SSR question - WarmGlass.com

SSR question

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Terry Gallentine
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SSR question

Postby Terry Gallentine » Fri Apr 05, 2019 12:51 pm

I built a bell kiln with 4 elements (240v). Each element has a load of approximately 9 amps. I ran wire to the kiln that took into consideration the standard safety margin for amp capacity. In other words, the wire handles 50 amps and the kiln actually has a load of 36 amps. The question that I have is whether I have to use a safety margin when selecting an ssr for the kiln. Will a 40 amp ssr work for the kiln?

Kevin Midgley
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Re: SSR question

Postby Kevin Midgley » Fri Apr 05, 2019 1:22 pm

Since heat is the enemy on all things electrical I would suggest going for the presumably extra capacity of the 50amp. What would your local electrical code say?
Further to the above, what is saved if the kiln stops running because of the smaller SSR you cheaped out on? Time and glass will be lost in all probability and they will usually have greater value than the cost difference.

Brad Walker
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Re: SSR question

Postby Brad Walker » Fri Apr 05, 2019 1:59 pm

The difference in price between a 40 amp and 50 amp SSR is under $10

Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Fri Apr 05, 2019 4:00 pm

I just asked because I have 40 amp relays already. I will look for 50 amp relays and see what I can find. I have always considered redundant relays (relays on both lines) to be the way that I would want to wire a kiln, in case one of the relays crapped out in a closed circuit position. I am wiring to an RTC-1000, would the extra relay be a senseless redundancy?

Baxsie
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Re: SSR question

Postby Baxsie » Fri Apr 12, 2019 6:36 pm

There is some good information on paralleling Solid State Relays (SSR) here:
http://www.crydom.com/en/tech/newslette ... series.pdf
basically, do not parallel SSR.

A properly installed 40-amp SSR will certainly handle a 36 amp load. There is no need to go to a 50 amp SSR.

The key is "properly installed". Unlike mechanical relays, SSRs do generate some heat when operating.

You really need to mount them to an adequate heat sink, with thermal compound between the heat sink and the relay.

As an example (it is out of stock) of what a "proper" heat sink size is, here is a 240v 40A relay and heat sink combination:
https://www.digikey.com/product-detail/ ... ND/2120180
Image

Here are some heat sinks that are specifically made for SSR:
https://www.digikey.com/products/en/fan ... /219?k=ssr
Unfortunately, they do not appear to have mounting holes, so you will have to get busy with the drill and tap -- or maybe a long screw.

On one project, I simply re-purposed an old aluminum CPU heat sink, threading holes in the old heat sink and securing it with two screws (and thermal compound), something like this.
Image
If you do not put a fan on the heat sink, then you want wider fins as shown, mount them vertically so convection can take away the heat.

If you use a heat sink with the thin fins that are really close together, like this one:
Image
Then you will want to rig the fan up to run any time the kiln is on--typically you can use an old 12v wall-bug will work fine. One of your computer geeek friends (or a computer recycling shop) will have an old heat sink & fan.

Best of luck on the project. What controller do you intend to use?

Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Fri Apr 12, 2019 7:09 pm

Thanks for the information on the ssr's. I was already planning on using good heatsinks with the ssr's and setting them away from the kilns to minimize ambient temperature. What I was thinking about doing to minimize the failure of the ssr was to use separate ssr's on each of the hot legs of the 240v. I am concerned about the single ssr failing in a closed position and thought that an ssr on each leg would minimize the situation If one relay failed. In such a case, they wouldn't really be considered to be in series or parallel would they? I am using the Bartlett RTC-1000 controller. Thanks again for the information.

Baxsie
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Re: SSR question

Postby Baxsie » Fri Apr 12, 2019 10:05 pm

If the SSRs are heat-sinked properly and used within their rating, then their reliability should be near infinite -- certainly orders of magnitude more reliable than a mechanical relay.

ref: https://media.digikey.com/pdf/Other%20R ... ations.pdf

The mean time to failure (MTTF) of SSRs is 100,000 hours, which varies with the operating conditions.


That is 13 years of "in use" time.

If you have one one control line controlling two SSRs (the control lines can be in parallel, this is not an issue), and each of those SSRs controlling half your load, then that would be OK. Since you can split your load you can get all the benefits of parallel without really paralleling them.

So basically you would have two independent switching circuits: each one with its own SSR controlling two of 9a coils for 18a which is great safety factor on a 40a relay.

I would not use one SSR on each hot leg. No benefit, and there is no reason to expect the SSRr to fail.

Here is a sketch of one way to lay it out.

