800 Ohms across SSRelay output. Normal?? 3

[tabletop]

Hello,
I am using a PID (Omega Cni16D22) to control a 30amp solid state relay for a motor. I've not used SSR's before and assumed they were the same as mechanical relays with respect to connections.
The output of the PID is also a SSR (internal) so I wired it such that the output would 'switch' a 12volt source to the control side of the 30amp SSR. It didnt work, the 30amp relay always stayed on regardless of state. I measured the resistance across the output of the PID- 800 ohms when in alarm state and 25MegOhm when normal.

I cant think of a simple way to make this work without an external circuit. The manual seems to indicate connection as one would for a mechanical relay but it is vague.There must be a simple solution, right?

 

[ScottyUK]

You need to have another look at the SSR: they come in AC switching and DC switching types. DC types usually use a MOSFET while AC types use a triac or anti-parallel thyristors.

The manual

http://www.omega.com/manuals/manualpdf/M3355.pdf

shows an AC connection to the SSR on page 11, while page 59 details the SSR spec as: 20V - 265V AC, min load 50mA, max load 500mA. You can't switch 12V DC with it.

You need an AC input SSR, or a standard relay or contactor. If you use an additional SSR you might need to add a burden resistor to get above the minimum load requirement.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Just as Scotty sez you need a different SSR.

You cannot switch DC with the '22' option you unfortunately have. You should've gotten the '44' option instead.

As soon as that internal AC-fed-with-DC output switches ON it will stay ON indefinitely as there is no zero crossing of the power source to allow the Cni16D22's internal AC SSR to shut off.

Get an equivalent "AC input" SSR and rewire your controller to feed AC to the new SSR. There are scads of them that are 85 to 260VAC input SSRs.

But to further improve your understanding of SSRs:

SSRs are are nothing like mechanical relays. SSRs provide the opportunity to switch their AC loads ON at the zero voltage point and OFF at the zero current point. This is not available with mechanical relays and can provide for much less electrical noise in systems.

Also you can switch them ON and OFF far more rapidly than mechanical relays. This can sometimes be an advantage in reducing temperature cycling of heating elements and systems.

They can last much longer than mechanical relays.

But, like most things, they have trade offs. The very BIGGEST is that they have a voltage loss across their outputs. This directly leads to heating whenever they're ON. Engineers spend some time making sure they will not over heat in their application. If they over heat they can fail spectacularly. They also cost 2 or 3X more than their mechanical counterparts.

If you actually are going to run your SSR with a 30A load you are going to need a serious heat sink. You may also be required to derate it. You may not be able to put it in a closed up box or enclosure, like you probably could with a mechanical relay.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[tabletop]

Gentlemen, THANK YOU!

I would never had found it and have been waiting for omega tech support for a week now. Thanks for putting me out of my misery. Page numbers and extra application info too.... Over and above.

 

[itsmoked]

No problemo.

What is your relay controlling and what frequency are you expecting the relay to switch at?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[tabletop]

Well I must confess, the relays are not controlling a motor at all the're controlling two 7amp 240volt heating elements for a small boiler. I wanted to ask in this forum cos I thought the knowlege base here was the most appropriate for my issue so I made it a motor! I know some folks get funny about off topic posts.
I will put the PID in auto and see what happens but Im thinking the switch frequency will be in the 2-10 second range. Having manually operated my DC relays I must say Im surprised at how little heat they generate.

ps, I just tested the PID with AC...success! Might try a rectifier with a small capacitor so can use my DC controlled SSR's.
Thanks again.

 

[itsmoked]

Never noticed the motor comment at all.. Would normally not control motors with SSRs.

If you are only running 7A thru 30A SSRs you don't have a lot of temp issues. That's about 7W of loss. Picture a standard 7W incandescent night light. Their small surface gets almost too hot to touch. But now translate that to a larger metal plate that is in turn touching even more metal. I'd estimate a 10~15F temp rise, (not much). If the SSRs are properly screwed down to something metal they will be just fine.

BTW: I never thought of a rectifier to 'DC' the controller's output. That should work great. Use a bridge rectifier and use a very small cap. You want just a wee bit of filtering and you want the current to drop to zero very quickly when the controller goes off. Maybe 0.1uF.

Keith Cress
kcress -

http://www.flaminsystems.com