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Protecting the plc!

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doctor electrons

Wed Aug 17 2011, 10:21PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Thanks for any help in advance!
I could use a little bit of help with an issue that i am having and if anyone has any ideas they would be greatly appreciated.
Heres the deal!
I am running a large tesla coil inside of a faraday cage. There are a few sensors inside the cage that monitor conditions
of the machine under test. These run outside the cage into an electrical panel where they are then hooked up to the inputs
on my plc (micrologix by allen bradley). On rare occasion, high voltage makes its way through these sensor leads, in turn, blowing
the input on the card. I have tried numerous solutions and none have worked. I have tried to crowbar the sensor lead at a terminal
before the input card. Using fuses, totally doesn't work because its high voltage and ultra low current. One other consideration is
space! The panel housing the plc is nearly full and crowbaring every one of the 16 channel input cards would take up half of the
panel space!! (5 16 ch cards)

Does anyone have any ideas that could short the sensor lead to ground before the card? Or just plain block the over voltage
condition period!!??
Any help would be greatly appreciated. I need the answer to be something small that would be reliable. Say a small circuit board
with all the sensor leads tied to it and back to the card. I just can't seem to figure it out!
The inputs are 24vdc and 120vac. The crowbar circuit i came up with had a .1uf cap, a 520ohm resistor and a zener diode.
It worked for about a thousandth of a second.

(waiting for the geniuses to make me feel more dumber...)

Forty

Wed Aug 17 2011, 11:04PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

how about tvs diodes?

doctor electrons

Wed Aug 17 2011, 11:18PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

I did try those! I actually used the same ones that i use in my drsstc. They don't seem to be fast
enough and the input blows as if they weren't even in there. I did run a test circuit that worked but it is
not practical because of the added space, heres what i did.
Same crowbar circuit as before but instead of running the crowbar to ground i ran it to a circuit breaker (line side)
with the load side to ground. With a normally closed aux contact on the breaker carrying the input to the card.
IE, crowbar kicks in, breaker trips, input to the card is broken until the breaker is reset. One problem though is that
i cannot predict the amperage at the breaker accurately. It is not always the same. The hv coming through is 20 to 30kv.
Low amperage but it seems a 100ma breaker catches it most of the time. (Pulls more hair out)

about 12 inputs left on the $900.00 card! I wince at every dead led!

Sulaiman

Wed Aug 17 2011, 11:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Which Micrologix model do you have?
(I have access to Rockwell knowledge base etc. so may be able to help)

doctor electrons

Wed Aug 17 2011, 11:33PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

The dc cards are 1762-iq16 and the ac cards are 1762-ia8 but the ac cards are much less prone
to frying for some reason. Right now i am using a micrologix 1400 processor. Onboard in and out are internal
to the control panel and the expansion cards are the ones running in and out of the cage.
Another idea i had was to try and filter out the freq from the hv, another epic fail if i must say.

doctor electrons

Mon Oct 17 2011, 09:31PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Well if anyone was interested, i found a solution.
The answer was to build the crowbar circuits with a one way tvs in place of the
2n5XXX diode that was in the original circuit. Testing resulted in the plc inputs and its outputs
being safe at up to 60kv feedback through the wiring into the test cage. My guess is that the other components
in the circuit kept the diode from being smoked. On its own the tvs could not hold up to 20kv. This new circuit
works on 120vac in and out, also 24vdc in and out.
Hurray!!!!!! Maybe i should patent it!!!!

Patrick

Mon Oct 17 2011, 09:36PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

doctor electrons wrote ...

Well if anyone was interested, i found a solution.
Hurray!!!!!! Maybe i should patent it!!!!

Yeah post an exact schematic here so the rest of us can see it, ive got a lot of special stuff that needs protecting.

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