[sdiy] soft PSU turn on circuit needed

[harry]

In rapid power cycling mode... this will kill you !!!

H^) harry

PS> I'm thinking of this Martin, I've done it before with two circuits, one
long
time constant, and one short time constant. I'll try to draw it tomorrow if
noone
gets there first.

Neil Johnson wrote:

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