Induction heating / cooking - and where to get low ESR high VAR caps.

Hmm, those are nice and tasty...

Nah, they won't explode -- hell, I've put 2A through some dinky MKPs (10mm lead spacing) I found for cheap at Allied. No failures, although they did get warm!

These Epcos parts (15mm lead spacing) are *rated* 2.3A, so I put a few together for my induction heater:

200 x 0.1uF = 20uF, 250VAC, 460A (max. VAR at ~15kHz). I've had them up at rated current before, they don't break a sweat. You could easily suck 3-4A through these things.

That bank was only $70 in parts, but maybe you'd like something a little less labor intensive.

The Chinese don't need no steenkin' ratings. :-) Also, they've been known to be exactly as cheap as I am... I've seen more than a few Chinese induction supplies that use a big mess of MKPs soldered on boards for the tank cap!

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Interesting.

naah - not really a problem - at least for playing around with.

Or they get ratings that are actually real - not produced by someone looking at a cap and thinking 'let's spec it to 100mA'.

Been playing with my new induction device today, violating its specs. :)

Some rough calculation said that skin depth for non-magnetic stainless at 50KHz was ~2.5mm.

As expected, most of my thick non-magnetic stainless steel pans diddn't work at all. I was not completely surprised to find that thin non-magnetic cookware - bowls, pans, ... worked fine.

Which is handy.

Pondering modding it to add auto-simmer. (high power until noises indicate boiling, then turn it down)

On a more DIY note - do you have any sources for the ferrite bars used to reduce the field under the pan?

Is the heavy stuff All-Clad? They sandwich copper into that stuff.

Except for the sides, which is really terrible for the omlette pan we have: the sides are raw stainless, so they get black hot before the rest of the pan is even sizzling.

Not to mention the general PITA that is a stainless surface -- *everything* sticks to it. I wish they could make a cast iron clad frying pan!

Regular steel or iron (or even aluminum) pan should work okay though. Speaking of aluminum, give it a try -- pick it up and wave it around, you should be able to feel a repulsive "bubble" over the coil due to eddy currents!

Piezo sensor or something? Neet!

Nope. You can find ferrite tiles and EMI slabs at various places, for $$$.

I've wondered if some of those EMI shielding products are useful (like the flexible mats that claim mu_r ~ 10), but it would probably saturate too easily, plus it would take a lot with the low permeability. And it's even more expensive than sawing your own tiles, oh well.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

reduce the field under the pan?

So, if you stick your head close do your metal fillings explode?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show

reduce the field under the pan?

A modern version of the old 'Hot Foot'? ;-)

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

hahah.

Very not.

It's not-quite-bottom-of-the range pans.

I found fringing a problem with subdiameter pans, which is a pity.

I'm unsure what the effect of a copper shorting ring round the top of the pot would be. I can argue that it would be effective to kill the fringing field, as well as it would kill the cooker.

You get them. Well - not cast iron - but iron.

I tend not to burn much on. And if I do - it comes off in moments with a stainless pan, as I can be _LOTS_ more aggressive than I could with a non-stick pan.

Nope - nothing. The hob - according to various app-notes - is pulsed for a few ms - and if it sees a load in range - it turns on.

Copper or Al, or indeed thick nonmagnetic stainless causes too much current to flow, even at the lowest operating frequency, and it won't turn on. (though why there isn't some magic distance at which it works, I'm unsure. At full power, with a large pan, raising the gap between the pan and hob changes it from ~2000W to ~1000W (before it turns off).

(magnetic stainless has a thinner skin effect)

Cheap steel non-stick works well.

Possibly, I was actually thinking more like a conventional mic. The banging around when it boils is quite obvious, though the fan noise may be annoying.

Possibly cheaper to rip apart a cheap induction cooker :/

I wonder if laminated 'pacman' shapes of foil and insulator would work to shield the E field without getting too much losses in it. I suspect not.

I need to hook up a scope near it to see what the frequency is.

reduce the field under the pan?

And wedding rings, metal watch straps etc

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show

at

reduce the field under the pan?

How resonant would a wedding ring or watch band be at 60 KHz? The wavelength is 5,000 meters. A quater wave antenna would be 1,250 meters.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

What does "resonance" of a symmetric loop (it's not a loop antenna) have to do with induction?

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

reduce the field under the pan? be careful when working around the induction coil area if you have any form of metal wrist band devices on you. Watches, bracelets and even a metal neck chain come to mind.

at

reduce the field under the pan?

I've heated the tips of small screwdrivers to red heat at 100kHz

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show

Efficiency.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

(10mm

did

up at

known

the

indicate

to reduce the field under the pan?

They are a solid, not a small loop.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

Resonance is not a factor at all. At least in the load. The induction cooker only uses a resonant circuit to reduce the drive requirements, and EMI. You can do a non-resonant stove - but you need 200A IGBTs, not 20A - for example.

Aluminium pans don't work - none of them. However.

Regular kitchen foil is indeed levitated. Moderately on medium, and quite strongly on high power. It also glows red, and fails quite rapidly.

A disk of foil in the bottom of a plastic bucket could be a handy water heater.

I do not have any fine enough copper foil to try - I would guess that foil about the thickness of PCB foil would levitate just fine (while also heating lots).

(10mm

did

up at

known

the

indicate

to reduce the field under the pan?

If anything, a small loop would heat faster

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show

(10mm

they did

up at

little

known

the

-

indicate

to reduce the field under the pan?

If it is inside the loop of an induction heater.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

It's not resonance. Anyway. Some tests with 'worst case' realistic tests.

A pan at the low edge of the diameter limit - with a stainless steel washer of diameter 25mm OD, 15mm ID, placed right next to the (merrily boiling) pan, the washer did not heat noticably.

There is very little induced current in a loop of a small diameter.

If the load was near resonance it wouldn't have to be right against the source.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'