Thursday, 11 September 2014
Ohm on the range
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I've been asked to make a large lasertag sensor harness for a client. This comprises of six Tagbits lasertag sensors on a single lead.
I was asked for four of these, three worked perfectly but the last one had a short between the power lines (+5V from a 7805).
A visual examination did not reveal the fault, and as I didn't fancy disassembling each sensor one by one I wheeled out my Polar Instruments Toneohm 700.
This is no spring chicken being last manufactured in 1988. But it's a great piece of kit having a 200mOhm (milli Ohm) range, and audio feedback with a higher tone indicating less resistance.
My normal multimeter in continuity mode indicated a short no matter where in the cable the probes were placed. But the Toneohm didn't even register a connection between +5V and 0V on any sensor until I tested the sensor that actually had the fault. e.g. It is so sensitive it registered open circuit along a 60cm piece of wire which was shorted at one end.
In fact, it's so sensitive it could detect which side of the PCB the fault was on:
Between the +5V and 0V connections on this side of the PCB, the resistance is 93.9mOhm.
But on the other side of the PCB, the same connections show 49mOhm, indicating the fault is on this side of the board.
And lifting the PCB showed a single strand of cable between 0V and 5V. Problem solved.