# help with peak voltage

Discussion in 'Homework Help' started by terrakota, Feb 21, 2005.

1. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
hi,
I'm a little confused when and when not take in account the peak voltage,

i understand that when i use pure DC i dont need to worry about the peak voltage value but if I'm working with AC voltage like in power supply applications i must take peak value in account, my question is at what point peak voltage dissapear? or no longer exists? ie, in a power supply application that converts AC to DC and "reduce" the AC voltage u use a bridge rectifier and some capacitors, to choose the rigth capacitor filtering i need to take in account the AC voltage(ripple?) but after capacitor filtering an AC voltage not exist any more so, theres no more a peak voltage?
with pure dc voltages like batteries?.

am i right? how i deal with peak voltage or when the peak voltage dont exists any more.

please excuse my poor english

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

There is a little history here. After Mr. Tesla invented the AC motor, there was some confusion as to how to relate AC motor capability to DC motors. Some figuring established that by taking the root of the square of the mean AC voltage (that's the RMS voltage), then AC power could relate to DC power. Nice for power calculation, but a bit of a pain for power supplies.

After rectification, the DC voltage on the filter capacitors should be close to 1.404 times the AC RMS input voltage. For instance, 12 VAC after rectification and filtering will produce about 16 VDC.

That seems to eliminate AC ripple - the peak voltage. Actually, when the filter is supplying current to a load, the voltage across the capacitor will draw down during the time no charging current flows through the recrifier. It will look like a small sawtooth waveform superimposed on top of the DC voltage. The top of the ripple will be the same as the AC peak voltage.

Normally, a regulator outputs several volts less than what is present on the filters. As long as the current draw does not couse the voltage on the filter to sag low enough to go lower than the regulator's set output, ther will be no problem. It is necessary to use filters large enough that the ripple voltage does not interfere with good voltage regulation.

Hope this helps...

3. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
ok i got it, but when i work with DC i dont need to worry about the peak voltage because is too little to worry aboutno ? or in the case of batteries is there some kind of peak voltge or is pure DC(a complete flat Line)?

thanks for your great help

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

Yes, with batteries the voltage is always at peak. No ripple to worry about.

5. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
ok thanks a lot for your help