# Optocoupler help!

Discussion in 'General Electronics Chat' started by elecbeg, Nov 7, 2009.

1. ### elecbeg Thread Starter Active Member

Sep 7, 2009
30
2
Hello everyone,

I don't know if its me not understanding how to read the datasheets, but I can't find on any of the optocoupler datasheets what frequency range they will work up to?

Basically the input to the optocoupler will be from a pic, the output will be switching a transister, which will at the end of the circuit pulse a coil.

But I cant figure out from the datasheets how fast the optocouplers can switch, there is a table chart that shows certian parts take x amount of microseconds, but I dont know how to convert that into a frequency.

Any and all input would be appreciated

P.S. I decided on a optocoupler as I plan on experimenting with coils and although I have got multiple safe gueards built into the circuit i.e. over voltage protection and current limiter, I didn't want any possible feedback from the coils to destroy the pic.

2. ### SgtWookie Expert

Jul 17, 2007
22,199
1,800
f = 1/t
t = 1/f
A square wave that is high for 1uS and low for 1uS for a total time of 2uS has a frequency of 1/2uS = 500kHz.

Also describe what frequency or time periods you want to operate at.

3. ### peranders Well-Known Member

May 21, 2007
87
0
A plain optocoupler is rather slow but you can speed it up. May I ask what you are up to?

You have also fast optocouplers...

4. ### elecbeg Thread Starter Active Member

Sep 7, 2009
30
2
I dont have the software at the moment to post a schematic, I have always used pencil and paper. Its crude but easy to work on no matter where I am.

A link to the datasheet for the optocoupler I am looking at is:

http://docs-europe.electrocomponents.com/webdocs/0028/0900766b80028883.pdf

( Hope that copied ok )

The frequency range is unknown at present as the project is to experiment with multiple coils at different voltages at different frequencies in different configurations and see how they interact with each other.

But I am looking for a top end of between 400Khz - 500Khz if possible. Higher would always be useful, but the above mentioned would be enough.

I hope that is enough info as I cant think of what else to put

5. ### SgtWookie Expert

Jul 17, 2007
22,199
1,800
You'll have a tough time getting an optoisolator to run that fast.

Take a look at the non-saturated switching times; Tr=2uS; Tf=2uS; 4uS total.
1/4uS=250kHz. That's just transitioning from on to off and back on again; no time spent in a high or low state.

Saturated switching times are far slower.

6. ### elecbeg Thread Starter Active Member

Sep 7, 2009
30
2
Actually, thinking about it, the frequency range I was thinking of is a bit insane, not sure where I got that figure from. If I could get between 150Khz - 200Khz then that should be more than enough.

When you say the "saturated switching times are far slower", how much slower, sorry for havin to ask, but the diff times it gives for the diff parts of the graph is a bit confusing. There are numbers there ranging from 1.6uS to 25uS.

Or if you could point me in the right direction to learn how to read what each part means, then I will try to learn myself if you think that would be better.

Last edited: Nov 7, 2009