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JAZZ2250 18th July 2002 09:56 PM

question on transformer and caps
What should be the rating of a power transformer and caps for the power supply if I want to run a car amp at home? Supposed that the power supply DC voltage will be 12V and that I will be using an amp w/ 2 x 20W and two 8 ohm speakers. Don't know how to figure out the current required. Thanks in advance.

Nelson Pass 18th July 2002 10:10 PM

2 X 20 watts means about 100 watts maximum draw,
and the transformer should be rated at more than
twice that, so 200+ VA.

You want a secondary DC voltage of 13.8 Volts, and so you
should figure on 12 volt secondary AC voltage with
a 20 amp rating.

paulb 19th July 2002 02:42 AM

Are you thinking of having a voltage regulator, or just a transformer / diodes / capacitor(s)? If the latter, you'll find that the voltage at low volumes is closer to 16 volts than 13.8. This may be okay, depending on your amp.

JAZZ2250 19th July 2002 04:13 AM

Will this do?
Thanks for your replies. I found a transformer rated at 12.6V/25A at allelectronics. Will it be ok? If so, what cap should I choose for transformer/full wave diode rectifier/capacitor power supply? I'd like to know how to calculate a proper cap value. Thanks.

AudioFreak 19th July 2002 04:22 AM

That should be fine so long as the amplifier is capable of dealing with the higher no-load voltage that will be presented at idle... 25V caps would be fine... i'm assuming the amp has a switching power supply as most car amps do .... in this case you dont need alot of filtering on the rail input say between 4700F and 10000F should be fine but go for caps with a high ripple current rating, low ESR and low leakage figures.

JAZZ2250 3rd August 2002 05:46 AM

role of capacitors
What's the role of caps in power supply? Is it only for filtering the AC to DC? Is it related to the maximum current which can be drwan from the power supply? I bought transformer and full-wave diode rectifier, and it's time to think about the caps. I have 3,300 and 4,700 uF caps. Both are rated at 50V.

seangoesbonk 3rd August 2002 07:58 AM

Re: role of capacitors

Originally posted by JAZZ2250
What's the role of caps in power supply? Is it only for filtering the AC to DC? Is it related to the maximum current which can be drawn from the power supply?
The diodes are what "rectifies" the AC into DC. Maximum current is determined by the current rating of the transformer.
The AC leaves the transformer, goes through the diodes, and comes out as DC on the other side. The DC that comes out of the diodes is not very steady at all(voltage-wise). The capacitors "smooth" out the fluctuations or "ripple".

The linear unregulated power supply is not that difficult to understand if you have diagrams to look at(but it is difficult to explain). I will attempt to draw you some pics tomorrow if I can get my scanner working.

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