Limiting in-rush current at start up.


2002-05-18 10:01 pm
I am in UK (240v AC) and am designing a power amp psu with a 560va torroidal transformer with 30v secondaries and 40,000uf smoothing caps. What value current limiting device (thermistor)would I need to limit the inrush current at switch on, but not limit the amps peak output( approx 100w into 8ohms) Thank you.
for a 560va transformer I wouldn't bother unless the psu caps are huuuge .....

I've been using a 650va toroidal psu for years ( about the last 15 or so ) without any failures of fuses or psu components ( use slow-blow fuses, or they _will_ fail....)

if it really worries you , or you have a massive cap bank, or very expensive caps , have a look at this site. they have a number of ideas:


...another point , you specify a power output of 100W into 8R.

with a 560va tranformer and 40,000 uF caps you should never reach a point where any thermistor is going to affect the voltage rails. even with a really inefficient amp design the full-power current draw is never going to reach the point where it exceeds the rating of the tranny.

you can probably get away with using thermistors in each rail to the caps , this'll halve the loading on each and still keep the initial charging current down.

the magnetic inrush current won't be too bad for a 560va toroidal transformer.