4 Heating Elements - 1 SSR, safest way to wire?

Hey y’all, novice electronics builder here.

I’m building a small oven with 4 ceramic heat elements in it that will be controlled via an Arduino digital pin & SSR.

The heating elements are from McMasterr Carr, they’re 150W/1.3Amp cylindrical cartridge heaters:

I plan on using 4 of them in a box like oven made from 4130 welded steel, one on each internal face, and insulated with a heat blanket/welders blanket.

Currently, I figure that a Crydom 25A SSR will handle the current fine. I have read/watched videos that touch on the safety hazard of using an exposed relay set-up with an Arduino, so I plan building some kind of simple acrylic housing and heat sink to house the exposed terminals in eventually.

My real question is, will this be as simple as using something like a 4 lead braided wire, with all 4 leads connected to/leaving from the single AC terminal of the SSR, and then the 4 leads on the other end connecting to the individual leads of the 4 cartridge heaters? And then the 4 ground leads of the cartridge heaters joining together at the ground terminal of the SSR? I get a bad feeling about it, but I know this can be accomplished safely somehow.

Thank you so much for your help in advance, I’ll draw up a quick photo and attach it below.

Parker

parker_adam:
will this be as simple as using something like a 4 lead braided wire, with all 4 leads connected to/leaving from the single AC terminal of the SSR, and then the 4 leads on the other end connecting to the individual leads of the 4 cartridge heaters? And then the 4 ground leads of the cartridge heaters joining together at the ground terminal of the SSR? I get a bad feeling about it,...

In your present scheme - where does the VAC figure in?
The relay will interrupt the Line that gets fed to your heaters. The other wire of the heaters should go to Neutral.
That "bad feeling" aforementioned - ominous

The SSR is a switch how is connecting heating elements across a switch going to do anything.

If this is your current state of knowledge then you are not safe to play with mains. Get some one to help you do the wiring.

Thanks for all of the help guys..
I definitely sketched this question up way too fast. My bad, I'll do some more research first.

http://www.powerswitchtail.com/Pages/default.aspx

I was thinking of using this instead of a conventional SSR. It would allow me to take my VAC from the wall and still use my Arduino. There are some examples online of using this, but not for controlling 4 heating elements with exposed wire leads.

Any thoughts or experience using one of these?

I would definitely consider putting a overtemp thermal switch in line with the heaters so if your software has an "issue" you don't melt the place down. Fires can really ruin your day (and house). Any of the wiring that has 110 (or 230) on it needs to be protected. Be careful with your idea of an acrylic housing - with that kind of heat it may melt if it is too close to the heat source and be the cause of a fire instead of protecting you.

Yeah, protected/shielded high temperature wire was definitely one of my early considerations. This oven is actually curing at only 100 degrees C (an organic polymer coating for photovoltaic applications), which I suppose is still 200+ degrees F but heat from the electrical itself could also be a factor I suppose.

Thank you!

Hi,
From the heater spec, I assume you are using 110Vac mains voltage.

Tom.... :slight_smile:

I guess my question really should be - does anyone have any ideas on building an oven safely? Perhaps I should look into a different type of heating element that can be connected in a straightforward manner with a plug rather than wiring up a socket and trying to use four elements that come with just leads in a working but not necessarily prime arrangement.

The reason I'm using four is to give each side of the wire predictable and even curing in our box shaped oven as the wire/fiber is fed through, because this step in the manufacturing process of our device is pretty crucial to efficiency and lifetime. I should add I'm an undergraduate researcher in Materials Science and no electrical engineer.

Using 4 cartridge heaters is fine. An easy way to use them is to buy a chunk of aluminum (like 6 inch x 4 inch x 0.5 inch) drill 4 holes in it. and install each cartridge heater in a hole. The aluminum will capture and spread the heat in a uniform fashion. On the SSR, install one 120 VAC hot wire to terminal 1 (or called L1) that can handle 10 amps (in the USA, about 14, 16, or 18 AWG size). Install another wire on the SSR outbound side (terminal 2 or T1), and attach it to the 4 cartridge heaters. The other leg of the 4 cartridge heaters is connected to neutral.

Install the SSR on a heatsink, since it will generate about 8-9 watts of heat. Put an on/off switch on the L1 wire, before the SSR, so you can turn it off and be 100% certain that it is off.

