Newb...2 elements 1 ssr, where did i go wrong? {help}

[Kawgomoo]

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.

 

[i8humans]

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?

 

[mattd2]

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

 

[jeffmeh]

4500w / 240v = 18.75a
18.75a x 2 = 37a

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

 

[Rbeckett]

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

 

[jeffmeh]

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.

 

[jCOSbrew]

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.

 

[Kawgomoo]

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!!

 

[alien]

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.

 

[Kawgomoo]

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.

 

[jeffmeh]

The 80a SSR should definitely be up to the task, but you may want to mount your heat sink externally. Fans can work also, but you need an air inlet as well as an outlet, and you will be pulling dust as well as air through your control panel. I favor the external heat sink.

 

[mendesm]

The wiring diagram is fine. The SSR was either defective or a <40a one labeled as a 40.

Either way, don't forget the thermal paste between the SSR and the heat sink.

 

[rcbishop]

Not sure how that SSR was rated, but a lot of things are not actually usable right up to their rating, because that requires you to keep the case of the device at room temperature.

An 80A SSR (and a brand-name one at that) is a step in the right direction, but be sure the heat sink is enough -- pay really close attention to how hot it's getting throughout the first run or two.

A fan greatly increases the amount of current you can safely switch with a device, so that might be a consideration too.

 

[Shockerengr]

Lack of cooling for the SSR probably doomed it. A lot of the heatsinks provided with the ebay SSR's aren't up to the task of cooling the SSR at full rated load, especially with only passive cooling (no fan) At high temps (like you saw) the electronics inside the SSR will start to break down and can fail spectacularly.

 

[Kawgomoo]

Well much thanks to all.

The Auber Instruments 80a is on the way... will it be any better? time will tell.

Thermal paste. Used Super Silver or whatever magic paste was the hottest thing when i built my pc's. Had enough left over to last life times.

Heatsink.. Basic piece of crap. I could cut out the back of the box where the heatsink mounts, as to allow most of the heatsink to dangle out in free air. But it seems this option would eliminate my oppurtunity to put a fan on it.

So is free air passive cooling, or inside the box with a fax the way to go? Always more decisions!

For what its worth i pried open the SSR to see what gives. Unfortunately it was all potted, so i couldnt see what mosfet was on there. However i did notice it was the resistors that exploded, and not the mosfet itself that failed...so far as i can tell. Not sure what that means, if anything. But it was worth a look i figure.

 

[alien]

You can mount a little PC fan externally if you want/need. The other advantage of mounting the heatsink outside is that you can feel how warm it is getting if you are worried that it is undersized.

 

[kal]

Can you take picture?

It sounds like you had the heatsink enclosed in a box? It won't cool very well at all like that. Best to have it on the outside of the box. If placed inside the entire insides of the box heats up to a certain temp and stays there. The only think dissipating heat is the heated air inside heating the case and wicking outside - not very effective at all.

Kal

 

[Kawgomoo]

Alright, well the SSR was replaced.

Turned out to just get a 50a locally from a industrial supplier.

The heat sink was enclosed in the small and rather crowded electrical enclosure. So i poked a hole in the back of the box, and mounted the SSR so its still inside the box, but the heatsink is dangling out the back of the box.

Couldnt manage to rig up a fan, dont have 12v source and no room to add it. Dont have a 120vac case fan on hand either.

So lets just hope for the best

Will fill the tank and fire it up tomorrow for a test run. I will try to get some pictures while im at it. Im sure you brewers will get a steady stream of us hydro dippers in your forums