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Andrew Davie wrote:Another bit of information I had forgotten - a 15V AC supply is actually measuring the RMS voltage. In fact AC goes from +15 to -15V
Still getting my head around how a rectifier can make 20+ volts from 15V AC though. Is there 30V "potential" there...? More studying/revising to do.
gary wrote:Andrew Davie wrote:Another bit of information I had forgotten - a 15V AC supply is actually measuring the RMS voltage. In fact AC goes from +15 to -15V
Still getting my head around how a rectifier can make 20+ volts from 15V AC though. Is there 30V "potential" there...? More studying/revising to do.
I think you have made a typo there Andrew, a 15V AC (RMS) goes (peak to peak) from (+15 x (the square root of 2)) to (-15 x (the square root of 2)) - that is (+21.2 to -21.2V).
The rectifier (diodes) *doesn't* make 20+ volts, it just makes the -21.2V part of the AC voltage +21.2V (full wave). The things that makes the DC 20+ volts are the whopping big capacitors that hold the peak voltage between peaks. Notice this results in "ripple" and that's where the voltage regulator comes in.
Andrew Davie wrote:gary wrote:Andrew Davie wrote:Another bit of information I had forgotten - a 15V AC supply is actually measuring the RMS voltage. In fact AC goes from +15 to -15V
Still getting my head around how a rectifier can make 20+ volts from 15V AC though. Is there 30V "potential" there...? More studying/revising to do.
I think you have made a typo there Andrew, a 15V AC (RMS) goes (peak to peak) from (+15 x (the square root of 2)) to (-15 x (the square root of 2)) - that is (+21.2 to -21.2V).
The rectifier (diodes) *doesn't* make 20+ volts, it just makes the -21.2V part of the AC voltage +21.2V (full wave). The things that makes the DC 20+ volts are the whopping big capacitors that hold the peak voltage between peaks. Notice this results in "ripple" and that's where the voltage regulator comes in.
Hi Gary thank you for your posting/clarification.
My understanding is good on all of it, except for perhaps the terminology of 15V RMS.
In the way I explained it, -15V to +15V AC would actually be 15/sqrt(2) = 10.6V RMS. But yes, I got that bit wrong.
I was assuming 15V AC had a range of -15 to +15 when in fact as you say it's -21.2 to +21.2 and the 15V "value" is the RMS calculated from those.
I'm sure I understand now. Already understood how the rectifier changed the negative part, and we had a series of positive hills, and how the capacitor "fills in" the dips to give a more even waveform, but with ripple. Appreciate your clarification! It's been 10 years and I'm re-learning all of this, but THIS time I'm going for *understanding* not just a working thing made by following advice. Cheers!
Andrew Davie wrote:I'm wondering how the capacitance of the ripple-fixing capacitors are chosen.
The 17V side has a "massive" 4700nF - and this came from a circuit diagram found/supplied during my original build.
The 21V side has a much smaller 1000nF one. Seems to me that there's little rhyme or reason here.
Andrew Davie wrote: I hooked up the oscilloscope to the output and saw no sign of ripple at all - even at 10us timebase.
gary wrote:If you are going to have a regulator you don't want the ripple dipping below the regulator output voltage + drop out voltage of the regulator (~1.5V from memory)
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