AC selonoid

[Roger Dodger old codger]

I have a system to fire 9 ac selonoids to release skeet. The Uc, when called on, turns on an OPTO-22 solid state relay with zero-crossing and built in snubber. It then momentarily turns on it's selonoid. The problem is that on power up of system sometimes 1,2,or three different selonoids will release. I disconnected the Uc from the sys. so it isnt the source. It seems as if stray ac or a surge on power up caused a SSR to fire or it leaks across it to the selonoids.
I tried a snubber on ea selonoid and this helped some what but didnt eliminate the problem. Any suggestions?<p>Thanks Roger the dodger<p>[ August 02, 2005: Message edited by: Roger Dodger old codger ]</p>

[jimandy]

Not knowing a lot about SSR's I am bumfuzzled when reading data sheets at the OPTO22 web site. I assume the relay goes "on" with a low voltage DC signal. When you disconnect the MP are the inputs held stiffly low? Also, are you testing this in the field, i.e. with wires running under ground or over distances to where the solenoids release the skeets?

[Chris Smith]

I would attach a snubber type circuit to the opto circuit. Any peaks on start up will be doused. <p>A simple Zener in line, set just above the actual operating voltage should do the trick. <p>That and a large Cap value on the positive line. <p>Your probably getting a surge above the operating voltage, and this can cause havoc from bounce.

[philba]

without a schematic, it is hard to say what your problem is. got one you could post?<p>How are you triggering the SSRs? If the trigger input is floating, you could be getting noise triggering the SSRs. I don't know which opto22 you are using but the DC ones pull in at 3VDC which is a lot of noise to deal with. As a remedial action, I'd try pulling the inputs low with a, say, 10K resistor. that should not effect the ability of the microcontroller to pull them high and trigger the relays but should improve noise immunity.<p>But, I think you should also track down the source of the noise. Obviously, some sort of startup transient. check wiring to make sure you dont have AC or unfilter DC near the SSRs and triggering circuitry. Check to make sure you have good common ground connections. I'd also make sure you are using bypass caps on all the digital logic.

[philba]

something jimandy said - if the trigger runs are long and there is power going through that bundle as well, you WILL pick up noise. I'd seperate out the triggers from everything else as well as pull them low at the SSRs. <p>I don't think this is a snubbing/back emf issue as the problem (according to your description) happens before any of the solenoids fire.

[rshayes]

One remote possibility is that power is being applied to the system too quickly. An SCR can be turned on if the voltage across it rises too quickly. Depending on the type of SCR, rates of rise faster than 10 to 200 volts/microsecond may cause problems.

[Robert Reed]

Roger
If I understand you correctly, you still have this problem when the microprocessor triggering is disconnected and out of the picture.This sounds like a Dv/Dt problem. Without further knowledge of the device you are using, I will assume it follows traditional SSR design. The opto triggers a sensitive triac in its out put.This in turn gates a hi power triac to activate the solenoid. Most solenoids operate on short but very hi bursts of current creating tremendous inductive spikes on turnoff. Power ups derived from 120 volt line can also produce some pretty good sized transients, depending on their ensuing power supply design. The sensitve triac in the opto is very succeptable to self turn on when seeing these transients imposed across the M1,M2 junctions, even when the gate is solidly grounded. This is due to the relatively small die area within this device. Small die areas translate to small juntion capacitances and consequent short RC time constants, allowing them to charge to maximum transient (Dv/Dt) very quickly. Its the rapid rise time of these transients that causes problems, maybe even moreso than their amplitude. The associated power triacs with there larger die areas are not nearly as prone to thi problem.
There are a dozen different ways to adress this problem-some work, some don't. Some work some of the time.
After completing design of a specialized machine magnetic starter, we were plagued by transient turn on from these causes. Every type of suppression technique we knew of or could get information on had only limited effect on this problem.
Yet lo and behold, the simplest and crudest form of suppression eliminated the problem 99.99% and that was to install a MOV of suitable value parelled by a 0.1 Mfd suppressor type of capacitor across the incoming power line as close to each individual device as possible..If transient turn on is your problem you might want to give this a try, as it is a cheap solution.

[Enzo]

Or add a breaker to the power line to the solenoids. Turn the controls and everything else on first, then flip the breaker on to power the solenoids. That way if there is a spike or other glitch, it can't fire the solenoids because they have no power yet.<p>If you don't want to flip two switches, then add a sequencer - Have a relay SS or otherwise control power feed to the solenoids. Make a small delay in its coming on.

[Roger Dodger old codger]

Hey Guys,<p>Thanks a bunch for the replys and all the help. I have been helped. I cant wait to give the suggestions a try.
I wish I would have came here sooner. It is good to know that help is available. Again thank you all very very much.
Roger Dodger and old codger