Image

Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Sat Apr 13, 2019 12:22 am

Thanks for the information. I was probably going on outdated information on the failure potential of ssr's. I had a friend have a relay in his controller fail many years ago. The kiln was white hot by the time he checked it. It kind of spooked me.

jim simmons
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Re: SSR question

Postby jim simmons » Sat Apr 13, 2019 2:24 pm

Me, being the ultimate optimist, :lol: the price of getting 2 ssr's is definitely worth the peace of mind.
MTF is a weighted average. Might last for 900,000 hours, might fail after 1 hour.
The other Jim

Baxsie
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Re: SSR question

Postby Baxsie » Sun Apr 14, 2019 3:47 pm

In my diagram, I showed the circuit split because the original poster said that they had 40amp SSR but did not feel that there was enough margin running a 36 amp load on the 40 amp SSR. In concept at least, running 18 amps instead of 36 amps would keep the SSR slightly cooler and thus increase its reliability--if only by a tiny amount.

In my circuit, I only broke the red/hot side of the circuit. If the OP had 4x of the 40 amp SSR, then they could certainly add SSRs on the two black/hot sides of the circuit as well.

Frankly, there is no problem running 36 amp through a 40amp SSR that is properly heat sinked, so the OP could use his original 2x 40-amp SSR to break both the red/hot (1xSSR) and black/hot (1x SSR) legs, then split that circuit 4-ways to his 4 coils.

Breaking both legs makes it more likely to have a "wont fire" failure and less likely to have an "always fire" failure.

I still assert that a quality SSR from Crydom or another big name that is properly heat sinked will have a negligible failure rate:

ref: https://www.electronicdesign.com/compon ... -its-place
When used as intended, SSRs will perform indefinitely, usually outlasting the equipment in which they're installed.


It is your kiln though -- won't hurt a bit to put one on each hot leg.

Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Sun Apr 14, 2019 6:25 pm

Thanks Baxsie. I appreciate the help.

Bert Weiss
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Re: SSR question

Postby Bert Weiss » Fri Apr 26, 2019 12:26 am

It is always a good idea to work within 80% of capacity only for wires, breakers, and relays. Putting 36 Amps through a 50 amp rated relay is probably how I’d go, but splitting them up is good too. In larger kilns, I always use 3 zones with 3 thermocouples, because the middle zone is always hotter than the outer zones.
Bert

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Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Fri Apr 26, 2019 11:56 am

Thanks Bert. I am going with a single zone because I tested the different outer zones with a temp gun and have found them not to vary much from each other or the center. I built the kiln with a very even separation of elements in the ceiling and heavy insulation. It seems to be pretty consistent. The only possible negative is that it comes down in temperature very slowly, but for some pieces that is a plus.

Ed Cantarella
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Re: SSR question

Postby Ed Cantarella » Wed May 01, 2019 11:36 am

I always use SSRs rated for at least 3 times the load as their real life rating drops quickly with a rise in ambient temps - see the pic. And the higher capacity is a trivial amount more imo.
relay temps.png
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lbailey
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Re: SSR question

Postby lbailey » Sat May 25, 2019 10:44 am

I need to upgrade my 220 VAC Oly kiln from mechanical relays to SSRs. It has 3 relays and is on a 60A breaker, rated at 48A I believe. I've replaced the current relays several times and just don't like them being so short lived. I've read this and other posts but am still a bit confused. I thought that SSRs maintained a low level of live voltage even when off and so most designs include a mechanical relay to shut that trace current off?

I didn't see that in the sketch in this string so was wondering.

I understand the basics of this plan but am not confident enough to attempt this on my own. Is there someone I can contact via phone or email to walk me through the upgrade? Happy to pay a fair price for their time and support. My new shop has a kiln room that can get warmish so I am planning for heat sinks, a fan and a remote box wall mounted away from the kilns.....

Thanks

Bert Weiss
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Re: SSR question

Postby Bert Weiss » Mon Jul 08, 2019 3:45 pm

A simple rule is always give at least an 80% margin. So a 36 amp draw would need at least 45 amp relay. 50 amp relays are quite common and inexpensive.

I recently got burned buying Chinese relays. They came with recommended light weight heatsinks. I had a fire, that luckily I could blow out with my breath. These were 75 amp relays on 50 amp draw. I bought bigger heat sinks, and will use a different interface between relay and heatsink. I used a gooey one. Next will be a silicone pad. To replaced the Chinese relays I bought some Watlows and sone Continentals. Crydom is another good quality brand.
Bert



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Terry Gallentine
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Re: SSR question

Postby Terry Gallentine » Wed Jul 10, 2019 11:34 am

Thanks Bert. Are your relays mounted away from the kiln? I plan on isolating them from the kiln so the ambient temp will not be more than room temp along with using heat sinks. The other question that I have is about the goo. Isn't the goo for ssr's designed to transmit temperature rather than insulate it? I would think that the silicone pad might act as an insulator, but I may be wrong.

Tom Fuhrman
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Re: SSR question

Postby Tom Fuhrman » Wed Jul 10, 2019 1:18 pm

don't dismiss putting a fan on them as well. In my experience, it helped a lot. Those small computer fans are very inexpensive and are well worth the effort to install.


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