24 V transistor switch acting strangely

Question

I am working on a project to switch a spring-applied brake (datasheet, page 84, MCNB 2GR) using a Teensy 3.5.

I only have limited electronics knowledge and am struggling with weird behaviour from the transistor. I have previously asked for some help in this post and have applied the help to create the following circuit:

As opposed to my previous question, I have switched to using a Teensy 3.5 which only supplies 3.3 V as an output on the digital pins.

The idea is to use a Sharp PC817 (datasheet) to isolate the Teensy and the 24 V circuit and then use a 2N3904 (datasheet) to turn on off the 24 V for the brake which is symbolised as R1.

The strange behaviour starts after a few seconds of sending a 3.3 V signal to the opto-isolator. At first the circuit works as expected and the brake turns on and off based on the current state of the output pin. However after some time (probably about 5-10 seconds), the brake doesn't turn off anymore (i.e., it still receives enough power to be in its on-state) when the output pin is turned off. The only way to turn it off is to turn off the 24 V DC power supply.

After some investigation, I found that the transistor gets extremely hot, hot enough to cause burn marks when touching it for less than a second. Considering that the circuit works normally after a little while, I think this issue is transistor related and has some relation to its temperature. I have also taken voltage readings on the transistor and have found the following:

When turned off: E: 0 V, B: 0 V, C: 23.96 V
When just turned on: E: 0.125 V, B: 0.84 V, C: 5.76 V
When turned off after being on 5-10 sec: E: 0.075 V, B: 0.493 V, C: 8.6 V

It seems to me as if the transistor does not completely turn off and still letting enough voltage/current through to keep the electromagnet in the brake engaged.

I am not entirely sure if the temperature is the problem. I am guessing that I may have either chosen the wrong resistance values in my circuit or that maybe the transistor is no good for what I want to do.

Is there a way to fix this without changing the transistor?

Answer 1

A brake is an electrogmagnetic component. It has a resistance but is mainly inductive.

When the brake is turned off the current will tend to continue flowing and a spike or back electromotive force will appear which will overheat or destroy the transistor after it is blocked.

Adding a freewheeling or flyback diode will allow current flow in this condition and protect the transistor:

Answer 2

In addition to the flyback diode problem, the fact that VCE (collector/emitter voltage) is 5.64V when the transistor is on means that it's not fully on. This greatly increases the amount of power dissipated in the transistor, which causes it to heat up more. Overheating can also damage a transistor.

You might need to increase the base current by decreasing R2, or even by adding another transistor stage between U1 and Q1 (if U1 is too weak). Q1's datasheet should tell you the needed amount of base current.