In article , chuck wrote:
CS wrote:
Looking to fit a resistor/relay combination in the supply line of a
3600VA 240v isolation transformer. I reckon the resistor should be 8
ohms - based on 200% loading, but anyone know what wattage is
required???

Thanks.


It is difficult, as you realize, to calculate wattage for the resistor
without some knowledge of the transient it will see. If you go with a
resistor, you might start with several in parallel, so as to provide
improved heat dissipation. If the inrush duration is on the order of 10
mS, I guess I'd use maybe four 36 ohm, 50 watt resistors in parallel to
start with, and go from there.

For 3.6 kVA transformers, NTC thermistors are perhaps more popular these
days than simple resistors. Here's a link that describes the design process:

http://www.epcos.com/web/generator/W...nlinearResisto
rs/NTCThermistors/PDF/NTCThermistors,property=Data__en.pdf;/NTCThermistors.pdf
NTCThermistors.pdf

Beside NTC, a quick look turned up other possible devices to do this.

greg