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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
With regard to quiz 6:
Why does the ripple current average to zero? The diode only allows positive current to pass so shouldn't the ripple always be above zero and hence the ripple average > 0? Or is the ripple above and below zero because the load is connected to both capacitors (which connects both diodes)?
As far as the RMS meter, I did not want to buy one; I wanted to know if you had any good links on the theory behind a RMS meter vs a regular "average" meter. From what I can tell, since reading about them, the RMS meter is simply "mismarked" to read the RMS values. A true RMS meter will measure ohmic heating of a resistive element, allowing it to calculate RMS for any waveform presented to the meter.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
According to KCL, the current through the diode is the sum of the ripple current through the capacitor and the load current. The ripple current goes both above and below zero, but when you add in the load current, you can see that the diode current ends up always positive or zero.
I guess another way of saying that, is that the positive half-cycles of the ripple come through the diode, and the negative parts flow to the load.
BTW, I seem to remember you were asking about where to get one of those meters that read RMS. They are called moving iron ammeters and you should be able to get one from eBay or a decent sized electronics dealer like Digikey or Mouser. I don't know how accurate the RMS really is for odd waveforms.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, the current through the diode is only in one direction. It's a diode after all, that's what they do. But when I say "ripple current" I mean the current through the capacitor, which is not the same as the current through the diode.
As far as I know, iron vane meters do really respond to the RMS of a waveform. They're not just mismarked like you say.
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Voltage double and current values (referrencing quiz 6)
