DC voltage from a PCB

dave11

I'm working on a treadmill whose motor won't run. All works fine, but the motor never tries to turn. The AC to DC conversion is done on the PCB. Tech at the company said it was almost certainly the circuit board, because the DC motors "almost never are the problem." But just on a whim, I put a voltage sensor near the motor's power wires when the motor was supposed to be running, and it blinked. It stopped blinking when the motor was supposed to be off.

So I disconnected the motor's power wires at the PCB and attached a DC voltmeter there. No voltage was present either way, running or stopped.

So I'm at a loss to explain why the voltage sensor blinks when the motor should be getting power, but why the voltmeter registers no voltage. I double-checked the voltage sensor and the DC voltmeter against other equipment, and they both read correctly.

Can anyone explain this?

Thanks.

Yoyizit

So it's a PCB problem or a shorted motor which then causes a problem for the PCB circuitry. Look for blown fuses on the PCB. Next check for a blown Triac [it'll be mounted on a heatsink
http://www.cartft.com/image_db/Passive_heatsink.jpg
].

dave11

The company is sending me a new PCB, so fortunately I don't have to troubleshoot that. My only question is why a voltage sensor senses voltage in the motor's power wires, but a DC voltmeter registers no voltage.

Yoyizit

Assuming your meter is good [check it on a car battery or a flashlight battery] more credibility goes to a hard-wired test instrument.

I suppose these testers work on capacitance or some other method for sensing an E field. Convenient, but they may lie to you.

And when meters lie to you it's time to pull out an oscilloscope.