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Solid State Relay problem: help with diagnosis.

Postby stevescapin on Sun May 02, 2010 9:59 pm

My recently pid Elektra has started to behave strangely. Everything will work fine and then the solid state relay fails to switch despite being energized by the pid. If I disconnect the output side, reconnect the wires, everything returns to normal for a few hours. This usually happens at the end of the day (i.e., the machine has been on for many hours). The SSR has a heat sink and I used thermal grease. The heat sink is not especially hot when this occurs. The SSR is rated for 50A, 480v; the Elektra is a 220 volt/20A,

I should point out that the same problem occurred with a first SSR I bought from Auber. I replaced the part with a Crydom SSR and all was well for about a month. But now the problem is back. Two faulty SSRs seems to be unlikely. I must be doing something wrong here.
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Postby JimG on Sun May 02, 2010 10:30 pm

Have you measured the DC input voltage? Maybe the problem isn't with the SSR.

Which Crydom did you use?

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Postby stevescapin on Mon May 03, 2010 7:03 am

The ssr is a 4850. I will check the dc voltage again, but it was okay.
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Postby lsjms on Mon May 03, 2010 7:23 am

The sink does not get hot- what about the SSR itself? From the cycles you suggest I would think it heat related. Where is the ssr located?

http://www.power-io.com/library/appnote...k-info.htm

Are you on 110v? Is the Elektra 220v ?
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Postby stevescapin on Mon May 03, 2010 9:58 am

The machine is on 220v. The SSR is not mounted in the machine; it's next to it with and I would think there is more than enough air circulation to dissipate the heat. I removed the the heat sink, reapplied thermal grease and screwed the heat sink back on the ssr this morning. I will monitor it's behaviour today.
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Postby Randy G. on Mon May 03, 2010 11:35 am

Just in case... it takes very little heat sink compound to do its job, and any air space left between the SSR and the heat sink will make problems worse instead of better. Making sure that both surfaces are flat for good contact can also help. A warped sink or a burr on the back of the SSR should be dealt with.
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Postby compton on Thu May 06, 2010 10:56 pm

Everything works ok, and after hours, it fails. Later, it works again. Same problem occurred before with a different SSR.

That sounds like a thermal problem. The SSR will generate about 28 watts of heat. Do you have a big enough heat sink? See: http://www.power-io.com/products/heatsinks.htm

The heat sink needs to be installed so the air can go up and through the fins. A heat sink that is installed horizontally is only half as effective. A "small" amount of thermal grease is needed. The SSR needs to be firmly screwed to the heat sink.

If an SSR overheats, it can "lock on".
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Postby Randy G. on Fri May 07, 2010 1:35 am

stevescapin wrote:The machine is on 220v. The SSR is not mounted in the machine; it's next to it with and I would think there is more than enough air circulation to dissipate the heat. .


Just to eliminate a possible cause, try putting a small computer fan over the heat sink temporarily just to eliminate that possibility. The other thing to try is if it fails in the off position, try disconnecting the input voltage (BE CAREFUL!) and apply the necessary voltage from another source to see if that triggers it. If that all fails, then when it stops working next time, jumper the output side to see if that works. My next guess involves a mojo hand and an aluminum hat. I got MY mojo workin'!
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