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Old 04-15-2013, 03:37 AM   #1
Kawgomoo
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Default Newb...2 elements 1 ssr, where did i go wrong? {help}

Alright folks. Apparently i have exceeded my understanding of all things electric.

I am using a PID to control 2 4500w 240v water heater elements. These draw 17amps a piece i believe.

They are wired parrallel into one 40a SSR. Which i thought would be OK.

When i tested the system everything seemed OK. As it approached the set value of 90 degrees the PID starts to "pulse" the output to the SSR, as it over shot to about 95 degrees the pulses became slower and slower until the until SSR was commanded "off". Right about this time, fireworks. Sparks and magic smoke let out of the SSR.

The heatsink was very hot when i checked it. Im stumped. 17 + 17 = 34. i thought i was good?

Can anyone please tell me what i did wrong? I dont want to order another SSR just to smoke it too.

can provide a wiring diagram if need be.

Thanks!!

p.s. mypin ta4, pt100 rtd, 40a ssr all ebay brand.

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Old 04-15-2013, 07:18 AM   #2
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What heat sink are you using? May be that it's too small or you don't have enough air flow around it. Is the sink inside an enclosure?

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Old 04-15-2013, 07:28 AM   #3
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SAying they are from ebay has me wonder if you actually got doen over by a chinesse seller with a lower rated SSR labeld as 40A

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Old 04-15-2013, 08:25 AM   #4
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4500w / 240v = 18.75a
18.75a x 2 = 37a

That said, a 40a SSR should still handle it. Definitely post the wiring diagram.

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Old 04-15-2013, 08:41 AM   #5
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I would have broken out the heating elements onto 2 seperate SSR's. I would have used 2x 25 Amps and isolated each element in cas one shorts or goes open so it is easier to trouble shoot and repair. That would give you almost a 50% safety margin to prevent the import SSR's from premature failure. Seems when they are run close to their max they fail a lot sooner than expected. That may also help with the gross overshooting of set temps too.
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Old 04-15-2013, 09:00 AM   #6
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Quote:
Originally Posted by Rbeckett View Post
I would have broken out the heating elements onto 2 seperate SSR's. I would have used 2x 25 Amps and isolated each element in cas one shorts or goes open so it is easier to trouble shoot and repair. That would give you almost a 50% safety margin to prevent the import SSR's from premature failure. Seems when they are run close to their max they fail a lot sooner than expected. That may also help with the gross overshooting of set temps too.
Wheelchair Bob
I would see your 2 x 25a SSRs, and raise you to 2 x 40a SSRs. For little additional cost, you get much more headroom and likely cooler operation.
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Old 04-15-2013, 03:37 PM   #7
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What is the heatsink configuration.
* Inside enclosure
* Inside w/ Fan
* Outside enclosure, etc.

Heatsink size and configuration will effect its ability to regulate the SSR temp.

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Old 04-16-2013, 12:10 PM   #8
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Here is a crude wiring diagram.

The ssr is on a heatsink with thermal paste. But it is inside the enclosure, with no fan. But i can change that.

Ordered an 80a SSR replacement from auber ins. i would have gone with 2x40 but done have the room to mount 2 ssr unless i lose the heat sinks.....or rebuild the entire control using a larger enclosure.

Heat sink was hot as crap after i touched it...which made me worry about the fan thing, which is an easy fix.

But what i wonder, is when do SSR get hot? do they get hot just passing energy? or is it when they cycle on and off is that what works them? just trying to understand this a little better.

Thanks for all the replies!!

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Old 04-16-2013, 12:18 PM   #9
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SSRs drop about 1.3V in use, so you need to dissipate 1.3W per Amp of power supplied.

When they cycle on and off, provided the cycle length is less than about 10 seconds, you can just multiply by the duty cycle to get the power dissipated.

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Old 04-16-2013, 11:53 PM   #10
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Well, i am not terribly certain how long the the pulse width is, or even how to adjust it.

I was assuming that running the "auto tune" feature once i got this thing up and running would prevent the temp overshoot and what not.

Auber says i can run two elements off one SSR, thats why i got the 80a ssr. I will add a fan to the case. And hope for the best. No clue if this is the right thing to do?

Does anyone see a glaring flaw with the wiring?

If i smoke another SSR i dont know what to do, except maybe go to mechanical relays instead of solid state devices.

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