How to fit a thermal fuse?

I have just upgraded the heatbed on my printer: I've gone from an aluminium Mk3 RepRap heater at around 120W to a 200W Keenovo silicon-encased heater stuck to 5mm aluminium tooling plate. As a result the bed heating time has gone down from approaching 30 minutes to around 5 minutes. When I was tuning the heatbed the firmware warned me that in the case of thermal runaway the heater could achieve 235C! Not a good thing to happen, although it was a warning, not an error message. The Keenovo heater has its own thermal fuse, but if that ruptures then I'm faced with replacing the whole heater - also, I don't know the fuse temperature rating (or if it's self-resetting).

So, as a result, I'd like to fit my own thermal fuse with a rating of around 150C - they are cheap to buy, after all. The trouble is, I have no idea how I should fit one to the heatbed.

The instructions from Keenovo were very good, and clear. The heat-pad has to be fixed firmly to a flat surface with no bumps or voids and they recommend using a roller to make sure it's in full contact all over its surface. So how exactly am I supposed to physically fit the thermal fuse and its wiring, considering that the device is about the size of a resistor or diode? And can these things carry the current that a heatbed requires (in my case, around 8.5A at 24v)?

Note that it's not the circuitry I'm concerned about here - it's the mechanical aspects of fitting such a device.

The other reason I'm asking this question is that I'm considering the fitting of a 230v heater via an SSR - in fact, it's almost a certainty. I have read a few warnings about the risk of the SSR failing in the 'on' state, so I'd also like a thermal fuse as a last-resort backup.

Just solder it in to the heat bed wires and then attach the fuse to the heated bed... How you do that is up to you, badly mounting it will make the fuse ineffective.

Perhaps some episodes epoxy putty?

Edited 1 time(s). Last edit at 09/22/2017 06:24AM by Origamib.

It was the "how you mount it" bit that I was most interested in!

There are a couple considerations. The heater is attached to the bottom of the plate. If the adhesive lets go and the heater falls off the plate, it will get very hot, very quickly. You can attach the TCO to the bed plate or to the heater. Attaching it to the plate assumes that the heater remains attached to the plate under all circumstances. Attaching the TCO to the heater allows the TCO to operate if the heater comes off the plate, assuming that the adhesive that holds the TCO on the heater keeps working..

You can attach the TCO to the heater using high temperature silicone, and assume that the high temperature silicone remains attached under all circumstances, probably a reasonable assumption..

You can attach the TCO to the bed with a clamp, and if you use bulldog clips to hold a piece of glass on the bed (ugh!), use another bull dog clip to hold the TCO on the edge of the bed (not the most reliable way to do things). Or you can make a screw-down clamp for it and attach it to the edge of the bed. If you use some sort of mechanical clamp to hold some sort of insulation on the bottom side of the bed you can just insert the TCO between the insulation and the bed.

However you mount it, it's best to crimp the leads to the wires. You can solder it but you have to be careful to put a heatsink, such as a hemostat on the leads when you solder them. You don't want the soldering iron to trip the TCO!. You should put insulation over the leads of the TCO for obvious reasons. Less obviously, the TCO's body is "hot" electrically and if you're going to clamp it to the bed you'd better put a few layers of kapton tape on it to insulate it electrically.

I made a clamp by drilling a hole in a piece of aluminum then cutting the piece in half, leaving a round channel running down the length of the clamp. Then drilled two more holes to screw it down, and two in the edge of the bed plate. I tapped the holes in the bed plate for 6-32 screws (sorry, metric hardware and tools are expensive is backwardistan). I covered the leads with teflon sleeving and wrapped the body of the TCO with 5 mil kapton tape.



Full details here: [drmrehorst.blogspot.com]

The TCO should have an operating temperature about 24C above the highest temperature you ever intend to operate the bed. Don't buy cheap knock-off TCOs via ebay, ali-express etc. Buy them from Digikey and know that you're getting safety certified, first quality parts. They only cost about $1 each. Buy a couple spares and keep them on the printer just in case.

I've had a heater on my second printer's bed plate for about 5 years and it shows no signs of letting go, so I am comfortable with the assumption that the heater will remain stuck to the bed and the TCO can do its job. I could be wrong.

Edited 2 time(s). Last edit at 09/22/2017 08:40AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Thanks for that - very useful info, and gives me loads to consider.

I hadn't realised that the shell of the TCO was live, although it makes sense when I look at the pictures of the ones I'm considering - one lead is fixed to the case. Is it safe to trust kapton tape to insulate the TCO from 230v AC? Even if it was a good-enough insulator I'd be scared that the tape would get pierced or worn through. My plan is to fix protective earths to the gantry and bed plate of course, but I don't like big bangs... maybe I'm just getting paranoid, but one of my jobs many years ago was to do with electrical safety, and it's not easy shrugging that off.

