Electrical – What value PTC Thermistor should I choose for over-current protection for a child’s electronics kit


What is an appropriate value for a PTC thermistor used in a 3-cell AA battery pack to provide over-current protection if the terminalsare shorted? Current draw during normal operation will be only a few hundred milliamps)

I'm looking to make something similar to this: SnapCircuits 3 battery holder so my son can experiment with connecting switches, leds, motors, etc. together without a risk of melting something.

Best Answer

I would use something like a 0.5A leaded PTC. For example, a Littelfuse RXEF050, which is a radial leaded type. It has a resistance of less than 1\$\Omega\$ when 'on' and a hold current of only 50mA. It will conduct at least 0.5A indefinitely without opening, and will trip in about four seconds at 2.5A (at reasonable temperatures).

With PTC resettable fuses (and fuses in general) there are a couple of other things to consider- maximum voltage (when open) and maximum current that the fuse can interrupt. Some battery technologies can supply enormous current, but I'll assume you have alkaline cells which can produce less than 10A. Checking the ratings on the RXEF050, we have:

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So it can interrupt a fault current of up to 40A at 72V, which should be more than safe for a few AA Alkaline batteries (perhaps not for NiCd types though).

If he leaves the short in place it will drain the battery relatively slowly if it trips, however if the batteries are getting weak and cannot produce enough current to trip the fuse, they will be drained much, much faster.