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I was changing the control board of my 3D printer. When testing the heatbed, I saw that the LED wasn't glowing (as with the old control board), when the heatbed was turned on. So I investigated with my multimeter.

The circuit is pretty simple (taken from the prusa3d GitHub, the wire on the right stands for the heating loops on the PCB).

enter image description here

So more appropriately would be:

enter image description here

Here my observations (during a long period, so there should be no PWM):

  1. The heatbed warms up in a timely manner
  2. Measured the voltage drop between the contacts (VCC to GND: 11.36 V) enter image description here
  3. Measured the voltage drop across R1 (VCC 10.36 V to pad of R1 towards D1)
  4. Measured the voltage drop across D1 (-2 V, both pads of the diode, however the value varies) enter image description here
  5. I checked the function of the diode with the diode setting on the multimeter

I understand that the diode won't glow with a negative voltage, but why is the measured voltage negative?

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    \$\begingroup\$ The circuit makes no sense. VCC is shorted directly to GND. Are you sure you are measuring drops in correct way, and it would help to know with what you are measuring and how, and are the multimeter batteries dead if you are using a multimeter. \$\endgroup\$
    – Justme
    Commented Jan 18 at 13:23
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    \$\begingroup\$ Hi @Justme, the circuit is taken from the prusa github. The wire on the right is the "heating" part, one can see it nicely in the image from Prusa raw.githubusercontent.com/prusa3d/Heatbed_MK52_magnetic/master/… I also added the information where I measured the voltages \$\endgroup\$
    – JoeyD
    Commented Jan 18 at 13:29
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    \$\begingroup\$ Then the heater should be shown as a resistor. If it doesn't have resistance then it can't dissipate power and won't generate heat! \$\endgroup\$
    – Transistor
    Commented Jan 18 at 13:31
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    \$\begingroup\$ Are you being consistent with your multimeter lead orientation? The V+ lead (red - if they're plugged in correctly) should always be to the top of the component being measured on your diagram. If you swapped the leads then that would explain your problem. \$\endgroup\$
    – Transistor
    Commented Jan 18 at 13:34
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    \$\begingroup\$ @Justme the schematic is not really drawn wrong, 3D printers use a long labyrinthine PCB trace as a large flat heating element so it is literally a short circuit, however it's a very long trace of a known resistance. The LED is placed across this. \$\endgroup\$
    – John U
    Commented Jan 18 at 15:06

2 Answers 2

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Your circuit was broken if the resistor was badly mounted and made no contact.

It means the LED anode was not connected anywhere, and LED cathode was connected to negative supply lead.

LEDs are light emitting diodes when power is applied, but they also work in reverse, acting as photodiodes or solar cells, only they are not very good at it as they are optimized for light emission.

So, in short, there was light shining on the LED, it was not connected anywhere, and the photovoltaic effect made voltage appear on LED termnals, which you were able to measure with a multimeter.

The polarity of the voltage was as expected for a diode junction acting as solar cell.

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By accident I was able to "fix" my problem. I changed the battery in my multimeter - just to be sure. While measuring once more, I "bumped" the resistor and the LED glowed again. I tried to reproduce the issue again - however strong I bumped the resistor again, nothing happened. LED is glowing now again. Sorry for all the hassle.

I still don't understand, why the voltage was negative.

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    \$\begingroup\$ This does not answer your question why the voltage was negative. You might want to delete this. Albeit this confirmed the LED being disconnected from the resistor and you were measuring the LED acting as solar cell, converting light hitting it into electricity. \$\endgroup\$
    – Justme
    Commented Jan 20 at 12:50

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