If the heating pad falls off then apparently it has a TCO of its own, but that's unlikely to happen as my heated bed has cork insulation and a metal backing plate underneath. Even if the glue failed the heater would be held very close to the plate - although there would probably be lots of hot spots. Maybe not so much better! Perhaps fixing it to the heater pad with silicone is the best and simplest option. After all, it is only meant to be backup protection for the worst-case scenario.

Crimping the mains wire to the TCO leads is a sensible idea, as is using high-temperature insulation as you have done.

I have a good UK supplier (called Rapid) who can sell me thermal fuses, a MeanWell PSU and a brand-name SSR with cover. They've always sold me genuine stuff in the past.

I'll have to ask Keenovo about the rating of the TCO they fit to their heating pads - last time I asked them a question I got an answer (complete with a datasheet) within 30 minutes.

Although the small cylindrical thermal fuses you can get have a live body, you can get thermal cutouts that have an isolated body. See [www.ebay.co.uk] for an example. This type of cutout is widely used in domestic appliances.

I have some smaller thermal cutouts with a ceramic body and wire ends, suitable for screwing to a heated bed plate, but right now I can't remember where I purchased them.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

In the end I found some very small thermal fuses that are plastic-encapsulated, small and can carry 2A (I need less than 1A). Hopefully one of them will do the job nicely if I use high-temp silicone to fix it in place. There will be cork insulation & a backing plate to keep everything safe & secure.

This is turning out to be an escalating job... started by ordering the heating pad, then a MeanWell PSU, an SSR with thermal mounting pad & protective cover, and now the thermal fuse. Realised that the thermal fuse needs crimp connectors and high-temp insulating sleeving, so they were a last-minute order from ebay.

And I haven't even started fitting anything yet!

I've been following along and have all of the parts for this, except one - teflon sleeving for the TCO leads. Any ideas on where to source that? Since the leads on my Keenovo heat pad are longer than I need, one thought was to cut some off the end, remove the internal wire, and slide that over the TCO leads.

Any other thoughts?

As far as silicone RTV for attaching directly to the heat bed, I'm assuming I can use the gasket silicone from an auto parts store?

Thanks

That silicone/glass insulation jacket from the heater leads should work fine.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I managed to find some woven (fibreglass?) heat-proof sleeving on ebay - smaller than I expected when I saw it, but it was big enough for the thermal fuse leads. I think it cost me about £0.90 - about $1.50?

Edited 1 time(s). Last edit at 10/16/2017 03:02AM by David J.

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David J
In the end I found some very small thermal fuses that are plastic-encapsulated, small and can carry 2A (I need less than 1A). Hopefully one of them will do the job nicely if I use high-temp silicone to fix it in place. There will be cork insulation & a backing plate to keep everything safe & secure.

What kind of heater uses less than 1A?

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

The thermal cutouts I bought were these [www.ebay.co.uk]. They have insulated bodies (the listing says plastic, but the ones I received are ceramic) and a convenient screw hole for attaching them to an aluminium bed plate. They are self-resetting, which means you can test them without writing them off.

Please note, the 15A rating is almost certainly for AC use, and will be much lower when used with DC.

Edited 2 time(s). Last edit at 10/16/2017 07:03AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

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the_digital_dentist

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David J
In the end I found some very small thermal fuses that are plastic-encapsulated, small and can carry 2A (I need less than 1A). Hopefully one of them will do the job nicely if I use high-temp silicone to fix it in place. There will be cork insulation & a backing plate to keep everything safe & secure.

What kind of heater uses less than 1A?

I've got a Keenovo 230v 200W heater; using I = P/V, that gives 200/230 = 0.87A.

I've recently used a 200W heater on my 200x200 heatbed and it's reached 120C in around 6 minutes (60C in 2 or 3 minutes). More than quick enough for my needs.

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dc42
The thermal cutouts I bought were these [www.ebay.co.uk]. They have insulated bodies (the listing says plastic, but the ones I received are ceramic) and a convenient screw hole for attaching them to an aluminium bed plate. They are self-resetting, which means you can test them without writing them off.

Please note, the 15A rating is almost certainly for AC use, and will be much lower when used with DC.

I'll bear those in mind for the future. I used these: ebay. Non-resettable, but I do have 3 spares!

Edited 1 time(s). Last edit at 10/16/2017 09:40AM by David J.

And to close this topic - I now have my 230v heatbed working nicely! It's good not to have the PSU fan noise all the time (the new PSU doesn't have a fan as it's a lot smaller than the old one - it just powers the Duet control board, motors and hot end).

I've taken the heatbed up to 120C and the thermal fuse hasn't popped, which is nice...