I think a transistor broke down. My knowledge of electronics is not sufficient to solve my issue, so it seems. Hopefully some better educated mind can help me on this.

This question is about troubleshooting a PWM signal to a fan of this board (Eagle file) from Arduino pin 7 to Transistor Q4 to PWM terminal. Below is the piece of the schematics its about: Arduino pin 7 via 1Kohm to BD679

Background info (not required to read though): I've got this PCB (Ultimaker 1.5.4 from 3D printer) and I think I short circuited a fan output. It is a couple of years ago already it happened. So I am not 100% sure, I hooked up an external power supply to power the fan instead. But now I want to have the fan driven by the PCB again and not by the external power source (as the software sometimes lowers the speed or turns of the fan completely, depending on what is printed). Anyway, currently it is not working to drive the fan by the PCB. It is driven by an Arduino Mega 2560 by a PWM (so says the PCB) through a BD679 FROM ST, which apparently is a complementary darlington transistor. I've only got some experience with MOSFETs and known that when the gate is driven by a voltage too high, the resistance between some pins become practically 0, meaning it broke down. Or they just exploded and it was very obvious what broke down.

Info of measurement: Looking at this datasheet. and a multimeter I read the following on the pins of the BD679. Resistance E-C=unlimited C-B=unlimited E-B=9.19kOhm

EDIT: With the diode test function I measure the following always starting with negative left, positive right:

  • E-C=1 (no read out)
  • E-B=1269
  • C-B=906
  • C-E=768
  • B-E=1 (no read out)
  • B-C=1 (no read out)

Question: Do you think that these are normal readings for this complementary darling transistor?

EDIT 2: I have tested the Arduino+PCB with an oscilloscope. While setup, I could send a GCODE to drive the PWM responsible to drive the fan. I see that the PWM signal at the Arduino (port 7) is correct, its duty cycle corresponds to what was commanded, and is 5V. The PWM pin of the Arduino to the transistor is connected with a 1Kohm resistor. The same duty cycle signal is found at the Base of the transistor at 1.28v. This I understand. And also at the Collector of the transistor at 960mv, this I do not understand. The signal at the Collector has the same frequency and duty cycle as the signal on the Base of the transistor. The emitter is equal to GND, which I understand. Now when I measure the PWM port that would be connected to a (12v fan) I measure 19.6v, the same as the power supply. The negative terminal of the fan connection shows the 960mv which is equal to the Collector of the transistor. That it is equal to the connector of the transistor I understand as it is directly connected. When I connect a 24vdc fan to the terminal it is not spinning. Although I measure 19.6v on the positive lead and the 960mv PWM signal on the negative lead of the terminal. Can I conclude that the transistor broke down because it is not sinking any current anymore?

Thank you.

  • \$\begingroup\$ Briefly, to answer your "resistance" question, you need a multi-meter that will forward bias a semiconductor junction. This ranges from applying 0.3 to 0.7 volts across the junction before it turns on. (Check to see if there is a Diode test setting on your multi-meter.) Not until then will you know if the junction is still intact. But, in the end, it is difficult to work out if a transistor is functioning normally with only a multi-meter. \$\endgroup\$
    – st2000
    Commented Apr 29, 2017 at 18:27
  • \$\begingroup\$ Use the diode test function of your meter, not the resistance range. \$\endgroup\$
    – Hearth
    Commented Apr 29, 2017 at 18:28
  • \$\begingroup\$ @user287001 I've added a picture. \$\endgroup\$ Commented Apr 29, 2017 at 18:32
  • \$\begingroup\$ @st2000 Thanks for the technical info about how the measurement works. \$\endgroup\$ Commented Apr 29, 2017 at 18:33
  • \$\begingroup\$ @Felthry Just added the measurements. \$\endgroup\$ Commented Apr 29, 2017 at 18:40


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