'Induction Heating Coil' - Huh??

[ppppenguin]

Interesting! And cheap.

One of the reviews says it will heat only iron and steel - not copper. Wondering why.

Possibly because copper has a much lower resistivity than iron?

[Nick]

ZVS drivers like this are very simple and we'll documented, however I'd have trouble building one at that price.

They normally run around 60kHz - 120kHz, but it is a tuned circuit and depends on the inductance of the coil and it's leads plus the caps... The loading presented by the workpiece will also lower the frequency from the predicted value....

Just Google "zvs induction heater"... Or " royer induction heater"

NOTE: The capacitors on these tend to heat up A LOT due to internal losses - you should allow decent cooling if used continuously for more than a minute or so... The 4HV forum (of which I was an active member for many years) has a large amount of DIY induction heaters - the most popular was Marko's - see https://markobakula.wordpress.com/power-...on-heater/...

[peter scott]

Hi David and Brian,

Thanks from me too. Triggered by Brian's tool hardening suggestion I've just ordered one too.

Peter :-)

[Ed_Dinning]

Hi Gents, larger versions of these have been sold to the motoring trade for some time now as a means of heating rusted nuts and bolts to release then. This is much safer than oxy-acetylene .
The commercial ones seem to come in at a few hundred pounds, this is way cheaper.

Iron and steel only as it is an eddy current effect that causes the heating.

Ed

[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

[pwdrive]

I'd wear eye protection if I was using one of those things.

Lawrence.

[peter scott]

Great product !!
By:jruibarroso 2016-04-14 08:02:56
Great product !! It can turn red a screwdriver in 30 seconds !! Power supply must deliver more than 10 amperes to get best results ;-)

Well! I received mine and soldered the coil connections in similar fashion to the above.
I connected the unit to a Vega 650 supply set to 10 volts (It has a maximum rating of 18 amps.)
The blue LED lights and the induction coil gets warm but the unit doesn't draw significant current and doesn't heat my small screwdriver.
I did wonder if the supply ramp up was too slow so allowed the supply to register 10 volts then connected the induction unit but still no result. I guess I either received a faulty unit or I killed it on first switch on.

It's oscillating at about 200kHz.

Peter

[peter scott]

Further persistence with it: Without proximity to iron it draws a little over an amp. I think I was just not bringing my small screwdriver close enough to the coil. If I place it inside the coil then the unit draws over 4 amps and I do get a red hot screwdriver. Perhaps the photo sent by another purchaser is deceptive or perhaps I should set my supply to 12 volts.

Peter

[Terry]

I assume that the rod has been removed from the coil after heating for a clearer photo ...

[pwdrive]

Yes, just before he got his fingers burnt.

Lawrence.