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Old 21st March 2003, 10:46 AM   #11
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Location: south yorkshire, UK
What do you mean when you say soft start?

if you use large psu transformers - especially toroidal ones - the inrush current on powerup can be excessive and blow fuses.
In the amp I'm building I have a 1KVA toroid which destroys any sensibly rated fuse , so I have to use a 'soft-start' arrangement to turn on the supply.
This works by supplying the initial mains power through a resistor to limit the inrush current. The resistor is removed from the circuit automatically after a couple of seconds ( effectively shorted out by a relay ) once the current has dropped to a sane value.
Some designs use a Thermistor to do the same thing. The cold resistance is high, and drops as the current causes it to heat up, thus restoring full power to the transformer.

I actually use both ideas. by shorting across the thermistor after a couple of seconds I prevent the thermistor staying hot. this means that it's instantly ready the start the cycle again if the amp is switched off and on quickly, and reduces the heat inside the amp slightly.

ray
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Old 21st March 2003, 12:06 PM   #12
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Default soft start

in Sorensen's high voltage, high current switching supplies they use something like a 10 ohm resistor with a relay to short it after a few seconds.

ADM1022, ADM1024, ADM1028, ADM1029, ADM1030, ADM1031 etc, -- just plug "fan control" into their search engine.
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Old 21st March 2003, 01:19 PM   #13
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in Sorensen's high voltage, high current switching supplies they use something like a 10 ohm resistor with a relay to short it after a few seconds.

yes, thats the sort of thing I mean , but I use a thermistor too.

probably overkill, but it's driving a big chunk of iron

ray
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Old 21st March 2003, 03:58 PM   #14
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Default Thanks guys

Thank you very much for the info! Fan control will be my first project since my amp can power up safely as it is. But I will be experimenting on that soft start circuit.

I still need to understand soft start more religiously since it involves tinkering with the mains powered lines.
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