relays for kilns

[Mark Wright]

I need to replace the relays in my Skutt kiln, relay #2139 (25 amp, 12V/240V, single phase). Wondering if there is a better, longer lasting relay out there? I didn't turn anything up with Google.

[tbach]

Check with Skutt customer service ... I had them replace my relays with a solid state relay, and I haven't had a problem since. Before that, I replaced my relays ... two at a time, since that what I was told had to be done ... 5 times over a three year period - the first one was less than 6 months after my purchase of the kiln. No more constant clicking ... just a reassuring "hum." Had to take off the controller - lots of digital pix so I would be able to put it back on the kiln - and then I took it to Skutt. In a very short time I was up and running again. Not a single relay problem in almost 2 years ... and for me, that's a record.

[Bert Weiss]

Kiln manufacturers are designing for solid state relays on kilns they ship to Europe. I have seen a design that places the relay inside the box, but has the heat sink, sitting outside the box. You need the right box to set that up. Mechanical relays are good for an average 100,000 clicks. Solid state are good for millions. The problem is that solid state relays are sensitive to over heating. On some controllers you can adjust for your relay to cycle 5 times a second, instead of every 7 seconds. This will reign in overshoot.

I advise everybody to call their kiln manufacturer and ask if you can add a high temp limit relay to your system. This will prevent a run away. All you need for this is a controller with a built in hi-temp limit capability and a double pole normally open contactor, I'm not sure, but I think the 12 key controllers have this built in. I know the Bartlett RTC1000 does. This often has the manufacturers name on it. I think the Ortons do as well.

[Rick Wilton]

In my mind a high limit setup is just another level of complexity and possible frustration that most consumers don't need and will cause more problems than solve. If your kiln has two relays then if one does get stuck the worst that'll happen is 50% of the available heating capacity will be on. That is not enough power in my mind to any real damage to the kiln. Yes it'll likely damage your project but your kiln should be just fine.

Adding a highlimit ads and another level of complexity to trouble shoot if and when there is a problem. In my kilns with thousands of firing I have not had a single SSR relay fail, I have fired my 24" x 60" kiln on average two firings a day for 8 years and have changed nothing, not elements nor relays.

Just my 2 cents worth.

[tbach]

When Skutt changed my controller to solid state relay, they also added a heat-sink on the front of the controller box. It was included in the cost of the conversion.

[charlie]

[quote="tbach"]When Skutt changed my controller to solid state relay, they also added a heat-sink on the front of the controller box. It was included in the cost of the conversion.[/quote]

dare we ask: what was the cost of this conversion?

[Morganica]

Had Skutt replace my original controller, too, went to the solid state on the GM1414 bathtub, didn't bother with the high limit. I ran mine almost constantly, doing everything from 1550F castings that lasted a couple of weeks or more to quickie flat fuses, for several years without a problem.

I think I paid a couple hundred bucks for mine, although that was several years ago, and I did the installation myself.

[Bert Weiss]

I have been running SCR solid state relays since the mid 80's. I have had 2 runaways. One was caused by controller failure, the other was an SCR stuck closed. Neither of these events was catastrophic for me. In my single zone kiln, it was the controller failure. Both of my kilns have nichrome elements which will fail around 2000ºF or so. The element broke, the insulation boards took some stress and shrunk, but with a simple stainless steel barrel connector it is back in action. In my 3 zone kiln, the stuck relay runaway was not able to reach a catastrophic temperature without the other 2 zones being on.

That said, I always design a hi temp limit system when I build for clients. It is not terribly complex or expensive. It will save grief, wear and tear. Only recently did I learn that there is a hi-temp limit capability built in to the Bartlett RTC 1000. So all that is needed is a set of double pole normally open relays. I set these up with a simple kill switch, so with that switch off, there is no power going to the elements. I think this is an excellent safety device. When you use regular single pole relays, there is always 120v AC potential if you touch the element to ground. I like the ability to remove that potential with one singe little switch. If you are a comber, this is really a necessary safety device. I've seen commercial kilns with this switch built in to the kiln door, so when you open the door, power gets removed.

Those of us who have been reading this board for a while know that many people have had catastrophic run aways. These can be avoided.

[tob]

A bit late to this, but the thread caught my eye while I was perusing the board, and since I have some experience with these things I thought I would comment.

