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'Opto protected output?'
1999\07\02@082212 by Love Shack

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Hi

I'm making up a small replacement device for a PLC and want to make some
transistorised outputs as bullet-proof as possible (switching 24V at a
couple of 100mA).

Planning to use a bulk standard optocoupler (eg. 4N35) to drive a small
output transistor, but I want this to be short-circuit and over-voltage
proof as possible.  Any pearls of wisdom or standard config. that you use?
(speed is not an issue here).

Thanks

Hamish

1999\07\02@083909 by Michael Rigby-Jones

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> Hi
>
> I'm making up a small replacement device for a PLC and want to make some
> transistorised outputs as bullet-proof as possible (switching 24V at a
> couple of 100mA).
>
> Planning to use a bulk standard optocoupler (eg. 4N35) to drive a small
> output transistor, but I want this to be short-circuit and over-voltage
> proof as possible.  Any pearls of wisdom or standard config. that you use?
> (speed is not an issue here).
>
> Thanks
>
> Hamish
>
Are these open collector outputs? You could use current sinks (or sources)
on the outputs to limit the maximum current, but the problem is dissipation
under short circuit conditions will be too high.  e.g. if current was
limited to 300mA under short circuit conditions the transitor would have to
dissipate 24 *0.3 = 7.2 Watts!

I think the easiest and safest solution would be to use the self resetting
semiconductor fuses such as the PolyFuse to ensure short circuit protection.
To protect against high voltages, a suitable zener accross the transistor
would shunt the voltage untill the polyfuse kicked in.

Regards

Mike Rigby-Jones

1999\07\04@183929 by Dennis Plunkett

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At 00:20 4/07/99 +1200, you wrote:
>Hi
>
>I'm making up a small replacement device for a PLC and want to make some
>transistorised outputs as bullet-proof as possible (switching 24V at a
>couple of 100mA).
>
>Planning to use a bulk standard optocoupler (eg. 4N35) to drive a small
>output transistor, but I want this to be short-circuit and over-voltage
>proof as possible.  Any pearls of wisdom or standard config. that you use?
>(speed is not an issue here).
>
>Thanks
>
>Hamish
>
>

Having been in this spot before I can not stress the benefits of relays
enough, no problems with polarities overvoltage etc



Dennis

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