I have the attached NPN transistor being switched by an MCU. I've made some measurements and done my calculations as a result of observing that the NPN is not fully switching off (the relay stays on).

Calculations show a negative base current and gain. Does this imply that the transistor is blown?

I've also just noticed that when off, Vin < Vb which does not seem right.

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  • \$\begingroup\$ Vce of 6.1v when transistor is supposed to be off - this is the problem. If there is less than ~0.5V on the transistor gate, Vce should be 12V. If not, the transistor has failed. The other calculations are erroneous if the device is failed. \$\endgroup\$
    – rdtsc
    Commented Nov 17, 2021 at 15:26
  • \$\begingroup\$ You probably don't have enough Vin to switch such a "high" load like 1k relay. \$\endgroup\$
    – user208862
    Commented Nov 17, 2021 at 19:30

2 Answers 2


The 6.1V voltage across the device when off suggests that you have the transistor in backwards.

Most silicon epitaxial bipolar transistors have a reverse base to emitter breakdown voltage in the 5V-7V range and a very low reverse Hfe (less than 10).

If you have put the transistor in backward when the transistor is supposed to be off the base-emitter junction (which is now connected where the collector should be) will breakdown and limit the voltage to ~6v-8v (the collector to the base junction is in series with the base-emitter).

When you try to make the transistor conduct it will have a very low gain. This doesn't account for the large voltage across the base to collector junction in that condition. I would expect it to be the usual 0.7V. It is possible that being used in reverse has caused damage to the transistor.

Schematic of relay driver with transistor reversed.


simulate this circuit – Schematic created using CircuitLab

Note: This is intentionally drawn with Q1 connections reverse!


Base voltage of 1.54 or 1.42V on a BC817 is way too high. It would normally be less than half of that with a small base current flowing. That alone indicates (assuming the measurement is correct) that something is very wrong with the transistor. The apparent hFE is also extremely low.

However, the fact it is appears to be doing much at all is a bit suspicious. I could explain this with a combination of measurement errors and an accidental swap of emitter and collector. I suggest double checking everything (connections and measurements) and then swapping the transistor out if nothing new shows up.


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