RUMBA heating resistor not heating up

Hi,

I have a RUMBA board (using Repetier firmware and client). The heating resister is not heating up.

It used to work OK but something (don’t know what) went horribly wrong and it overheated & blow the T0 Thermistor connections (so they no longer acknowledged when a thermistor was been connected). I solved that by changing pins.h to use the T1 thermistor connections. The thermistor readings work just fine now.

But now neither the T0 nor (having changed pins.h again) T1 Heating resister connections heat up the resister (I've tried several and checked the connections are OK).
I connected a multimeter.
- With the ‘Heat resister’ button off it reads: resistance
- When I ask Repetier to heat up the resister it reads: 0 resistance
(I’m thinking I should be checking voltage or something but didn't get anything helpful – but quite possibly since I wasn't sure what to set my multimeter to)

So, it’s trying to do something but not enough power is getting through????
Is that fried? (T0 and T1 heating resistor connections?)

Any ideas?

Thanks
Pete

With multimeter on voltage, measure between the two wires from connector of mosfet (heater). When heater is off - wires measure 12v or whatever psu voltage is there, because mosfet does not conduct and one wire is 0v and the other is psu voltage. When the heater is on - wires measure 0v to each other, or anyway a very low voltage, this is because mosfet does conduct so the wires are "almost" short circuit to each other.

Also you can check that mosfet does receive the command properly, that is by measuring mosfet gate pin with respect to ground (see mosfet datasheet to locate the gate pin). When you press heater on, the mosfet gate pin should read 5v. When the heater is off, the gate pin should be at 0v.

Hi,

Your advice sorted the problem (though not quite as I'd hope). I was checking voltage as you say (v helpful) but still couldn't work out reason for discrepancies. Err.....then realised I had it running only on USB power, so there was no umph to heat the resister. So, all sorted now.

Thanks for help

Pete