steady 9.5 volts AC in, horrible DC out of bridge.

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someone help.. !!!!

its a 250VA, 9 volt transformer, and outputs 9.5 volts with a slight load. this is rectified with a 35 amp bridge, the resulting unloaded DC is about 12.5... only problem with a slight load, this drops to 11.5 volts, and under heavy load it drops under 11 volts, while the AC remains at 9.5... SOMEONE HELP ME!!!!

if its the bridges. then WHY can a 35 amp bridge not even rectify ac at higher current... please help...
 
Hi,
What do you mean by 'Heavy Load'
A 250VA, 9Vac transformer will produce 27.7Amps (AC)

Allowing for the losses / ineffeciencies in the Recitfier, 10Amps DC is still a very heavy load. You would need a lot of filter capacitance, and short, heavy wiring? Have you considered resistive losses in your wiring?

Also, if you don't have enough capacitance, and you are measuring with a DC voltmeter, then as the ripple voltage increases as you increase the load, your voltmeter will read the average DC voltage, and the voltage reading will drop.
Try measuring the AC voltage on the output of your power supply. This will give an indication of the ripple voltage. (Better yet, get an oscilloscope!)

Adrian
 
SkinnyBoy said:
someone help.. !!!!

its a 250VA, 9 volt transformer, and outputs 9.5 volts with a slight load. this is rectified with a 35 amp bridge, the resulting unloaded DC is about 12.5... only problem with a slight load, this drops to 11.5 volts, and under heavy load it drops under 11 volts, while the AC remains at 9.5... SOMEONE HELP ME!!!!

if its the bridges. then WHY can a 35 amp bridge not even rectify ac at higher current... please help...
This is quite normal. Low current, 0.5-0.6V across each diode. heavy load 0.7-1.0V (or even more) across each diode and remember you have always to in series.... so the voltage will drop. In your case you loose 0.5 volts and that is pretty normal I think.
 
SkinnyBoy said:
someone help.. !!!!

its a 250VA, 9 volt transformer, and outputs 9.5 volts with a slight load. this is rectified with a 35 amp bridge, the resulting unloaded DC is about 12.5... only problem with a slight load, this drops to 11.5 volts, and under heavy load it drops under 11 volts, while the AC remains at 9.5... SOMEONE HELP ME!!!!

if its the bridges. then WHY can a 35 amp bridge not even rectify ac at higher current... please help...

How many amps are a "slight load" and a "heavy load"?

For grins, try checking the connections on the bridge. Look for voltage drops from the pin itself, NOT the connector, to the rest of the circuit. This one bit a hardware engineer a few years ago at my dayjob - he was days chasing a 0.2 volt drop to the CPU until we looked at the circuit board trace, which lost at least half that going from one end of the board to the other.


Cheers,
Francois.
 
"if the AC is stable, shouldn't the DC also be??"

I don't think you understand how power supplies work. AC is not a constant voltage. The capacitors are there to try and fill in for the time where the AC is not at peak voltage. And for 10000uF is not even close to enough to do a few amps w/o a voltage drop. If you wanted a 25A load, I would say you should start at 10x that and work your way up from there. Or use a SMPS.
 
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