vac 11Aac (=200VA) to 22Aac (=400VA).

If after building the new PSU you find the voltage is a bit low you can add a few turns to the toroid secondaries to raise the voltage to 9.5Vac or 10Vac or 10.5Vac.

The wire you choose must be capable of passing that rated output current. Do not use a single very thick wire (2.5mm or 3mm diameter). Instead use parallel 1.6mm diameter enameled wires to give the same total area that you need.
1.6mm diameter can pass upto 6Aac.
If you need 11Aac (200VA) use two 1.6mm wires. If you need 16.7Aac (300VA) use three 1.6mm diameter wires.

[Merlinb]

Originally posted by Cyberzim
Simon H.A.

Considering one half of the supply first of all:

For a full wave rectifier ignoring diode drop,
Vout = Vpeak - Vripple
Vripple = Iload/(2fC)
Vout = Vpeak - (Iload/2fC)

Vout = 12.6V, Iload = 4A

Say you can tollerate about 1V ripple.
C = 4/(2*50Hz*1V) = 0.04F = 40000uF on each supply rail.

Vpeak = Vout+1V = 13.6V
Also add a little for diode drop, say 3V, = 16.6Vpeak.

Vac = 16.6 / ^/2 = 11.9Vrms
So you are probably looking at a 0-12, 0-12V transformer, 300VA minimum.

[Cyberzim]

Hi Merlinb,
Thanks for the calculations!
Now I know what to look after, so that's just great.
In my country we use 230vac/50hz, but I looked on digikey, and I found this transformer.
It can use both 117 and 234vac 60/50hz, and gives a parallel output of 22VAC @ 10,22A so that should do the thing.

Regards,
Simon H.A.

[AndrewT]

no.
average Vout = Vpeak - {Vripple/2}

Minimum Vout = Vpeak - Vripple.

Maximum Vout = Vpeak.

Vpeak will vary with load.
Vripple will vary with load.
Average Vout will vary with load.

The load will vary from DC (continuous load) to AC (sinewave following the sinewave at the output) to transient spikes to half sinewaves and saw tooths etc.