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I am working on building a kiln (approx 7.5 kW) and am using a PID to control an SSR that drives a heating element I made from Kanthal wire.

The problem I am having is that the heating element is a coil and there is potential for large voltage spikes that could end up on the mains lines in my house from back EMF.

I am aware that there are devices such as TVS diodes and MOVS, as well as power line filter devices. However, I have never used any of these before so I am unsure about which ratings of any of these I should use.

Can I use TVS Zener diodes to "clamp" the voltage across my inductive load? How do I know what value to use considering this is on mains power (240 VAC)? Or should I use something else?

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    \$\begingroup\$ A heating element is liable to be a self-snubbing coil. Its resistance is most likely to overwhelm the inductance. There should be online inductance calculators where you plug in the dimensions and number of turns if you want to check. (Or even wind it non-inductively) \$\endgroup\$
    – user16324
    Commented Mar 7, 2020 at 23:11
  • \$\begingroup\$ Not quite sure how its own resistance would overwhelm it's inductance. Consider that I am making this from Kanthal wire, and that even though it has some resistance associated with it it will still act as an inductor. Not sure what you mean. \$\endgroup\$
    – Ryan Kall
    Commented Mar 8, 2020 at 1:17
  • \$\begingroup\$ Unless you have hundreds of turns or an iron core, the inductance is likely to be negligible. \$\endgroup\$
    – SomeoneSomewhereSupportsMonica
    Commented Mar 8, 2020 at 1:55
  • \$\begingroup\$ 1) Rectify to DC then 2) Classic freewheeling diode. By the way, what is your actual inductance? \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Mar 8, 2020 at 2:31
  • \$\begingroup\$ I am making the coil today. So I can get the inductance soon, but, it will be around 200+ turns and will not have a core. Thank you for the suggestion of rectifying it first, I am going to look to see if this is reasonable (cost) considering how much power I will be using. \$\endgroup\$
    – Ryan Kall
    Commented Mar 8, 2020 at 13:56

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Kiln coils probably don't have enough inductance to cause a serious issue. But, if you are concerned, consider using zero voltage switching which greatly reduces EMI. These can still be somewhat proportional, for example to get 33% power, turn on every third cycle. This company has a good description.

https://www.avatarinstruments.com/wp-content/uploads/2015/08/Understanding-SCRs-by-Paul-Evalds-President-Avatar-Instruments.pdf

For a home design, you probably don't need an EMI filter (if you zero switch). Adding a filter won't hurt, there are others here with more experience than me regarding EMI filters, someone else will probably chime in.

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  • \$\begingroup\$ Inductive loads will draw less inrush current when switched ON at power line peak, which corresponds to minimum current at 90 degrees phase lag. And an SSR with TRIAC or SCRs will continue to conduct when turned off until current drops below hold current. But, as mentioned above, inductance in a heating coil will be negligible. \$\endgroup\$
    – PStechPaul
    Commented Jul 4, 2022 at 19:10

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