LJTick-RelayDriver repair

I managed to blow up both of my LJTick-RelayDriver boards. These are little boards that plug into a LabJack and allow it to drive relays. Since I wanted to drive a relay to turn the battery charger on and off, I had to investigate. The circuit is quite simple, a ULN2003AI Darlington Transistor array and 3 resistors. R1 provides a return path for the base current by and also connects the relay power supply ground to the LabJack's ground. LabJack's datasheet suggests it should be 10Ω, but on both my units it was open circuit. I guess what happened was I put the relay power across this resistor (perhaps by using the a supply sharing a ground with the computer and connecting it backwards) which would cause a great deal of current to flow. If it was 12V, you would expect about 14W to be dissipated in a tiny surface mount component. No wonder it went open circuit.

I replaced this resistor with a 120Ω 1/2W unit which will still be overloaded by a 12V fault, but will last a lot longer.