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Please don't laugh but I am building a small working oven for my daughter and need a heating element that can heat it from 70 to 220 degrees celsius via a thermostat.

I've found this heating element http://uk.rs-online.com/web/p/ceramic-heating-elements/3762795/ (ceramic, 230V, 500W, 122 x 60 x 31 mm)

and this thermostat http://uk.rs-online.com/web/p/products/7334726/ (230V, 16A)

but am not sure these are suitable and would work together. The oven will measure about 15cm wide x 8cm hit x 10cm deep but I don't have to stick to this. Any help or advice would be amazing. You're laughing aren't you? :)

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  • \$\begingroup\$ 500W seems a bit excessive for that size. Of course, this depends on how well insulated the oven is. What are you using to insulate it? \$\endgroup\$ – Dirk Bruere Jan 16 '17 at 11:21
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    \$\begingroup\$ Traditionally this has been done with lightbulbs ("e-z-bake"). \$\endgroup\$ – pjc50 Jan 16 '17 at 11:29
  • \$\begingroup\$ I'm laughing - go buy a small oven if you at all value your time. How old is the child BTW? \$\endgroup\$ – Andy aka Jan 16 '17 at 12:01
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    \$\begingroup\$ amazon.com/Easy-Bake-A8720S00-Easy-Bake-Ultimate/dp/B00JMCJH4W/… \$\endgroup\$ – Spehro Pefhany Jan 16 '17 at 12:14
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    \$\begingroup\$ I'm not sure why you are bothering to make something that you can buy, so easily. There are well-made and thoroughly tested convection ovens and a variety of other options open to you at reasonable prices. They just work and they are UL/CE approved, too. There's little reason to re-invent this wheel. So there must be some deeper reason you are troubling yourself and placing everything at risk. You haven't disclosed that reason. You should. What is your motivation here??? \$\endgroup\$ – jonk Jan 16 '17 at 14:22
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Good old LEBs (light emitting bulbs) make great heaters. They are quite inefficient at making light, but that's because most of the power gets converted to heat.

Incandescent bulbs aren't quite as available as they used to be, but you should still be able to get them. Advantages are that they are intended to handle the heat, make nice indicators to show you when they're on, are obvious when they get blown out, and cool down rather quickly when power is removed. Lighting up the inside of the oven while it's baking so that you can see what's going on inside is a nice extra.

Definitely include a interlock so that both terminals to the bulb sockets are shut off when the door is opened. Some sort of over-temperature shutoff independent of the temperature controller is also a good idea. Stuff happens, and you want extra redundancy so that no single failure causes harm.

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  • \$\begingroup\$ I suggest an approved thermal fuse for the safety- they are very reliable and cheap. One-time- they melt and directly interrupt the supply current. \$\endgroup\$ – Spehro Pefhany Jan 16 '17 at 12:15
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Any appliance designed to be used by children should work with SELV below 25V.

If you do not know what SELV is, you probably should stop reading because you do not want your daughter dead and you in jail.

If you continued reading, I assume, you are an EE. The power in SELV appliances must be (at least in the EU) limited to 20A. A transformer supplying 24V at 20A is an expensive thing, think about at least 300,- €. You then need a heating element designed for this voltage and current. Furthermore, your thermostat has to be designed for the same voltages and currents.

Then it is not sufficient to have a thermostat for limiting the temperature. You have to add a thermal fuse to prevent a fire in case of the failure of the thermostat. A thermostat cannot be considered a safety mechanism, so a thermal fuse is mandatory.

What more is vital, if you design such an oven?

  • Make sure no part of the SELV-circuitry is connected to any conductive parts on the outside of the oven
  • Use a SELV-transformer with sufficient protection circuitry against overcurrent and short circuit. (i.e. an electronic transformer)
  • The thermal design of the housing is a vital part. You have to prevent that the outer surfaces reach dangerous temperatures to prevent fire and/or burns when using the oven.
  • you need silicone or FEP insulated wiring inside the oven to power it
  • you have to design it to reach certain IP ratings (e.g. IP 54) to deal with boiling fluids
  • all parts used in the oven must be heat resistant (metal/ceramic). And you have to take into consideration the thermal expansion of all parts at different temperatures. This is not an easy task.
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    \$\begingroup\$ Would a PC power supply be safe enough? Because those are far less than 300€ for a couple of hundred watts. \$\endgroup\$ – 0x6d64 Jan 16 '17 at 12:33
  • \$\begingroup\$ I would consider a PC power supply safe. BUT it is probably not compliant to typical regulations for toy appliances. Keep in mind what could happen if something happens! \$\endgroup\$ – Ariser Jan 17 '17 at 8:08
  • \$\begingroup\$ Why should the appliance use SELV everywhere, not just in exposed parts? Source for that? \$\endgroup\$ – immibis Jan 24 at 2:15
  • \$\begingroup\$ @imibis There is no "rule". But for a hobbyist's project it seems rather cumbersome to me to make sure all creeping distances and double insulations between SELV and non-SELV circuits are conforming to all regulations. Not to forget testing everything with finger probes and selecting reasonable IP and IK ratings. \$\endgroup\$ – Ariser Jan 24 at 9:04
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Some considerations:

  1. Heater itself: tungten wire wrapped on ceramics? Sealed tungsten unit as found in hot water pots? Oven rods? Electric stove coils? Lamp-heated? Etc. What exactly do you intend here?
  2. Oven chamber: If lamp-heated, then nickel plated is cheapest, though gold plated is better. If convective, any material may suffice.
  3. Insulation and/or Cooling: Regardless, you may want to use insulating materials between the inner walls and anything your daughter might touch. There are a variety of materials (ceramics, and any number of other options.) If lamp-heated, you may want to consider a simple quartz water cooling jacket.
  4. Closed loop control system. Obvious.
  5. Protection: GFCI, door inter-locks, bimetal thermal breaker, over-current, iover-voltage, etc.

I've built a home FAB oven that was lamp-heated and used a quartz-jacket water cooled system with nickel chamber inner walls and which used a PID closed loop control system and non-contact temperature measurement. Thousands of dollars invested in that. And it worked to develop 300 C per second ramp rates. But there is no way this was safe for a "daughter" to use.

Despite that experience, I've no idea what to recommend for you. You haven't disclosed WHY you are taking this route instead of just buying something relatively cheap, tested, and ready-to-use. There are too many good/useful products out there right now which already do all that you need and do it for a very, very reasonable price. Home-brewing this just seems very odd, unless you have some driving reason that you haven't stated.

That said, do take a look at the products available from Cotronics. They aren't a solution, but they do provide unusually useful products that may be part of one. If I were crazy enough to consider what you are considering and had some driving reason of my own, I'd probably go there are part of my research.

Speaking of which... what research have you actually done on this? Beyond just finding a heater, I mean?

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