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Old 22nd September 2005, 01:51 AM   #11
Arius is offline Arius  United States
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Hey Wes,
Nice to see you lurking around here. I'm "CapJohn" from the Ninja250.net forums.

I'm doing the layout for my power amp now. When that's done, I'll work on my soft-start board. Like Richieboy, I find the thermistor method too crude. My goal for soft-start is not only as soft-start but to reduce AC wiring (i.e. no AC wires to multiple devices and from back to front of case). Schematics done, no layout yet.
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Old 22nd September 2005, 11:59 AM   #12
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Chris --

Oh ho, of course! Short out the thermistor with the relay! See attached revision.

The NTC thermistor is critical here, as it allows a smooth voltage rise in the transformer; then when the voltage is high enough, the relay kicks in. Once the relay kicks in, the thermistor is out of the picture; the relay stays on by virtue of it being on. Very appealing in a self-excited sort of way.

I think this circuit would work if tuned properly; one concern I have is if the transformer's output voltage isn't linear during core saturation, the circuit will turn on prematurely.

CapJohn --

Hey, good to see a familiar face! Now you know why I haven't been posting much... to busy building my GainClone! I thought the amp part was going to be tricky; turns out the hard part is the freakin' chassis!! If I tap another freakin' crooked hole, or break another $4 drill bit, I think I'm gonna scream.

My (untested) dislike to the thermistor startup is about the same as Richie's -- I'm concerned that there will be a delay turning on full voltage when the amp has been idle, and that it will add extra noise or choo-choo-ing in the system.

My *like* for the thermistor startup is the symplicity, and zen-like nature: it's on when it needs to be on.

These are the two issues (aside from cost and part count) I tried to address in my circuit.

Wes
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Old 22nd September 2005, 12:46 PM   #13
anatech is offline anatech  Canada
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Now to simplifiy the circuit more and protect from other problems ....
The relay runs off the secondary DC supply. Resistor as required to limit the coil current. The best part is that if you have a short, the secondary voltage may drop enough to drop the relay out. The resistor then acts as a fuse and opens. Of course the relay can be opened with a protection circuit as well.

-Chris
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