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4hv.org :: Forums :: General Science and Electronics
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Induction heater from Instructables, or what determines the current draw?

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Ash Small
Wed Mar 26 2014, 09:14PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
IamSmooth wrote ...

I'm referring to the circuits on my tutorial website: the 3kw unit and the 12kw unit. BOth have PLLs.

The circuit on instructables is not using a PLL

Sorry about the confusion, I was referring to this circuit:


1395868011 3414 FT1630 Circuit


No PLL, a pot cpntrolling frequency, and a pot controlling dead time.

Am I correct in assuming the 'dead time' pot controls PWM, or have I missed something?
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IamSmooth
Thu Mar 27 2014, 01:39AM
IamSmooth Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I haven't used that chip.
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Ash Small
Thu Mar 27 2014, 03:14AM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
IamSmooth wrote ...

I haven't used that chip.

My mistake, I Am, I assumed that this was your circuit.

If I'm correct, this circuit is brilliantly simple, no need for any feedback.
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Ash Small
Thu Mar 27 2014, 05:11PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've looked at the TL494 datasheet here: Link2 and it looks like it's just one pot for the frequency (maybe with the ability to change the capacitance value as well, a rotary switch and several capacitors, maybe), and one pot to control the duty cycle, from 4% to 48%, if I remember correctly, by controlling voltage between 0V and 3.3V.

I imagine what Dr DC says above regarding the series resonant topology is a valid comment, and the circuit may perform better with parallel resonant topology, but apart from that is does seem to be a 'brilliantly simple', high power circuit with only one expensive part, the tank capacitor.
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Artlav
Thu Mar 27 2014, 09:00PM
Artlav Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
There is only one pot that matters - the frequency control one.
You want to put a good multi-turn one there, otherwise it's too touchy.

The dead time pot does not do anything useful - the dead time does not propagate through the GDT.

The circuit works nicely with some fixes - damping resistors and zenner clamping on the IGBT gates, and careful layout of the driver board.

It's quite touchy - waving your hand near the driver or the timer gets the frequency off.

With a 10-turn pot it's quite easy to tune the resonance.
No idea how hard would it be to track it across things like curie point - i haven't ran it over 40V (~100W) yet, and at that level the steel bolt is heating up slowly, and aluminium stays metal cold.

I don't see anything bad on the IGBTs when the tank is off-resonance, so detuning is a valid power control method.
It seems to be cleaner if the frequency is higher than the resonance than when it is lower, which seem to be the same as in IamSmooth's design.

So far, it looks good...
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Ash Small
Thu Mar 27 2014, 11:20PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Artlav wrote ...


The dead time pot does not do anything useful - the dead time does not propagate through the GDT.



Of course. Maybe some type of centre-tapped matching transformer setup could work? (single ended topology)
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Artlav
Sat Mar 29 2014, 08:40PM
Artlav Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
Meanwhile, things are going along nicely with the build.
The power section is now built solid, and i'm testing at 50V.

It draws 5A unloaded, 4A with a chunk of aluminium (which heats up eagerly now), and 2A with a steel bolt.
So completely opposite to a ZVS heater, which consume nothing unloaded and a lot loaded.
Is that normal?

Another peculiarity - with a big chunk of steel it looses power, but keeps the frequency.
With a large chunk of aluminium it keeps the power, but the frequency goes up.
Why does this happen?
I sort of expected the opposite, since steel is ferromagnetic.

Then, i've done some tests for possible failures.
Shorting out the work coil does not seem to unsettle the bridge - some ms after the short there are spikes about 10V above the 50V, but nothing that appear major.

Turning off the gate drive while the power is applied does not produce anything catastrophic, but for some while the gates are driven at lower voltage and then chaotically as the capacitors discharge and the GDT currents decay.
This might get the IGBTs into linear region or make a few short shoot-throughs, thus doing some burst heating.
So, control circuitry must be on before the power is applied, and turned off last.

Am i missing any failure modes or potential issues with modes described?

Ash Small wrote ...
Of course. Maybe some type of centre-tapped matching transformer setup could work? (single ended topology)
Not sure what you mean here.
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Ash Small
Sat Mar 29 2014, 09:11PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Artlav wrote ...


Ash Small wrote ...
Of course. Maybe some type of centre-tapped matching transformer setup could work? (single ended topology)
Not sure what you mean here.


I think I'm going off at a tangent a bit here, I was just thinking that you could use a TL494, and two mosfet drivers to drive two mosfets (or IGBTs), one on each end of a centre-tapped matching transformer primary, and simply turn up the power using the PWM function on the TL494.

Maybe I should sketch a diagram to illustrate this? I can't see any reason why it shouldn't work, I was even thinking a similar setup could be used to drive the primary of a TC, if it's not already been done.

I'll try and get a diagram together later.

Which circuit are you using? is it the one in the first link in your first post?
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Artlav
Sat Mar 29 2014, 09:50PM
Artlav Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
Ash Small wrote ...
Maybe I should sketch a diagram to illustrate this?
You should, since i still don't quite follow.
Drive each diagonal pair of H bridge or each side of half-bridge from a separate driver?

That might work, but is it worth it?
Detuning seems to work fine as a power control method.

Ash Small wrote ...
Which circuit are you using? is it the one in the first link in your first post?
The one in the first post, with the H bridge.
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Ash Small
Sun Mar 30 2014, 10:08PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Artlav wrote ...

Ash Small wrote ...
Maybe I should sketch a diagram to illustrate this?
You should, since i still don't quite follow.

I started a new thread here: Link2 in order not to further pollute your thread with OT stuff.
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