Monitor the temperature in your oven and pulse the SSR accordingly. It is always a good idea to have the monitoring device have an alarm output that can trip a mechanical relay that is installed near the on/off switch. Then, if an SSR fails ON, the system will turn itself OFF.

You're an undergrad researcher? At some kind of university? This is a project for work? There should be a whole department of people that build this kind of experimental apparatus. Even if you have no funding to use them on your project, they can tell you what you need to know in 30 minutes conversation.

You may even have a building maintenance department which employs an electrician. That person could give you better advice than the internet.

Paulcs:
Using 4 cartridge heaters is fine. An easy way to use them is to buy a chunk of aluminum (like 6 inch x 4 inch x 0.5 inch) drill 4 holes in it. and install each cartridge heater in a hole. The aluminum will capture and spread the heat in a uniform fashion. On the SSR, install one 120 VAC hot wire to terminal 1 (or called L1) that can handle 10 amps (in the USA, about 14, 16, or 18 AWG size). Install another wire on the SSR outbound side (terminal 2 or T1), and attach it to the 4 cartridge heaters. The other leg of the 4 cartridge heaters is connected to neutral.

Install the SSR on a heatsink, since it will generate about 8-9 watts of heat. Put an on/off switch on the L1 wire, before the SSR, so you can turn it off and be 100% certain that it is off.

Monitor the temperature in your oven and pulse the SSR accordingly. It is always a good idea to have the monitoring device have an alarm output that can trip a mechanical relay that is installed near the on/off switch. Then, if an SSR fails ON, the system will turn itself OFF.

Thank you so so much!

I’m going to include a picture of what I have drawn up so far, it should be as complete as possible.

Should I use an extension cord from the wall with 3 conductors (hot, ground, neutral) or simply one with 2 conductors (hot and neutral)? I don’t see the earth wire factoring in here…

Thanks to all for your help, I hope that this schematic makes sense (see attachment). Only thing I forgot to include is that the SSR will be on a heatsink that I’ll steal from an old PC tower.

One last question: If I'm using cartridge heaters use to heat an aluminum bulk, will I have to ground the aluminum chunk?

Or does the insulating layer & sheath fully insulate the resistive heating wire on the inside of the cartridge from the metal that I am using to radiate heat? From what I've read on Wikipedia and a few other sources, MgO does the job of electrical insulation very well in an industrial setting, so a ground will not be required unless I decide to include for safety reasons such as the sheath failing.

Here is a schematic of the common cartridge heater similar to the ones I plan on using:

I assume you are referring to a white ceramic cylinder with holes in it ?

parker_adam:
One last question: If I'm using cartridge heaters use to heat an aluminum bulk, will I have to ground the aluminum chunk?

Nice Image. That's really helpful.

If there was an electrical connection between the heating elements and the outside of the case, the heater would be very awkward to use. The Al block would be "live" at mains potential and a serious electrocution hazard. So yes, the thing is insulated.

However, is it double insulated? That's the standard of insulation required when you are building equipment without an earth pin in the power plug. There may be something in the datasheet which makes this claim. But then, you have to make sure that all your other wiring is constructed to the same standard. If just one of your wires rubs against a sharp edge of the heatsink and punctures the insulation, you have the electrocution hazard again.

The Al block and any other major pieces of metal chassis should be earthed with the earth wire from a 3-prong power plug. That wire should be continuous (no joins) from the plug to the first grounding point on the chassis, secured with shakeproof washers. That way if insulation rubs through somewhere, the fuse will blow and let you know there is a problem with your equipment.

You don't normally connect the circuit ground to earth, but often it's a good idea.

FYI, your drawing in Reply #11 is one of the best I have seen on the forum by non-electronics people.
Nice job !

Your drawing shows a "3D printed housing to protect user". Yes, you could do that. Some people would just buy an existing electrical box like this:

to make this:

Obviously, buying a bigger box (such as a 4 x 4 inch) gives you more space. The electrical box would be grounded to the green wire on your extension cord. If you buy an even larger metal box, you can mount the on/off switch, SSR, Arduino, etc, all in something that is 8x8x4 inches.

raschemmel:
FYI, your drawing in Reply #11 is one of the best I have seen on the forum by non-electronics people.
Nice job !

Thanks!

And thanks Paulcs and others for all of your input. I think I'll stick with a 3 conductor extension cord for that extra degree of safety.

@Paulcs, that is an excellent idea!

I'll make sure to post something here when I have it all working.