Thermistor for an 120VA toroid transformer

[imfree707]

5 Amp Start Surge

Looks like about 5 amps of start surge. That's not really very bad on the power switch and harmless to the residential branch circuit. The pictured 47 ohm, 20 watt resistor could be changed to 100 ohms to limit start surge to 2.3 amps, probably a better match to the amp's needs.

 

[DF96]

Yes, it will handle them, in the sense that it can cope with the current. But the smaller the transformer, the bigger the resistor to have any useful effect. For 120VA at 220V you need 100-150ohms. 47ohms won't do any harm, but it won't be very soft. The initial DC current could be 80% of what it might be with no resistor, although it will decay a bit quicker. Hardly worth doing.

 

[eladmi]

I have difficulties getting a proper resistor in Germany. I could get one with 11 watt 120 ohm. Would that work?

Edit:

I found one with 17 watt 100 ohm or 150 ohm. Would that be enough?

Thanks

eladmi

 

[DF96]

Any of those would do. Pick the one which best fits the space on the PCB.

 

[jitter]

One of our customers doesn't use soft start to power up a 300+ VA toroidal and still use a pretty low rated fuse of 1.6A. They get away with this because they use a very high I2t value fuse (>20).