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# Bridge Rectifier

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#### fireworks1

##### New Member
This is a post that is going to be ridiculed but I need to ask the
question. I have just tested the Bridge rectifier in my transformer-less
power supply that uses the 12VAC input voltage from an actual power supply.
The reason for no transformer is that it is a school project and we are just trying to
understand basic circuit function of Voltage Regulator, Capacitors and the
Bridge Rectifier. However I believe I have returned
with what seem like strange results... or maybe I am just being
stupid and can't quite realise it at the moment.

I input 12VAC (I measured it to be 11.7VAC) to the bridge rectifier.
On the output side of the bridge rectifier I get about 15.2VDC.

It seems to have stepped up the voltage...?

The only thing I believe may be correct is this.

Say the input voltage is 12VAC, then that means 12V RMS.

12 * 1.4 = 16.8 Vpeak AC

Considering that there is a 1.4volt drop across the
diodes in the bridge configuration, that means there will

15.4 Volts output.. DC?!

Is this correct or am I on the wrong track?

Last edited:
You are correct. A bridge rectifier with a capacitive filter on the output will have an output approximately equal to the peak AC voltage minus the two diode drops.

If you add a load to the bridge rectifier output, then the voltage will drop some, depending upon the resistance of the AC source and the amount of load.

Thank god I'm not as dumb as I thought I was.

The voltage won't just drop some, it will drop dramatically and quickly. The final state output of an unregulated supply should be considered in ALL states, not just the ones you expect, especially with high power supplies.

Sorry to tack a question onto the end of a thread, but while on the subject, when using a bridge rectifier and a voltage regulator, should the voltage regulator be on the DC side? Probably a stupid question but I haven't seen any examples of their use together so I wanted to be sure; don't know enough about voltage regulators to know if they're designed for DC or otherwise.

Sorry to tack a question onto the end of a thread, but while on the subject, when using a bridge rectifier and a voltage regulator, should the voltage regulator be on the DC side? Probably a stupid question but I haven't seen any examples of their use together so I wanted to be sure; don't know enough about voltage regulators to know if they're designed for DC or otherwise.

hi,
You are correct, the Voltage regulator should be on the 'dc' output side of the bridge rectifier.

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