Converted to Split 12V/24V - No Communication HELP
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There is an LED and resistor connected to the heater output, that's why you can measure VIN less the voltage drop of the LED at the heater output pin.
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The bed heat LED is off on your first photo but lights on on second one. Maybe this is a cue?
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That was my thought as well. I am unsure why connecting the grounds of the bed heater control and the VIN should allow current to flow between the two.
I have no bed heater connected in these photos, so in my mind there should be no reason for the voltage to get from the bed heat + to the bed heat - unless something has gone wrong.
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That was my thought as well. I am unsure why connecting the grounds of the bed heater control and the VIN should allow current to flow between the two.
Because your multimeter doesn't have infinite resistance.
I have no bed heater connected in these photos, so in my mind there should be no reason for the voltage to get from the bed heat + to the bed heat - unless something has gone wrong.
As David says, with the bed heater off you should see the ground of the bed heater at (Vin - LED voltage drop), because the LED is connected between the + and - terminals of the bed heater terminal strip.
With the bed heater off, the positive terminal will show Vin and the negative terminal will be floating, pulled up to Vin (less the forward drop of the LED) by the bed heat LED and its resistor.
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If the multimeter is in good shape it would have input resistance in order of mega-ohms so it can't provide an essential current to light LED and to make such a big voltage drop across LED. I suspect that heater's nMOS switch is partly fried.
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@zov:
If the multimeter is in good shape it would have input resistance in order of mega-ohms so it can't provide an essential current to light LED and to make such a big voltage drop across LED. I suspect that heater's nMOS switch is partly fried.
Nope, I have a perfectly functioning Fluke 179, it provides enough current to light the LED enough to be visible.
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Look at the spec's of my multimeter (Fluke 289), input impedance 10MOhm < 100pF so the current through LED is about 12 uA in our case. Don't think this current is able to fire up any perceivable glow in LED. To speak frankly I don't check it by myself so can be mistaken and contemporary LEDs might be such effective.
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zov: I think what elmoret is talking about is the "diode test" mode, available on most multimeters. In this mode the multimeter provides typically 3V on the probes, and limits the current to 2-3mA. If you put your probes onto an LED in this mode, the LED will light up dimly - but it will definitely emit light! This mode can be used to measure the forward & reverse bias voltage of diodes (and light-emitting diodes of course).
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If you look at photos of OP you see he is measuring voltage between 2 terminals not diode test.
ADD: And I have mistaken in the post above: the current through LED should be about 1.2 uA not 12 uA. So it makes the multimeter much more unlike as a shunt to lights LED on. -
zov: I think what elmoret is talking about is the "diode test" mode, available on most multimeters. In this mode the multimeter provides typically 3V on the probes, and limits the current to 2-3mA. If you put your probes onto an LED in this mode, the LED will light up dimly - but it will definitely emit light! This mode can be used to measure the forward & reverse bias voltage of diodes (and light-emitting diodes of course).
No, wasn't talking about diode test mode. Regular DC voltage mode.
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@zov:
If you look at photos of OP you see he is measuring voltage between 2 terminals not diode test.
ADD: And I have mistaken in the post above: the current through LED should be about 1.2 uA not 12 uA. So it makes the multimeter much more unlike as a shunt to lights LED on.My multimeter also has a 10Mohm input resistance, and it is enough to light the LED, faintly. You can see an LED's light output at as little as one microamp.
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Won't argue with you as I don't have any practical experience of using LEDs with such low currents. Anyway thanks for a new knowledge you gave me.