There’s a good reason why electro-mechanical relays should be considered a “wear item” and not expected to last forever. The electrical contacts of an EM relay typically have an expected mean time before failure (MTBF) of roughly 100,000 operations. Their mechanical life is usually rated at about 1,000,000 MTBF, but unless you can replace the contacts when they die, that longer mechanical life doesn’t matter much.

Both electronic power controls - SSRs and SCRs - and mercury relays have much longer life expectancies than EM relays. The increase in mercury relay operational life is about tenfold, and the life of SSRs and SCRs is largely indeterminate when installed correctly. (Bajillions of operations?)

Let’s do some quick math to illustrate. Starting with a 10 second cycle time, which I believe is what the Orton 3 key controller - and Paragon, and Sentry, and virtually every other one with a similar look - uses. (They’re all the same controller with different nameplates.). At any power output higher than zero but less than 100 percent, the relay operates 12 times per minute. (6 contact closures and 6 contact openings each minute.)

So…

12 operations per minute = 720 operations per hour (12 x 60 = 720)

100,000 operations (MTBF)/ 720 operations per hour = 138.9 hours MTBF

That’s right, an average expected life of less than 140 hours. So it should be no surprise that these relays need to be replaced frequently. These figures will also vary depending on the load current and the ambient temperature. Lower is better for both of those. Obviously, the calculations do not take into account the time the kiln is at 100% power (full heat) or 0% power (trying to cool faster than the kiln will allow). Those times won’t count against relay life because the relay is either full on or full off, and not switching.

So, for the sake of argument, let’s arbitrarily say that the time with power full on or full off equates to 50% of the kiln’s operating time. This would double the life expectancy of the relay to 280 hours (rounded). Still not very good if the kiln is used frequently.

So don’t expect ANY electro-mechanical relay to last very long before it needs to be replaced. That’s just the nature of the beast. Fortunately, they are pretty cheap in comparison to mercury relays and SSRs. But they make up for their low price with low reliability. My guess is that the Orton 3 key controller was originally intended to power a mercury relay, and that's why the cycle time is so short. As illustrated above, a 10 second cycle time will eat up an EM relay pretty quickly. Most relay manufacturers recommend a minimum cycle time of 30 seconds or more.

Regarding overtemp controls, I agree with Bert. They are a good idea and add a very limited amount of extra complexity to a control system. And if you already have a door switch circuit, an overtemp relay can usually be wired in with little trouble. (Assuming your controller also has an alarm output.) All it takes is the prevention of one meltdown with a full oven to more than pay for the components and the installation labor.

[Mark Wright]

Can anyone suggest who might sell a SSR for this kiln (Skutt GM-22-CS). I contacted Skutt and here is the answer I got:
"Unfortunately, we don't offer solid state relays for this kiln and don't have a design to put solid state relays on it."
Also tried Allied Electronics. No luck.
Here are the requirements:
DPST Normally Open, 30 amp, 12vdc control voltage.

[Brad Walker]

Can anyone suggest who might sell a SSR for this kiln (Skutt GM-22-CS). I contacted Skutt and here is the answer I got:
"Unfortunately, we don't offer solid state relays for this kiln and don't have a design to put solid state relays on it."
Also tried Allied Electronics. No luck.
Here are the requirements:
DPST Normally Open, 30 amp, 12vdc control voltage.

Not exactly the same situation, but when I rewired my Paragon Pearl 22 I used the following components, all purchased from Allied:

Allied Stock #, mfg number, description

693-0003 84134020
Relay; SSR; Zero-Switching; Cur-Rtg 50A; Ctrl-V 3-32DC; Vol-Rtg 24-280AC; GN Series

682-0064 HS-2
Heatsink; Aluminum; 8 to 32 Threaded; 1.875 in.; 4.12 in.; 5.0 Oz. (Typ.)

661-0057 860-60G
Heat Transfer Compound; Silicone; white paste; 2 oz tub

[Bert Weiss]

http://www.eurothermonline.com/Zelio_SS ... anel_Mount

Like Brad posted you need the relay, a heatsink, and either a pad or liquid heat transfer goo.

They just changed brands starting in January, so call and ask which model you need.

