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Old 26th February 2005, 06:51 AM   #1
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Default steady 9.5 volts AC in, horrible DC out of bridge.

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...
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Old 26th February 2005, 06:54 AM   #2
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Ok, I'll ask an obvious question..... do you have filter caps after the bridge?
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Old 26th February 2005, 07:16 AM   #3
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how much would I need?
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Old 26th February 2005, 08:16 AM   #4
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I have about 10,000uF on the DC, and it still drops to within the 10 volt range, while the AC remains constant..

if the AC is constant, then the DC should be too.... even if there isn't enough capacitance.
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Old 26th February 2005, 08:45 AM   #5
dnsey is offline dnsey  United Kingdom
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Check the volt drop across each leg of the bridge under load. You'll probably find one high reading corresponding to a faulty diode.
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Old 26th February 2005, 08:50 AM   #6
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and what should I measure across them?
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Old 26th February 2005, 06:14 PM   #7
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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!)

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Old 26th February 2005, 08:10 PM   #8
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Default Re: steady 9.5 volts AC in, horrible DC out of bridge.

Quote:
Originally posted by SkinnyBoy
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.
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Old 26th February 2005, 08:27 PM   #9
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Yep, voltage drops with loads are normal. The way around it is to regulate it (in which case it won't be 12.5V anyway), or just beef it up ridiculously. It's probably best to watch what's going on with an oscilloscope if you can get one.
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Old 27th February 2005, 12:45 AM   #10
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I have a cro.... it broke...

but my point is... if the AC is stable, shouldn't the DC also be?? even if it doesn't read stable with my multimeter...

alright... if I need more capacitance, how much would I need assuming a 25amp load on the supply?
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