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Suggestions for relay control opto-is...

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 17 October, 2010 » Suggestions for relay control opto-isolation circuits. « Previous Next »

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stubble
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Username: stubble

Post Number: 20
Registered: 02-2010

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Posted on Saturday, 25 September, 2010 - 01:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,

Would any one have a simple circuit for controlling relays using opto-isolation, I have some CNY17-3 and some 4N25. Iím using a 16F872 and the relays are 12V DC.
Could not find any circuits on the WEB to go by.

Many thanks Lee
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muskrat
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Username: muskrat

Post Number: 302
Registered: 06-2009


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Posted on Saturday, 25 September, 2010 - 01:50 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If within the drive limits of the PIC, connect the output lines, through a limiting resistor, to the LED side of the isolators.

Use the output side to switch a driver transistor that switches the relay on when the PIC line is on. Remember to use a diode across the relay coil to reduce the possibility of fried driver transistors.
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stubble
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Username: stubble

Post Number: 21
Registered: 02-2010

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Posted on Saturday, 25 September, 2010 - 04:01 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Some thing like this if I have understood it correctly.

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epithumia
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Username: epithumia

Post Number: 787
Registered: 06-2006

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Posted on Saturday, 25 September, 2010 - 04:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Grr, I was going to post this this morning, but the site seems to me having trouble remembering that I'm logged in.

myimage

(Message edited by Epithumia on 25 September, 2010)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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gordon
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Username: gordon

Post Number: 618
Registered: 05-2005

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Posted on Saturday, 25 September, 2010 - 05:12 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi, Epithumia's circuit is the simplest, as long as the relay coil current is less than 50mA - for the 4N25. Your circuit should work, but there is no base current limiting resistor present. This could damage the 4N25 and the second transistor, and will draw excess current from the 5 volt supply. If you connect a 1K resistor between emitter of 4N25 and base of Q2, that should do it.
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epithumia
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Username: epithumia

Post Number: 788
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Posted on Saturday, 25 September, 2010 - 05:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You have good points, Gordon.

stubble's circuit needs an extra resistor. And since we've trying to isolate from the 5V, the optotransistor should run off 12V.

On the other hand, my circuit probably does only work up to 50mA. I was working on 150mA because that's the maximum rating, but I was in error, because the limited current transfer ration probably will only offer 50mA.

Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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gordon
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Username: gordon

Post Number: 619
Registered: 05-2005

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Posted on Saturday, 25 September, 2010 - 05:58 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I missed the opto isolation bit in Stubble's circuit, well spotted Epi.
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stubble
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Username: stubble

Post Number: 22
Registered: 02-2010

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Posted on Sunday, 26 September, 2010 - 09:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks,

I forgot to say that the relay side will have a separate power supply which will have 5v and 12v. The relays are power hungry.

I also, as pointed out forgot the current limiting resistor.

Hope I have got it right with this new circuit.
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gordon
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Username: gordon

Post Number: 620
Registered: 05-2005

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Posted on Sunday, 26 September, 2010 - 02:56 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

That new circuit should work okay. You could simplify it further by just using the 12V isolated supply and a 2K2 resistor.
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stubble
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Username: stubble

Post Number: 23
Registered: 02-2010

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Posted on Sunday, 26 September, 2010 - 06:30 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thank you very much. As an armature Iím learning everyday thanks to the useful people on this forum sharing there expertise.

Thank you. Lee

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