Optocouplers

[ericgibbs]

hi,
A quick run of a LTspice simulation of your existing circuit shows the BC640 dosnt saturate and dissipates about 2 to 3 Watts!

Do you realise the LTV827 is a dual channel Opto coupler.?

 

[FunkMasterBob]

Hm. So what would solve the problem? Is the transistor at fault, or simply the value of the resistances?

I am aware of the dual optocouplerness of the LTV827. I am using the other one to switch on a buzzer. Thanks for your help thus far.

 

[ericgibbs]

Hm. So what would solve the problem? Is the transistor at fault, or simply the value of the resistances?

I am aware of the dual optocouplerness of the LTV827. I am using the other one to switch on a buzzer. Thanks for your help thus far.

Basically the opto cannot drive the base of the BC640 hard enough to fully saturate it.

I would have chosen a P MOSFET, not a transistor. If thats not an option. you require an additional transistor between the opto and BC640.

If it helps I could post a circuit tomorrow.

 

[FunkMasterBob]

Bother. Bad time to be figuring all this out. Suppose that's my fault though.

So why can't the optocoupler drive the transistor? Data sheet says optocoupler can deal with currents up to 50 mA, and you said I only need a 20 mA current. Would that be sufficient?

 

[ericgibbs]

Bother. Bad time to be figuring all this out. Suppose that's my fault though.

So why can't the optocoupler drive the transistor? Data sheet says optocoupler can deal with currents up to 50 mA, and you said I only need a 20 mA current. Would that be sufficient?

hi,
The reason for only approx 2mA to the base is the 4k7 resistor.

Say you have a Vbe of 0.7V for the BC640 and a Vsat for the LTV, approx 1V say.

Then [12Vs-1V]/4/7K = 2.3mA..
You could try reducing the 4.7K to about 470R thru 1K, this will give approx 23mA thru 11mA.

Choose a emitter drive resistor to give 10mA to 15mA thru the emitter diode.
Check that the drive voltage across the emitter is less than +0.6V when in the off state.

The typical LTV827, CTR for 10mA/15mA drive is 150%.

Points to consider, if the motor current while running with a light load is say 200mA, on heavy load and while starting up could be close to 1Amp.

Also the power supply must be capable of supplying these high startup currents.

I would suggets the following tests.
Disconnect R2 from the opto output, replace it with a 470R.
Connect one end of the 470R to 0V common and the other end to the BC640 base, [keep the 10k in circuit] ,
the BC640 transistor will switch on and drive the motor.

Connect a 1K resistor between the opto output and +12V, drive the emitter with 15mA. The opto output should go close to +0.5V

Also check that the opto output rises to +12V without any drive to the opto emitter.

These are simple tests that will prove the LV827 and BC640.

If any of these tests fail they should indicate if you have a faulty device.

Lets know what you find.

EDIT: couple of sims to show the effect of under driving the transistor. Used close equivalent types.

 

[FunkMasterBob]

In your schematic, you attached the 10k resistor directly to the base. Why did you change this from the other schematic?

I'm going to go through the tests now. I'll let you know what I find. Thanks a bunch for your help.

 

[ericgibbs]

In your schematic, you attached the 10k resistor directly to the base. Why did you change this from the other schematic?

I'm going to go through the tests now. I'll let you know what I find. Thanks a bunch for your help.

hi,
Ref the 10K, I have not posted 'another' circuit.

The 10K acts a path for any leakage current from the opto, for low leakage currents the voltage drop across the 10K is too low to turn on the transistor.

 

[FunkMasterBob]

Well, I tried the first test and the mtor managed to switch on, but the transistor was still running very hot. This doesn't seem to mesh with what you said about the transistor, well, not running hot. It seems I have a problem there.

I did the third test as well, and I didnt get twelve volts. I got around 3 when the motor was running (bad sign that the motor was running, yes?) and 1.7 when it wasn't (to turn it off I had to lower the voltage and then raise it back up.)

I also may have done these tests wrong. I left the circuit in tact while I did them, only altering the things you explicitly said to. Help?

 

[FunkMasterBob]

Well, I was referring to how you changed the location of the 10k in comparison to the circuit posted earlier in this discussion by kmoffet. Is there a reason for that difference?

 

[ericgibbs]

Well, I was referring to how you changed the location of the 10k in comparison to the circuit posted earlier in this discussion by kmoffet. Is there a reason for that difference?

hi,

Its my preferred way, when using the opto output as a 'current switch'.

When the opto is conducting, why shunt 1mA down a dead 10K, its better to maximise the current that flows into the base circuit.

 

[FunkMasterBob]

So from the results of the test, (three posts back), do you think my transistor is messed up, or did I do something wrong?

 

[ericgibbs]

So from the results of the test, (three posts back), do you think my transistor is messed up, or did I do something wrong?

OK,
Read the post results.

Look at this image and measure the points marked and post what you find.


BTW: for info only every post has its own serial number.. on the righthandside.

 

[FunkMasterBob]

Thanks for the help.

Due to a reduced amount of supplies and a desire to stop ruining more equipment, I just gave up on the optocoupler method for this portion and switched to a TIP 120 power transistor. Probably not the best method, but its sufficent. Thanks for your help.

 

[ericgibbs]

Thanks for the help.

Due to a reduced amount of supplies and a desire to stop ruining more equipment, I just gave up on the optocoupler method for this portion and switched to a TIP 120 power transistor. Probably not the best method, but its sufficent. Thanks for your help.

Your welcome , good luck with your projects.