Mains power puzzle

Ther Carver "Mag Amp" had a triac in series with the mains on its power transformer to keep the transformer from saturating because it was physically too small for the job without limiting the volt-seconds somehow.

Maybe others have followed this design ?

boB

I've used a series resistor, shorted by a triac, in series with a toroidal transformer primary. It softens the turn-on and can even be used as a rough pre-regulator.

It's really tough on the resistor!

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Any specific examples you can tell us about, with resistor details?

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 Thanks, 
    - Win

Fuse fatigue over the years?

I started with one of the giant gold wirewound metal-case military style resistors, but they fatigued and died, sometimes shorting internally to the case.

This style of porcelain on steel thick-film, on a heat sink, took the strain:

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

For the carver amp, they just put a light-dimmer in front of the transformer...

I have a sorta-similar issue now. A DDS sine wave generator will drive a class-D 240 watt amp, into a custom toroidal transformer. Weird things happen at instant sine startup, so we'll program the DDS logic in our FPGA to ramp up and down the sine wave amplitude, basically slew rate limit the sine envelope.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I mis-remembered. The DC is blocked by large electo caps (some mF) and the diodes are merely there to protect the capacitors.

This article explains it:

Allan

.

OK enough guessing... what's the answer ???

Thanks, Allan, very interesting!

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 Thanks, 
    - Win

Darn, I thought I invented that.

Pity there are no high-C 1-volt electrolytic caps around. We usually use 6.3 volt parts for this. There are only a few lower voltage ones around.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

The writer claims that this circuit won't allow DC through the diodes but I fail to see that logic. You would have two diode drops in each direction, essentially giving roughly 1.414VDC drop in each direction so the capacitors are pretty much shorted out above that voltage. If you scoped across the diodes you would see a waveform that has a slight gap at the zero crossing point if I am not mistaken...

People have used the diodes wired as shown (25 or 35A rectifier) as a

1.5VAC voltage drop to reduce the power to incandescent bulbs used in pinball games where the lighting secondary voltage was higher than the called for 6.3VAC.

John :-#(#

"A quick look inside revealed what was going on.".

Not obvious when you don't have the schematics nor have the unit sitting

in front of you - too many variables. Perhaps there is some TRIAC or SCR

involved in the supply side of the circuit which might account for the odd readings.

John :-#(#

The inrush to a toroid can fry a front-panel power switch, so sometimes one uses a tiny switch and a relay or a triac to switch the AC line.

I've heard the wiring in the wall make an audible thump when a big toroid was switched on. So some soft-start is useful too, a second triac+resistor, or an NTC thermistor.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

The DC component in mains is usually way less than two diode drops. The capacitors should be large enough that given the maximum normal operation DC level + current*reactance does not cause diode conduction. The waveform thus does not have gaps.

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mikko

Absent further guidance / information from P.A., along with the concomitant insults, name calling and general nastiness, I've been thinking about the DC-blocking scheme.

Rod Elliot's experiments prove that it's the capacitors that are really doing the heavy lifting, and the diodes are merely there to protect the capacitors. That's because it's not practical to make the capacitors large enough to handle the transformer's maximum operating currents. At higher currents the diodes take over. As John Larkin pointed out, in order to get as much capacitance as possible, you choose low-voltage electrolytics, like 6.3 volts. This means that if one of the diodes fails, the capacitor(s) will simultaneously experience excess voltage with high currents, and fail. I imagine that it'd fail as a short, disabling the DC-blocking scheme, and allowing the transformer to make the unwanted buzzing sound. But not blowing the fuse. But if it fails as an open, aha!

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 Thanks, 
    - Win

Phil has been SED silent for 86 hrs 49 minutes, it would be a shame if we never got the answer.

Mikek

Maybe someone, (maybe even I ?) guessed the correct answer and since Phil hates me and others here, he has gone away to pout ?

:

ng

CR

So Phil - what is the answer?

Unenlightened people want to know!

John ;-#)#