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Problems with optoisolator and grounding

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randazzm

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Hello,

I have a micro controller that has a 5v output and 5v input. (which is normal). I have 2 circuits that I need help with.

1) I have a circuit that takes the 5v from the microcontroller and steps it up to 24v to a PLC. I have an optoisolator circuit running that circuit. My problem lies in the grounding. For my PCB, i have 24V coming in, and that's it. Only 2 lines. Does one half of the optoisolator circuit need to be completely isolated, or are there different alternatives?

2) My second circuit is pretty much the opposite. I have 24V coming in and it needs to be transformed to 5v (I do this 8 times, so space needs to be saved) Otherwise i would just use a voltage regulator. (which could be possible if anybody thinks that is the best method).

Any help would be greatly appreciated! Thanks!

-=Razzledazzle=-
 

Re: Optoisolator help!

For your 1st case, it depends on if you need the 24V supply's ground to be separate or floating as compared to the 5V ground. If you want them separate then yes. Otherwise if the 24 and 5V grounds are tied together and you're just using the optocoupler for the level transition then no.

For the second problem, if you need isolation use optoisolator. It sounds like you dont though because of the regulator idea (very bad idea). If you're not looking for high speed or power saving you might try just connecting a series resistor and 5V zener diode (if your signals can put out enough current).
 

Re: Optoisolator help!

So, according to your answer for problem number 1, (and this is just for my verification to see if i understand you), that it IS possible to use an optoisolator to transition from 5V to 24V with the same ground?

For the 2nd problem, the 24V is coming from a PLC (so it will have enough current) and is going to a ATMEL microcontroller. So you're saying that a resistor and diode in series will do the job?

by the way, thank you for the fast and very helpful response!
 

Re: Optoisolator help!

Hi,
Problem 1
If you have a common ground interferance might become a problem, however over short distances this should not be the case. Optoisolators cannot translate 5V to 24V it only protects the 5V circuit from the 24V one.

Problem 2
Depends on what currents you require on the 5V side, for low currents small surface mount regulators can be used BUT if they fail your circuit would be unprotected. Optoisolation is still the best idea and again some surface mount packages contain severl isolators so space is conserved.

Cheers
Slayer
 

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