Gallery, Projects and General > How do I??

poorly seig c3

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PK:
The problem with specifying a fuse is that it has to:
a: blow at a current * time value (it's energy that blows a fuse, not current)
b: withstand the inrush current at startup. which can be 10-100 times the operating current if you have a DC power supply (like those little lathes do)

For a circuit that always draws the same current, you fix this with an NTC thermistor.  This device has a high resistance when cold and this resistance drops as it heats up. What happens is that the high resistance limits the inrush current at startup to sane values (maybe only 4 -5 times operating current) and then heats up.

For a circuit that draws a variable amount of current (eg a motor speed control on a lathe) it's somewhere between VERY hard and impossible to pick an NTC thermistor with the correct characteristics.

So what do you do?
The cheap way is to pick the smallest fuse that has an INRUSH rating up to the task and just live with whatever its maximum continuous rating is.

We went through this process recently where a product did, in fact, draw a fairly constant current of about 100mA at 220V. However it needed to operate from 24V-250V.  As such it had a big bank of capacitors (to deal with the low voltage application). We measured  inrush at around 150A when we ran from 250VAC...

 

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