SSR keeps blowing on 24V Heated bed

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SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Fri Mar 17, 2017 12:57 pm

awhile back I upgraded my onyx 12V bed to 24V using 35amp power supply and SSR. My problem is that I my SSR's keep malfunctioning where the SSR will stay in the closed position permanently. Checklist:

1. does SSR have acceptable current rating. Yes, I have used 40A and 100A with same result.
2. Is circuit wired properly. Yes, I have triple checked. this. Bed indeed heats, Led lights, but it does this no matter what input side of SSR is doing.
3. I have not put a fuse inline yet. maybe I should

Question, I am purchasing fostek SSR's from amazon. Could this be the issue? My SSR's are just junk and I am throwing money down the drain? Has anyone else been experiencing this issue?
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Re: SSR keeps blowing on 24V Heated bed

Postby Jimustanguitar » Fri Mar 17, 2017 2:28 pm

Do they get hot? Are you mounting them on a suitable heatsink?

You could also try turning on "bang bang" instead of PID to cycle it less. Maybe that's what's making them unhappy?

Also, you're using a DC-DC SSR, right?

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Re: SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Fri Mar 17, 2017 2:44 pm

No not getting hot. They just get stuck in the closed state. It was strange as I had it working awhile back very happily. Then all of a sudden not so happy.

Yes, SSR is DC-DC.

I do not think the issue is with the PID or frequency related because SSR stays in closed state no matter what I do. I currently have one on the bench with same result. All I can think is the SSR's are garbage. But you would think it would stay in the opened state if it was indeed an internal malfuction. That being said, I tested SSR before installing and confirmed proper operation. I am wondering if there is a way to reset an SSR.

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Re: SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Fri Mar 17, 2017 3:09 pm

Jimustanguitar
Thank you for the input on this. I have one last question. Would the more rapid cycle kill my SSR? I have set PID to 2, now all I need to do is wait for new SSR to arrive. I will post result.

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Re: SSR keeps blowing on 24V Heated bed

Postby Xenocrates » Fri Mar 17, 2017 5:50 pm

In theory, any relay has a limited life span. however, the solid state ones have a very long cycle life. I would not be worried as they were developed for industrial use with PID loops, which means the same sorts of switching rates we use.

I think the big problem is almost all the SSR's on amazon and fleabay are garbage. I recommend you buy the Auber model rather than a Fotek knock-off. Their 100A DC-DC SSR is ~20$, and is very well regarded on here, as well as it's what I personally use.
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Re: SSR keeps blowing on 24V Heated bed

Postby IMBoring25 » Fri Mar 17, 2017 10:22 pm

Swap the wires on the output terminals. They are not interchangeable.

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Re: SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Mon Mar 20, 2017 12:41 pm

Xenocrates,
I Will certainly take your advice and use the better quality SSR. Thank you very much for the help.
However, I cannot be sure this is the root cause. as yet another SSR blew immediatly upon energizing device. So, before I fry a quality SSR I need to sort out the real cause of my heartburn...

Note:
I had also taken Jimustanguitar's advice and changed teh HEATED_BED_HEAT_MANAGER setting to bang bang. (2) I also reconfirmed wiring is correct.

Here are my settings/findings:
1. heatbed set to bang bang,
2. SSR is DC-DC 40A (latest cheap amazon knockoff I had lying around)
3. Power supply voltage set to 20V.
4. Current is about 28amps @ 0.7 ohms
5. One item I have noticed that could be a problem is the Onyx Heated bed itself. I measure about .7 ohms. The documentation states this should be about 1.1 ohms. The strange thing is the resistance when measuring with multi. ohms will start at about 1.5 then will slowly drop to about .7 where it stablizes. Not sure if this is a real symptom but it might be worthy of note to the guru. I am going to go ahead and purchase a new bed just for kicks. Good idea to have spare around anyway I would think.

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Re: SSR keeps blowing on 24V Heated bed

Postby IMBoring25 » Mon Mar 20, 2017 1:36 pm

I don't think you're blowing anything. Heat stuck on is exactly the condition I had when my SSR was wired the way yours is. Swapping the output terminals left it functioning correctly.

Another thing... You can't ohm out the bed while it's connected to the circuit.

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Re: SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Mon Mar 20, 2017 2:29 pm

Hi IMBoring25,
I think my problem is a bit more sinister than having output misswired.
1. I had actually tried this awhile back and confirmed wiring was correct. LED lite on is correct wiring.
2. Bench test of SSR before installing into system proved proper operation.
3. After attempt to run on system, SSR output permanently stays in the closed position powered or not. (output removed from system)
4. Ohm test of Onyx is with leads removed from system.

After researching the SSR, I have learned that if the output stays closed and does not respond to energizing or not energizing means it is defective or has malfunctioned. I am starting to suspect there is a massive current spike when SSR is energized before it goes back to some normal state. It would be interesting to know how you have set your system up. Maybe I am doing something completely wrong. Do you by any chance know what resistance your heat bed is currently?

Thanks for the assistance.

a-

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Re: SSR keeps blowing on 24V Heated bed

Postby gchristopher » Mon Mar 20, 2017 6:08 pm

(I can't help with real electrical issues, but,) there is a firmware setting in Repetier for the PWM frequency that might help. 15hz is the slowest before you have to switch to bang-bang control. I notice that the cheap ebay/amazon SSRs don't even have a rated frequency listed.

