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Old 11th July 2008, 10:56 AM   #1
Mooly is offline Mooly  United Kingdom
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Default A75 PSU Mains switching

Been having a look at the PSU for the A75. The Triac switching is of particular interest because it lends itself to remote control ( suitably isolated of course, probably an opto coupled Triac in place of SW1).
A few thoughts really,
Do you feel the inclusion of the thermistor could be audible. It's a great feature but the resistance even when hot is magnitudes more than any mains wiring etc. ( If used with a class AB amp rather than a class A with it's constant current draw )
Any thoughts on the viability of a slow start using the Triac itself ?
Most important of all are any harmonics apparent on the secondary voltages due to the commutation of the Triac around the zero crossing.

Thanks for any comments ---- Karl
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Old 11th July 2008, 11:06 AM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
The Power Thermistor can be left in circuit. It does modulate the voltage supplied to the transformer.
I much prefer to short out the soft start function after a few hundred milliseconds. Due to the poor reliability of my rural electricity supply, I use resistors rather than Thermistor which does not work well if still hot/warm.

You can use a Triac with timer control to short out the soft start, if you want to stay solid state for the whole shooting match.
regards Andrew T.
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Old 11th July 2008, 12:17 PM   #3
Mooly is offline Mooly  United Kingdom
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That's what you get for living in the outback Andrew. Worth it though
It is surprising how long power thermistors take to cool off fully, degauss circuits in tellys can take half an hour before they work properly after being on, even if it was only a few seconds. They work the other round of course, current dropping to zero.
I was really wondering whether the Triac itself could be used for the soft start, you know, bring it into full conduction over a few dozen cycles, like bringing a lamp dimmer to full very quickly. I don't know what the triac would make of that ( hefty tranny and discharged caps etc ) but I suspect it's do-able.
The big question is whether the Triac distorts the mains current at all around the zero crossing.
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