Hot Transistor - Is inductor increasing voltage?

Thread Starter

Tornado Bones

Joined Nov 8, 2014
I have a design concept for a device that uses light signals to operate a relay. I'm having some trouble and I'd appreciate some expert advice.

I've been making these circuits and they seem to work fine but once in a while I notice one of the transistors will get hot. Also, once in a while, one of the circuits will just fail. When analyzing the failure, I find that the same transistor is toast. Replacing the torched transistor restores the circuit to its normal function. The circuit runs off of 12VDC, either from a regulated wall adapter or a 12V automotive battery. The transistors are rated for much higher than that. I've tried putting the circuit into spice but I'm not an electronics engineer and I'm still trying to teach myself how to understand the simulations.

I'm wondering if the inductor is affecting the voltage or current to Q1. If so, how do I fix?

I've pasted a picture of the circuit below. I didn't find the actual semiconductors in Spice, so I substituted something I thought was close for the sake of the example. The real components are Q1=NTE159 PNP Transistor, Q2 and Q3 are NPN NTE0289A. Q1 is the one that seems to burn out. ALso, there is a phototransistor across C2. I didn't find that component in spice. It pretty much acts like a switch, I think. The inductor L1 is the coil from the relay. The 12V coil resistance is 400ohms and 360mW.

Thanks in advance for any help. I don't know anyone else who cares about electronics, so I don't know who else to ask!
upload_2016-7-30_1-51-38.pngiGate Switch.jpg


Joined Jun 19, 2012
I don't see where the circuit has an input or output? Can you explain what the circuit is supposed to do exactly?
I could not draw a more confusing diagram if I tried.
Q1 seems entirely backwards?


Joined Jun 4, 2014
I agree Q1 emitter and collector are swapped.
The relay would be better replaced with a 400 Ohm resistor for the simulation.
If the circuit overheats Q1 with the 400 Ohm relay in the circuit (and not the inductor) then I don't know how that happens.

blocco a spirale

Joined Jun 18, 2008
There are so many things wrong with this schematic I don't know where to start.

It would help if you redraw the schematic in a conventional and logical way; it looks as though it has been drawn with the intention of making it difficult to follow or as an example of how not to draw a schematic.

And, what's going on with your resistor values: 4.99k, 49.9k, 499k ! ?

Here are a few tips:
- Use standard component values
- Place inputs on the left and outputs on the right
- NPN transistors should have their emitters at the bottom and PNP transistors at the top
- Place the highest voltage supply rail at the top and the lowest at the bottom
- Don't cross wires unnecessarily
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Joined Mar 14, 2008
That certainly is a poster child for a confusingly drawn, upside-down schematic. :rolleyes:
What on earth are all those parts supposed to be doing?
Is V1 the power supply?
Please state exactly what this circuit is supposed to do.