'Induction Heating Coil' - Huh??

[Nick]

The commercial ones seem to come in at a few hundred pounds, this is way cheaper.

That's because essentially, it's a toy with no in-built safety protection at all. It's based on a simple Royer oscillator - professional systems use PLLs to track and adjust the frequency needed as the work coils & work items change... they also tend to have lots of safety features and power control etc. (typically uProcessor controlled).

This unit has no UVLO (under-voltage lock out) - without a decent UVLO, if the input voltage drops below the full-on voltage for the FETs (due to not being beefy enough etc.) and the FETs drop into the linear region, they can self-destruct by over-heating. Another UV failure mode is lock-up, where one of the pair latches on and burns out.

Do not exceed the maximum stated input voltage - the FETs experience voltage spikes several times the input voltage - generally, low voltage FETs have to be used to get the necessary switching speed for the required currents, so the FETs are easily damaged if the allowed input voltage range is exceeded - there is not much headroom.

The coil is not cooled in any way - even DIY induction heaters use small-bore hollow pipe as the heating coil and tend to use CPU coolers to pump chilled water through them during use. Coils can melt too...

The cap bank also experiences very large peak currents and will warm up and get hot with continuous use - explosive failure of caps is not unusual - generally, the cap (properly termed an "MMC" - "Multi-Mini Capacitor") bank is cooled by a small fan, not sealed in a plastic box - decent, low-loss, MKPs should be used - I don't know what type this particular one uses. Even removing the box and allowing convection cooling would be a help.

So...

• Don't run it for more than a minute or so
  
• Make sure you have a beefy enough power supply to prevent voltage sag
  
• Do NOT let the voltage drop below the lower input limits (try to run at the higher end of the input range)
  
• Do not exceed the permitted maximum input voltage 
  
• Keep an eye on the temperature of the FETs, the coil & the MMC bank
  
• Do not leave the unit running unattended...
  
• Do not let the work coil touch the work piece and/or short any turns
  
• Operate only on a non-conductive fire-proof surface !
  

I've built a few of these over the years, and the above points are borne of the experience of myself & many others...  you'll find the same comments elsewhere on the web...

With the above caveats, all should be fine