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

component-valuescurrent-limitingthermistor

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.

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).