Power Supply question [bridge rectifier]

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Sorry,
I have been reading all replays of this thread, but nobody mentioned the link to source (original schematic), so I never seen this schematic before.

Just now I'm reading article but I not found answer
to all my questions mentioned in my previous reply.

Anythig, thanks for link.
 

It's all explained there! - but presumably you know what an earth or ground loop is?. It causes hum in audio systems when they are earthed more than once - as in the case of an amplifier and a mixer, both earthed individually, and then connected together via a signal lead - the earth connection via the signal lead means both are earthed TWICE.

The 10 ohm resistor means that the amplifier isn't directly connected to earth, it goes via the ten ohm - this stops the earth loop problem.

However, if something nasty happens you need the earth connection to pass a high current (to blow fuses or whatever) and the ten ohm would prevent this - putting parallel diodes across the ten ohm allows high current under fault conditions, and makes it safe.

High current bridges are cheap to buy, and it's cheaper to use that (with four diodes) rather than buy two high power diodes. In practice they should NEVER actually do anything, unless there's a serious earth fault - like the transformer breaking down primary to secondary!.
 
When you are passing a lot of current through the bridge, the diodes take a little longer to turn off and for a very tiny instance in time, you effectively put a short circuit across the transformer. This does not harm the transformer but the result is very short spikes that manage to get through the smoothing and regulators. I know as I have seen them on a fast oscilloscope when I was designing heavy current power supplies. The cure was to place a capacitor across the AC input to the bridge, it has to be a high voltage working, say 400V minimum, believe me they are very large spikes!
A value around 220nF to 470nF should do the trick, some higher class equipments install the capacitors where others leave them out. If you see a capacitor across the bridge on a circuit then this is the reason it is there. High speed diodes help minimise the problem but it is still worth adding the cap if you expect high currents to be drawn. The diodes must be rated much higher than the expected current demand because you have to add the smoothing capacitor ripple current, I am not surprised they ask for 35A here but the additional bonus is that these larger bridge rectifiers have a metal base so enabling more effective heat transfer from the diodes to chassis.
Always put some heatsink compound on them as well and make sure they are bolted down firmly but not too tight!
 

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ArcaneDreams said:
Well here is the schematic:
I'm not sure what type bridge rectifier to use, there are so many to choose from. I want a reliable one.

Hi ArcaneDreams,

The fuse rating is generally double of working current, it is believed. thus the load at primaary could be 250VA. taking an efficiency of 80%, secondary may be arond 200VA. Perhaps the load on each output may be around 100/35V. actually 35 volts wont be there by the time it is loaded. thus a bridge rectifier of 7 amps ( a max of 10 amps including safety) is more than sufficient. Finally, I feel that each diode need to carry 5 amps, as this is a full wave rectifier configuration using a bridge and centre tapped transformer to deliver +/- outputs.

the second bridgecoupling the centre point and the ground may be a sort of protective device? i am not sure yet.
 
mvs sarma said:
the second bridgecoupling the centre point and the ground may be a sort of protective device? i am not sure yet.

Am I just wasting my time explaining it?, and giving the link for the original explanation?.
 
Hi Nigel,
Sorry!. I really missed your MAIN letter just and acted.. Yes I should have read it.
Nice explanation was there. on the referred site.

Thanks for guidance.
 
Last edited:
N.G.:
Your explanation is very clear. I understood, even tough my english is pour
and curently I use dictionary to help me. You are more explicit then article.
Thanks.


...but presumably you know what an earth or ground loop is?

I know. Is a basic question.

Regards
 
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