[sdiy] soft PSU turn on circuit needed

[Neil Johnson]

Martin,

One trick used in SwitchMode PSUs is to put a highpower NTC resistor in
series with the primary.  At turn-on the high cold resistance limits
current flow and heats up the resistor.  As it warms up its resistance
drops, so the current flow ramps up to "normal", keeping the resistor at a
high enough temperature to keep its resistance low.

I think all this happens in about 1sec.  Also not sure what would happen
in rapid power cycling mode (apart from "Don't Do It!")

Neil

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Neil Johnson :: Computer Laboratory :: University of Cambridge ::
  http://www.cl.cam.ac.uk/~nej22           +44 (0) 1223 334 477