With solid state relays you can configure some controllers to be on or off 5 times a second, instead of the standard 7 second cycle. I know Bartlett RTC 1000 controllers (sometimes a company brand name) can do this. I will guess that the Ortons can too.

[tob]

Can anyone suggest who might sell a SSR for this kiln (Skutt GM-22-CS). I contacted Skutt and here is the answer I got:
"Unfortunately, we don't offer solid state relays for this kiln and don't have a design to put solid state relays on it."
Also tried Allied Electronics. No luck.
Here are the requirements:
DPST Normally Open, 30 amp, 12vdc control voltage.

Based on the wiring diagram - http://www.skutt.com/pdf/glass_kilns/Gl ... agrams.pdf - for the Skutt GM22CS, it appears you will need three SSRs - one to replace each of the relays it uses. If the relays that are on it now still work, it would be wise to retain them in the circuit as door switches. And if they no longer work, it would still be wise to have a door switch to kill power to the elements when the kiln lid is open.

As far as SSRs go, I have two brand new Chromalox 4115-40900 solid state power controllers right now that I can part with. (They're actually SSRs mounted on heatsinks, but Chromalox likes to call them "solid state power controllers", probably because it sounds cooler). One is in the original box with the user's manual. The other has no box or manual, but it is also brand new/unused. I was in the process of mounting the second one in a control box I was building for someone when I hit a deer on my motorcycle last spring. That deal subsequently fell through because she needed the controller before I was able to finish it. (My two broken hands didn't help speed things up any.) That's why there's no box for that one. I don't know what happened to that manual. I looked, but it seems to be hiding from me.

The specs are:
Rated maximum current: 90 amps
Rated current in 104F still air as mounted on it's heat sink: 50 amps (This rating can be increased by using a small fan to help cool the heatsink.)
Voltage rating: up to 480 Vac
Input control: 3-32 Vdc

I can let these go for $65 each, plus shipping, which should be about $10. But since you'll need three, there is one on eBay right now for about $23, with shipping. (Although the seller actually has two of them.) I have no experience with this seller, so can't vouch for him or the SSR condition. The mounting screws look a bit rusted in the listing photos, but it's still a good deal if working. The ones on eBay are here - http://www.ebay.com/itm/Chromalox-4115- ... 0829781044








The other Brad

[tob]

Bert already alluded to this, but I will also mention that if your controller allows it, you should set the power on/off cycle time to the shortest possible setting when using SSRs. (Assuming the controller itself isn't using an internal relay for output.) Unlike electro-mechanical relays, shorter cycle times actually improve the reliability of SSRs. A short cycle time also improves the temperature control of the kiln and extends element life. So it's a win-win when using SSRs.

Some controllers allow a cycle time as short as 1/60th of a second. (A single AC line cycle on a 60 Htz line.) Others have a much longer minimum.

[Bert Weiss]

I saw somewhere that one of the major kiln manufacturers came up with a control box design (on the side of a hot kiln) where the aluminum heatsink sits on the outside of the box, while the relay itself is situated inside the box. They routinely ship these to Europe.

[tob]

That’s nothing new or unusual, Bert. Manufacturers and equipment builders have been offering that option for many, many years. In fact, the Watlow Din-A-Mite SCRs that you bought from me a very long time ago have always offered the option of what Watlow calls a “through-wall” heatsink. The SCR works the same as yours, but instead of mounting completely inside a control panel,the heatsink goes on the outside. One simply needs to cut an appropriately sized hole in the side of the panel and bolt the SCR in place.

It’s very easy to do the same thing with an SSR. You just need a heatsink that has a flat side on which the SSR mounts, as well as a control box that has one side that is large enough and flat. Then cut a hole in the wall of your control box at least big enough for the SSR to fit through, so the SSR can be inside the box while the heatsink fins are outside. Last, drill a few matching mounting holes in both the heatsink and panel, and bolt everything together. Quite easy. In fact, right now I’m in the process of building a couple of temperature control boxes that use this same style of heatsink mounting. Nothing special, but it does make cooling easier by getting the heatsink out into the cooler (hopefully) unenclosed air.

Here are a couple of photos. The first shows several different style Watlow SCRs, with a through-wall SCR on the top left. The other is a control box I have that mounts the SSR inside the box and the heatsink on the outside.