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Re: SSR keeps blowing on 24V Heated bed

Postby Renha » Wed Mar 22, 2017 5:11 am

Xenocrates wrote:I think the big problem is almost all the SSR's on amazon and fleabay are garbage. I recommend you buy the Auber model rather than a Fotek knock-off. Their 100A DC-DC SSR is ~20$, and is very well regarded on here, as well as it's what I personally use.

Do you know some good SSR's on amazon which aren't garbage? I prefer buying there very much, and haven't found Auber ones there.
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Re: SSR keeps blowing on 24V Heated bed

Postby DeltaCon » Wed Mar 22, 2017 11:07 am

I got my Crydom through Ebay, but I would not do that again. I have absolutely no problem with my SSR, but reading these forums there are a lot of examples where fake items were being received. Most likely you are suffering from fake-items as well... Since this an item that poses serious hazards when failing, I would never take the chance of getting one over ebay or amazon unless it is absolutely a non-fake one (and you can never be sure of that!). Make sure you get one that is normally open, your's is apparently failing in a closed state and you don't want that!

This is the one I got, and is generally considered as one of the best:
http://www.digikey.com/product-detail/e ... 39-ND/7519
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Re: SSR keeps blowing on 24V Heated bed

Postby timskloss » Thu Mar 23, 2017 8:02 am

Renha wrote: Do you know some good SSR's on amazon which aren't garbage? I prefer buying there very much, and haven't found Auber ones there.


Since this much heat (500W) could start a fire I chose to spend the extra $$$ on the Crydom from Digikey. I also built a safety circuit using a normally open 24V relay and thermal fuses. The circuit is below. I run 24V through a chain of 150C thermal fuses to power the NO safety relay coil. If any of the fuses blow due to an over-temp then the relay shuts off and opens the circuit to the SSR and bed. I sleep MUCH better knowing there are thermal fuses on the bed (two on the bed in series) and one on the SSR. I used Permatex Ultra-Copper high temperature silicone to glue the thermal fuses in place.

http://www.digikey.com
Thermal fuse: 317-1136-ND
Safety relay: PB1012-ND
SSR: CC2158-ND

The SSR can go on the 'high' or 'low' side and I chose the 'high' side. It isn't supposed to matter which side you use, but with cheap imitation SSRs out there they may not like one type and fail. I also added a 1k-ohm resister between the control inputs on the SSR to make sure the input is pulled low when the RAMBO signal is off. This may be overkill but it doesn't hurt.

I also found that the switching speed of the RAMBO to the heated bed is very fast, about 2 kilohertz. The Crydom just barely reaches this speed even though the rated frequency is 0.9 kilohertz. It works for me using the stock RAMBO firmware, but it may not work well on other brands of SSRs. I agree that bang-bang would be the place to start with other brands of SSRs.

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Re: SSR keeps blowing on 24V Heated bed

Postby boarddesigner » Fri Mar 24, 2017 5:39 pm

All,
Thank you for the valuable input on this. I have resolved this issue using the instruction gained here.

1. bought Auber relay. DC DC 100A. (actually bought 2 just in case) Thanks Xenocrates!
2. boughht new Onyx ver7. However, after removing original from system I did a new resistance check. it appears to be ok. I am not sure why I was seeing the odd reading previously. Maybe I was reaading while it was still hot.
3. I am now convinced it was indeed the garbage SSR's I was purchasing from amazon. Lesson learned.

One item of note. When upgrading to 24V system I had run into an issue with the wires being to thick to easily fit under nylon standoffs of Onyx. I finally have gotten around to solving this by buying longer 4-40 flat head machine screws and printing a nice robust custom standoff @10mm instead of the 4.15mm that are used currently. Anyone else ever run into this?

Putting in a N.O relay is a good idea..... Back to the bench...

FYI,
I hope anyone who reads this thread realizes the value in never buying a cheap SRR. They are not cheap......

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Re: SSR keeps blowing on 24V Heated bed

Postby Andre B » Fri Mar 24, 2017 5:54 pm

Has anyone put a scope on the circuit?
Perhaps there is enough inductive kickback to eventually kill a cheap SSR. Maybe just need a snubber circuit.

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Re: SSR keeps blowing on 24V Heated bed

Postby gchristopher » Fri Mar 24, 2017 8:21 pm

timskloss wrote:I also found that the switching speed of the RAMBO to the heated bed is very fast, about 2 kilohertz. The Crydom just barely reaches this speed even though the rated frequency is 0.9 kilohertz. It works for me using the stock RAMBO firmware, but it may not work well on other brands of SSRs. I agree that bang-bang would be the place to start with other brands of SSRs.

Wow, I had it turned down to 15 Hz in Repetier Firmware. When it starts to PWM, it's pretty audible. Is it quieter at higher switching frequencies? I could experiment with turning that up.

I noticed that most cheap SSRs don't even list a rating for max switching frequency and I was thinking that's probably one big difference between an $80 Crydom relay and cheaper ones.

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Re: SSR keeps blowing on 24V Heated bed

Postby DeltaCon » Sat Mar 25, 2017 5:22 pm

gchristopher wrote:...I had it turned down to 15 Hz in Repetier Firmware. When it starts to PWM, it's pretty audible. Is it quieter at higher switching frequencies? I could experiment with turning that up.

I have this crydom SSR and use 15Hz frequency (standard in Duet). I nevee heard it make any noise though. Something I do hear, is the fan of my PSU going on and off in the rythem of the heater going on and off
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