Convert Skutt Model 145 to Digital Control

[paulcook]

I'd like to convert my old kiln to a digital control. It is a Skutt 145. It's a bit odd, as it uses each hot leg of the 240V 4-wire outlet to run it's own set of 120V elements, sharing a common neutral.

Does the attached schematic seem like a reasonable way to wire this?

[neilestrick]

You can't just put digital control parts into that box. It doesn't have any sort of insulating baffle and isn't deep enough to add one. The easiest way to add digital controls is to use an external wall mounted box that the kiln plugs into. Being wall mounted, it will stay cool and last longer. Wire it up with the proper 4 wire configuration going to an outlet on the box, and plug the kiln into it. Set all the manual kiln controls to high and let the controller cycle the power. I also recommend using an existing controller made for kilns, such as the Bartlett Genesis. It's got far more kiln-specific programming than you'll be able to do yourself with a general purpose controller. You want to run each hot leg through an SSR and both hot and neutral through a mechanical relay. The mechanical relay gets wired to the safety output on the controller- it comes on and the beginning of the firing and turns off at the end. The SSR handles the cycling. You'll also need a fuse for each leg. Either get good heat sinks for the SSRs, or put a computer cabinet fan in the box. I like the fan because it generally does a better job around the heat of the kiln and it'a a lot cheaper and easier to install.

[Bill Kielb]

12 hours ago, paulcook said:

I'd like to convert my old kiln to a digital control. It is a Skutt 145. It's a bit odd, as it uses each hot leg of the 240V 4-wire outlet to run it's own set of 120V elements, sharing a common neutral.

Does the attached schematic seem like a reasonable way to wire this?

Looking at your drawing, I would say no. You are leaving the sitter in place which is a good idea for safety …. Still use cones. The SSR’s would typically take the place of the three heat switches. Since SSR’s can get stuck on as well as off a safety relay is usually placed before them and enabled by a safety circuit. Additionally PID’s work but can be difficult for most to program. Neil is right about design, cooling, and fabrication. If you are not familiar with kilns, SSR design, fail safe operating designs and electrical wiring practices this might prove to be a huge challenge.

Using a manufactured kiln controller might prove easier in the end and provide a more pleasant operating experience with built in safety features.

[paulcook]

Thank you Neil and Bill.

I will be building out the control box separate from the box already on the kiln. It will, in essence, be what you described Neil, with conduit connecting the boxes.

I will be adding physical switches to each element.

I won't add the relays, but will add in-line fuses to each hot.

And will still use cones Bill.

BOM is about $200 of parts - all-in. I can't justify $600+ on a purpose built kiln controller (that will still require additional cords/ thermocouple / etc...).

 

[paulcook]

12 hours ago, neilestrick said:

It doesn't have any sort of insulating baffle and isn't deep enough to add one.

PS - I was planning on lining the outside back face of the control box with ceramic fiber insulation and mounting the box about 1" off the kiln (thickness of the insulation).

[neilestrick]

1 hour ago, paulcook said:

I can't justify $600+ on a purpose built kiln controller

A Genesis Mini is $329, and IMO the ease of use and features are worth every dime- cone fire modes, stored programs, diagnostic tools, wifi connectivity and software updates, etc.

I would ditch the switches completely and wire the elements directly to the Sitter. The switches are just another part to fail and serve no purpose in the system. I would also hard wire the interbox receptacle, another weak point.

Mechanical relays are inexpensive and an easy safety addition to the system.

[Bill Kielb]

11 hours ago, paulcook said:

I won't add the relays, but will add in-line fuses to each hot.

Fuses will not protect against human contact with energized parts. Fuses to protect SSR’s are very fast devices and SSR manufactures provide a way to calculate or specific requirement based on time to blow. Crydoms SSR protection:

 https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx?pdfUrl=D8008051EDDB4C06A22AE4C371BFBECE

It’s good to explore and create new things, unfortunately the things you don’t know carry the potential for fire or electrocution. A controller such as Neil mentions has a lot of engineering and safety built in and 12v kiln relays are economical, established performers and relatively safe.

[Bill Kielb]

11 hours ago, paulcook said:

PS - I was planning on lining the outside back face of the control box with ceramic fiber insulation and mounting the box about 1" off the kiln (thickness of the insulation).

Ceramic not the best for conduction but high temp. Air gap is essential and anything to block the radiation from the kiln (Shiny reflector) very productive. Kind of like the difference between blazing Sun and shade. If you mount it external, no need to come up with special hardware. 

[neilestrick]

The nice thing about mounting the box to the wall rather than to the kiln, in addition to it staying cool, is that it makes construction, maintenance and repair simpler. Simply unplug the kiln from the box and you can work on the kiln without having to unmount it or disconnect a bunch of wires from it.

[paulcook]

8 hours ago, neilestrick said:

Simply unplug the kiln from the box and you can work on the kiln without having to unmount it or disconnect a bunch of wires from it.

Is there a controller with a 14-30 receptacle that would be compatible with my kiln? The Genesis Mini and others do not seem like they are plug-and-play?

[neilestrick]

2 hours ago, paulcook said:

Is there a controller with a 14-30 receptacle that would be compatible with my kiln? The Genesis Mini and others do not seem like they are plug-and-play?

Orton makes a bunch of plug and play control boxes, but they're not cheap. See HERE. Building your own system with a Genesis Mini will end up costing you about $600 in parts.

Commercial kilns controllers like the Genesis Mini run on 24 volts, so they require a transformer like THIS ONE to power them. It takes the 120 or 240 volts coming in and knocks it down to 24 volts. You'll also want a 0.5 amp fuse on the hot leg between the transformer and the controller. A basic panel mount fuse works fine. The Genesis Mini has one 12V output which you'd use to control the SSRs. There is also a safety output that you would use for the mechanical relays. The safety turns on at the start of the firing, off at the end. The mechanical relays should last forever since they're only switching twice during a firing. Run the hot leg through the fuse, then SSR, then mechanical relay. Run the neutral through the mechanical relay.

The SSRs, as cool as they are, complicate the system. You could build it much easier without the SSRs and just use a pair of mechanical relays. It would eliminate the need for fuses and a cooling fan. There's very little benefit to using SSRs unless you're doing crystalline glazes that require a little more control than the mechanical relays provide. You'll get somewhere between 200 and 600 firings from the mechanical relays, and they're cheap.

[paulcook]

@neilestrick  Those Orton boxes are very cool!

I'm going to (perhaps foolishly) proceed with my plan. I can't justify putting $600+ of equipment on a 40 year old $450 kiln.

I'll report back on how it goes.

The kiln is for firing investment plaster. It will only be used by me, maybe 5-6 times/year, supervised, with fire extinguisher nearby  In a studio environment, I wholeheartedly agree that a DIY solution is not ideal.

Really appreciate everyone's insight and help!

[paulcook]

Wired this up over the weekend - it works!

Only took it up to about 200F, but gets to temp and maintains well.

Was bummed to find out that it is not programmable, but don't know what I expected for $25 (with thermocouple and SSR included!).

Max temperature is also a bit of a question mark at this point (the documentation is not very good). But I believe it is 1300C or (oddly) 1999F.

I did replace the thermocouple with a Skutt brand.

Also, made it remote / not mounted to the Kiln and that was great advice from Neil. Nice to not have to worry about how hot it's getting in the box.

[neilestrick]

Glad it's working!

6 hours ago, paulcook said:

I believe it is 1300C or (oddly) 1999F

The first digit of the display can only display a '1' or nothing, so the F scale is limited to 1999.

[paulcook]

@neilestrick Totally the reason - smart man!