The other Brad

[Mark Wright]

Can anyone suggest who might sell a SSR for this kiln (Skutt GM-22-CS). I contacted Skutt and here is the answer I got:
"Unfortunately, we don't offer solid state relays for this kiln and don't have a design to put solid state relays on it."
Also tried Allied Electronics. No luck.
Here are the requirements:
DPST Normally Open, 30 amp, 12vdc control voltage.

Based on the wiring diagram - http://www.skutt.com/pdf/glass_kilns/Gl ... agrams.pdf - for the Skutt GM22CS, it appears you will need three SSRs - one to replace each of the relays it uses. If the relays that are on it now still work, it would be wise to retain them in the circuit as door switches. And if they no longer work, it would still be wise to have a door switch to kill power to the elements when the kiln lid is open.

As far as SSRs go, I have two brand new Chromalox 4115-40900 solid state power controllers right now that I can part with. (They're actually SSRs mounted on heatsinks, but Chromalox likes to call them "solid state power controllers", probably because it sounds cooler). One is in the original box with the user's manual. The other has no box or manual, but it is also brand new/unused. I was in the process of mounting the second one in a control box I was building for someone when I hit a deer on my motorcycle last spring. That deal subsequently fell through because she needed the controller before I was able to finish it. (My two broken hands didn't help speed things up any.) That's why there's no box for that one. I don't know what happened to that manual. I looked, but it seems to be hiding from me.

The specs are:
Rated maximum current: 90 amps
Rated current in 104F still air as mounted on it's heat sink: 50 amps (This rating can be increased by using a small fan to help cool the heatsink.)
Voltage rating: up to 480 Vac
Input control: 3-32 Vdc

I can let these go for $65 each, plus shipping, which should be about $10. But since you'll need three, there is one on eBay right now for about $23, with shipping. (Although the seller actually has two of them.) I have no experience with this seller, so can't vouch for him or the SSR condition. The mounting screws look a bit rusted in the listing photos, but it's still a good deal if working. The ones on eBay are here - http://www.ebay.com/itm/Chromalox-4115- ... 0829781044








The other Brad

Correct me if I am wrong but it looks like the Chromalox 4115-40900 you have are SPST, not DPST that I need.
Mark

[tob]

Removed my accidental double post...

[tob]

Correct me if I am wrong but it looks like the Chromalox 4115-40900 you have are SPST, not DPST that I need.
Mark

You are correct that like nearly all SSRs, these are SPST. But you are incorrect in believing you need DPST SSRs to control your kiln. (And good luck finding one, they are pretty rare.) Virtually all SSRs are single pole. This is probably the thing that is most confusing to those who have never used SSRs or SCRs. They are only used to control one leg of a two leg 240Vac power circuit. Controlling one incoming line is all that's necessary to control heating elements on a 240Vac circuit. It's also why I recommended keeping the electro-mechanical relays you have now as door switches. They will cut the power to both incoming power lines when the kiln door is opened. And because they only operate (switch power on/off) when the door is opened or closed, they will be much more reliable than when they are the main power control device. I used the same ones on my glassmaking furnace for 14 years without a failure. (Although I did clean the contacts once a year.)

Even if you used a two pole SSR on your power lines you still need something to mechanically disconnect power to your elements when the door is open. No type of solid state power control device - SSR or SCR - will safely remove power from heating elements. They ALL leak enough electricity to hurt you.

Here is a diagram of the correct way to wire 240Vac elements when using SSR power control. Note that the SSR is on only one incoming line.




TOB

[Bert Weiss]

I agree with Brad here, but I do it only slightly differently. I use a mechanical DPDT contactor attached to a hitemp limit circuit with a light switch, and a SPST SSR controlling the kiln. This setup allows you to remove all power to the elements when you want safety, and control the elements the most efficient way. On the last system I put together, I used the high temp feature on the Bartlett RTC 1000 controller instead of a redundant hi temp controller.

BTW, I bought some used Watlow Dinamite SCR's from Brad around 17 or 18 years ago. Not one has failed on me in all that time. They are firing away as I write this.

I once made the mistake of buying 480 volt SCR's, thinking that meant up to 480v. They don't work on 240v. I have a set of 6 if anybody is working with a 480v setup.