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Bridge rectifier

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techy_not

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Is a bridge rectifier supposed to cut voltage or regulate it. I have a power suppluy that has the input voltage at 120 but it leaves at 70. Is that normal.
if i disconnect the dc ends from the board i get the desired voltage but once connected to the board i get ZAP,0,NOTHING. any ideas as to what might be causing the short on the dc side of the board. or is the short taking place on the ac side. Please send me your comments
 

tcmtech

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A bridge rectifier only changes alternating current into direct current. 120 Volts AC put in will give you 120 volts DC with a big ripple voltage. By adding a smoothing capacitor you should get around 168 volts peak DC as your output.
Either your rectifier is not connected right or you have bad diode in it.

70 volts open circuit would suggest you only have one diode not working out of the four that make up a full bridge rectifier. you are essentially only getting about 1/2 of the average voltage.
 

techy_not

New Member
Thank you i realise i was testing it wrong. Thanks so much tmctech. Any notes on fixing a switchin power supply? something is wrong on the dc side. i believe something might be shorting. the ac voltages are okay up to the transformers 1)5v 2)24v. I get no dc voltages from them when connected to the board but once i de-solder them and test them they work.
 

tcmtech

Banned
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Just a guess but a common problem with switching power supply's is for capacitors to go bad. If you have any with swelled up or rounded tops or what looks like burnt goo that is around the base replace them and put in new ones.
The short high current pulse widths and high frequency's are naturally hard on electrolytic capacitors and can cause internal failure by cooking them to death from the inside out.
Plus many switchers are built with low grade capacitors to start with.
 
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