Bridge rectifiers in PSU

[Speek]

Hi! This is my first post on this forum. I'm a complete newbie to electronics, so please bear with me :-)

I've just received the PCB from Rod Elliott for the P3A amp. The PSU will be the Project4 PSU: http://sound.westhost.com/project04.htm. I'm now making a list for ordering the components. Rod mentions that the two bridge rectifiers should be rated 35A. The company where I'm ordering the parts doesn't have that. Maximum rating they sell is 25A. Would that be enough? I'll be using a 300VA +-30Vsec. trafo. How to calculate the rating for the bridge rectifier? TIA,

Wim

[Peter Daniel]

How much capacitance are you using, because that will influence longevity of your bridge.

[skaara]

It will be enough, but put it on the heatsink, because it will be warmer than 35A model would be.. You can calculate the current drawn with ohms law (I = U / R)

[Speek]

Thanks for the replies. I'll be using 4 10.000 uF elco's (2 per bridge rectifier). And there's no space for the rectifiers on the heatsink, but I can bold them to the bottum of the case. I know of Ohm's law, but what U and what R should I put in the formula?

[Peter Daniel]

25A would be plenty.

[tiroth]

Bolting them to the case works great.

[Kilowatt]

P3a is 120W into 4 ohms with +/-35V supply. About 22V RMS may appear across the speaker load, so 22V/4ohms = 5.5A. 5.5A x 22V = 121W. This is just the current going to the speaker, this is perhaps 65% of the total current, equating to maybe 8.5A total draw, certainly not more than 5A/rail, as the rail fuses are rated 5A. This is very vague, and it may well draw more, and it will certainly draw WAY bigger peak current. The filter caps, on start up, can draw as much current as they want, it all depends on their ESR, and more so how much current the transformer can supply. This is fairly brief, much less than 1 sec in your case. Also you want the bridge to be plenty overrated so it doesn't get to hot. In short, a 25A bridge should work fine, a 35A bridge would be good for a pair. bolting it to the case (with heatsink compound, of course) is great, especially if the case is heavy aluminum.

http://www.alliedelec.com has 35A bridges, if you would rather use that. In fact, they have them up to over 100A if you get into 3 phase ones and 1/2 bridges. They also have descrete diodes up to 1170A, 1600V, for what it's worth. I have found no better place for getting electronic parts.