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Dumb rectifier question

MadMechanic

Aug 28, 2012
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I know this is a dumb question, but ive read mixed results. What is the voltage drop across a normal generic full bridge 4 diode bridge rectifier? I have heard 1.1V per output lead, some say 1.1V per diode, and some say 1.1V for the whole rectifier. If I have 12VAC plumbed into a 4 lead full bridge rectifier, what will be the estimated VDC output? I have not had a chance to meter this yet or I would already know. Thanks,
 

CocaCola

Apr 7, 2012
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There are other variables that come into play, so there is no 'definitive' answer...

With 12 VAC in you should get about 10.6 VDC out... That assumes a forward voltage drop of about 0.7 per diode, and the current going through 2 diodes at any given point...
 

MadMechanic

Aug 28, 2012
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There are other variables that come into play, so there is no 'definitive' answer...

With 12 VAC in you should get about 10.6 VDC out... That assumes a forward voltage drop of about 0.7 per diode, and the current going through 2 diodes at any given point...

I see this makes more sense now. So even though there are 4 diodes in a full bridge rectifier, The distance from ACin to DCout is one diode at any given point in the rectifier, thus 0.7 x 2 diodes = 1.4V loss from AC input pole to DCout pole.

In theory, if the AC generator on the AC side of the bridge is rated at 12VAC....and I were to increase the rpm input of the generator to compensate for the voltage loss across the bridge to bring my DCVout up to 12VDC....this would mean the generator output load would increase to 13.4VAC correct? Thus exceeding its rating? I am assuming the rectifier is treated as a resistor.
 

davenn

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increasing the RPM of the generator may increase the output voltage but it will definately increase the frequency of the AC signal, that may cause other problems
where a system is expecting 60Hz/50Hz depending on your country

generators are generally designed to run at a constant RPM to keep that freq constant

Dave
 

john monks

Mar 9, 2012
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Is your 12VAC in RMS?
In this case your output should be about 14.7 volts.
Your are cursed to have to rely on the datasheet. And if this is not sufficient you calculate in the diode impedance. I used the formula .025 divided by the peak forward current to get the impedance and figure the actual output voltage up or down using this impedance.
This is an estimate but may be helpful.
 

MadMechanic

Aug 28, 2012
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Thanks, That does help. In some cases the VAC is listed as RMS voltage, depending on the manufacturer. I am planning to take a 2-phase stepper motor and wire each set of leads through a bridge rectifier to create a voltage doubler circuit. The main output I am aiming for is at least 8Vdc, 1A. Many of these motors are low voltage but capable of 1A with both phases combined. I just need to make sure I can achieve a constant voltage above 5Vdc 900mA so that my voltage regulator will work @ 5V 900mA which is the power of USB for charging devices. Now I just need to figure out which motor will provide what I need, even with the loss at the diodes.
 

CocaCola

Apr 7, 2012
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I just need to make sure I can achieve a constant voltage above 5Vdc 900mA so that my voltage regulator will work @ 5V 900mA which is the power of USB for charging devices.

Many regulators need an excess voltage to remain steady, as high as +2V in many cases...
 

MadMechanic

Aug 28, 2012
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Many regulators need an excess voltage to remain steady, as high as +2V in many cases...

True, this is my concern as most of these stepper motors seem to deliver on the borderline of what I need with the losses.
 
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