quote:Originally posted by iakobos:quote:Originally posted by stan2004:
So to be clear, with no motor connected, using Diode mode you measure each NPN between pins 2-3 (and then 3-2 backwards) and in each case you measure 0.0V (like when you touch the test leads together).
To be precise it's 0.6V black on 2, red on 3; and then 0L red on 2, black on 3
"That's a relief" says Detective Thomson (without a p). The NPNs aren't shorted and order is restored to the universe. You're measuring the protection diodes placed externally across the transistors - standard practice in a motor bridge.
I don't know which path you want to pursue (swap processors, testing optoisolators, just changing all transistors, etc.) but here's what I'd do:
Using the DC-Volts mode, with the engine powered up and motor connected, somehow hold or affix the black test lead to one of the NPN emitters (the two emitters are shorted as established before). With the engine commanded to forward and reverse, measure the red test lead voltage at each motor terminal. Obviously this will be trickier in reverse since it wants to move but perhaps not so bad with a low enough track voltage.
Here's how it works. In Fwd, one PNP switches positive voltage onto the "+" motor terminal and its mating NPN puts a less positive voltage onto the "-" motor terminal. The other two transistors are off. The motor sees a net positive voltage and spins forward. In Rev, the first pair of transistors are off and the other PNP puts positive voltage onto the "-" terminal and its mating NPN puts a less positive voltage onto the "+" terminal. The motor sees a net negative voltage and spins backwards.
In making the measurements as suggested, I believe you will be able to see which transistor has failed (if it's just one). That is, if either the PNP or NPN does not switch on, both motor terminals will read the same voltage set by the working transistor.
Then we can work back to confirm it is in fact a bad transistor or if whatever is driving the transistor (the MOC optoisolator, and before that the microprocessor) is busted.
