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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Scutarius on July 06, 2010, 04:52:00 pm

Title: optocouplers ( doubling Vce?)
Post by: Scutarius on July 06, 2010, 04:52:00 pm
Hi, I want to use some optos that I have for a 100Hz application but I need a 100Vce

So, if have these optos with a Vce of 70v, that just work fine with my application (why?) can I put 2 in series to be sure that will survive the stress for a long time?

thank you
Title: Re: optocouplers ( doubling Vce?)
Post by: Zero999 on July 06, 2010, 05:46:10 pm
No, you can't put two in series and expect it to be fine because you can't guarantee that they'll both switch on at exactly the same time.
Title: Re: optocouplers ( doubling Vce?)
Post by: Scutarius on July 06, 2010, 05:55:56 pm
do you mean that I can not say that if I put in parallel the As and Ks of the LEDs and in series the E and C of the transistors I wouldnt get the same response?

thank you
Title: Re: optocouplers ( doubling Vce?)
Post by: Time on July 06, 2010, 07:31:22 pm
Why not just use the output side of the of the opto couplers to drive something more robust that can handle the 100V

Quote from: ee.jcesar on July 06, 2010, 05:55:56 pm
do you mean that I can not say that if I put in parallel the As and Ks of the LEDs and in series the E and C of the transistors I wouldnt get the same response?

thank you
Title: Re: optocouplers ( doubling Vce?)
Post by: Zero999 on July 06, 2010, 08:32:01 pm
Quote from: ee.jcesar on July 06, 2010, 05:55:56 pm
do you mean that I can not say that if I put in parallel the As and Ks of the LEDs and in series the E and C of the transistors I wouldnt get the same response?
Suppose one transistor turns on before the other?

You'll have one 70V transistor with 100V across it. It might survive for a short length of time but it might fail in the long term.

Anyway, you shouldn't put the LEDs in parallel, they should be in series.

Title: Re: optocouplers ( doubling Vce?)
Post by: Scutarius on July 06, 2010, 08:56:42 pm
thank you, I will try another opto then.

:D
Title: Re: optocouplers ( doubling Vce?)
Post by: Time on July 06, 2010, 10:31:21 pm
I worked on something similar with 2 optocouplers in series on the LED side.  The optocouplers worked in parallel on the (push-pull in the case of FOD3180 I think it was) output to provide gate current for 2 SCRs rated at 1.5 kV with a total voltage of 2 kV across them both.  The jitter  caused by the 2 optocouplers was enough to cause the SCRs to have a very short lifetime.  Since they could not be triggered at precisely the same time the over voltaging took its toll.

The company who made the SCRs stopped making them but the device was still being used readily in the lab - of course this was bad!  Soooo... my boss told me to build something better to replace.