18 Jan
2010
18 Jan
'10
12:08 p.m.
Tony Duell wrote:
You would expect ot see 60Hz waveforms from a half-wave rectifier (one
diode) with no smoothing, and 120Hz waveforms from a full-wave rectifier
(either 2 diodes with a centre-tapped transformer winding or a 4-diode
bridge rectifier) with no smoothing. The open-circuit capacitors would
account for 'no smoothing', but if you have what appears to be a
full-wave circuit giving 60Hz ripple, I would suspect faulty diodes too.
Do you mean AC, or DC with excessive ripple. Did
any of the lines go the
wrong side of ground? (my guess is they didn't, but it's best to be sure).
DC with excessive ripple. Nothing on the wrong side of ground but 60 and
120 Hz waveforms almost down to ground. Yes, there is an 18 Ohm 2 Watt resistor between the +9
and +5 (the input
and output pins of the 7805). I was wondering what that is supposed to do.
Is this resistor causing the +5 side to "look higher" than it really is?
No, the voltmeter is genuinely measuring the voltage on the 5V rail. It
is 9V. The circuit _may_ be working -- if you incres the load, the
output voltage may come down to 5V and then _stay there_ for increasing
loads (that's more current, so lower resistance load, of course), until
you get to tyhe point where the 7805 goes into current limiting.
I explained the action of said resistor in another message, but the
poster there said it was 15 ohms. If it's 18 Ohms, the principle is the
same, you jkust have to re-do the calculations.
Is everything actually OK, even though my VOM and
scope both agree the +5
rail is at +9.2 _WITH_ automobile light dummy loads?
What sort of car bulbs? What wattage?
-tony