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Induction heater 4 year project continued.

gary350

Well-Known Member
Thread starter #1
I have not worked on this project since last summer when I made an new choke with 8 toroids taped together wound with 15 turns of #14 solid copper THHN house wire. It value is 6mh tested on my best meter. I also used the mosfet heat sink tab as the gate wire if you saw this last summer amps are up from 35 to 50. This circuit use to blow mosfets at 35 amps using the gate wire but now it runs fine at 50 amps using the tab as the gate wire.

Today I made a new coil. 10 turns, 1.125" diameter, 1.5" long, #10 solid copper THHN wire with the plastic insulation still on the wire. Insulation prevents coil from getting shorted out an blowing mosfets. Notice black marks on the capacitors I have blown several mosfets. LOL. Old coil was, 8 turns, .875" diameter, 1.375" long, #10 solid copper wire.

It is very interesting the new coil is keep amps lower 10 to 20 amps depending on what I heat up. A 1/4" diameter solid steel rod turns red hot in 8 seconds just like before but with 1/2 the amps as before. The new coil must have made the circuit more efficient??? A solid 3/8" diameter steel rod heats up red hot in 14 seconds. With no metal rod inside the coil amp meter shows 2 amps. In the past the 8 turn coil got hot enough in 45 seconds to unsoldering itself but now the 10 turn coil hardly gets warm. Heat from the Red Hot steel rod is heating up the black plastic wire insulation it is still not bullet proof yet. I need to buy high temperature fiberglass wire insulation 2 pieces about 2 ft long each.

I have 8 capacitors rated .47uf 1000v in parallel, I bought new replacements capacitors rated .47uf 500v I will test soon.

Here are photos I uploaded on Craigslist to copy and paste here I hope you can see them until someone flags the AD and gets it deleted.

I know someone will complain again, WHY are you still screwing around with this thing. I am still learning & having FUN that is what a retired person does. LOL Soon I want to start the 1GBT project building a larger induction heater.

I wish this work bench was white color so I don't have to cover it with paper to get a better photo.

15 turn 6 mh choke coil


10 turn work coil


The whole induction heater


Circuit drawing
 
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rjenkinsgb

Active Member
#3
Should'nt there be a by pass capacitor on the "circuit" side of the 6 mH RFI choke?
I suspect it needs that for commutation, with the simple self-oscillating driver.
The changeover between the two power transistors is not going to be very clean and the inductor will limit the current surges.

I'd be tempted to try adding a small cap and series resistor across each schottky diode, so the gate switching is helped by the changing voltage rather than the devices only turning fully off when the opposite is fully on.

It would also need a zener or other diode protection on the gate to prevent negative or overvoltage peaks, but should improve efficiency and reduce transistor heating to some extent.

[Interesting device, I am rather impressed with the heating effects!]
 

gary350

Well-Known Member
Thread starter #4
I could use some help making this circuit better & more efficient. I took electronics in college 50 years ago but never had a job working in electronics I have forgotten more than I ever learned. I still remember and understand the simple things but I don't under stand things like why the circuit is helps with a parallel capacitor connected to the 6mh choke & why it is helped with a resistor & capacitor across the schottky diode? What capacitor do I need across the 6mh choke? I will test it to see what happens. What about resistor & capacitor across the diodes what do I need there? This circuit use to oscillation at 201 KHz the new 10T coil has probably changed that I need to test it to see what it is now. Capacitor bank is built like a rail gun capacitor bank with the mosfet connect in the center of the cap bank this works better than it did with the mosfet connected at one end of the cap bank. Now I have the heat sink connected to the circuit in 2 more places mostly to keep heat sink from moving around this seems to work better too. A capacitor in parallel with the 6mh choke will make an LC circuit does the Hz of the LC need to = the Hz of the circuit? I had to look this up in my college text book, XL=2piFL = 6.28 x 200K x .006 = 7536 ohms inductance. XL are about 6 times what it use to be with the other choke it was 1256 ohms. The correct capacitor at resonace Hz will block line current. I guess line current will be max for a split second until the circuit oscillates. All we ever did in college lab class was test resistors with a ohm meter, connect a speaker to a freq generator &make noice, build a project I built the 8 watt amp in the back of the tube manual it worked great but I never learned anything about why parts need to have certain values in a circuit. I am learning this stuff now.
 
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rjenkinsgb

Active Member
#5
The way it appears to me, there will be a significant part of each cycle where both transistors are turned on, so taking current (and getting hot) without doing anything useful.

Each is only turned off when the other is turned on hard enough to pull the gate down via the diode.

A capacitor across the diode will vary the gate voltage dependant on the change-of-voltage on the other collector - as one transistor turns off and it's collector voltage rises, that will be coupled to the other transistor and boost it's turn-on.

Likewise with the first turning on, the reducing collector voltage will force the gate of the other one low faster, ensuring that turns off quickly.

The series resistors in combination with the existing bas resistors would act as voltage dividers so the gate voltages do not get high enough to damage the transistors.
Plus a high-speed diode from each gate to emitter so the gates cannot be forced negative enough to do harm.

If you try it, add the diodes first - they should have no obvious effect.
Then add eg. a 1n cap in series with a 2k2 resistor, across each of the existing schottky diodes in your schematic.
That will only feed about 10% of the collector voltage swing back to the gates; you ideally need a 'scope to check the waveforms as it runs, or possibly add 12V or 15V zeners across gate-emitter as well to avoid any chance of overvoltage.

Then try eg. 1K in place of 2k2

It may work better with a lower gate bias, with the added cap/resistor feedback - eg. possibly five to six volts, as the caps circuit will increase as well as decrease the gate voltage, if you get the values right.


I don't thing you need the capacitor on the supply choke as it stands; possibly with the modified circuit to improve switching, but not without.
 
#6
Hello Garry 350.

In France we have make this "induction heater" some of us in the forum "usinages.com".
This scheme is a scheme of the "De Royer" oscillator .
It's a sort of an astable oscillator which provide power across an LC parallel circuit .
So ,for that I have understand ,...it's a very simple circuit , but as all simple circuit ...it have some conditions for oscillate .
The first condition , a power supply upper than 12 Volts ..if not MOSFET are destroyed .
Our friends on our forum have make fuse aluminium and copper in the "fusion" coil, with a 48 Volts power supply and apropriate MOSFET ...near 1 kWatts consumption.
But for fuse steel and rise in this case the 1300/1500° C ...nobody make a successfull try .
The convenient of this scheme is the efficiency of the machine , many calories goes in the capacitors and make warming them.
It seams that , it don't work with all MOSFET ...to verifie.
Perhap's a solution,with a circuit working , make progressivly , the "ZL" very lower than the "ZC" ...even If the wave are near square .
You can go on this french site the name is " chauffage par induction très simple ", if french is a language you know .
It's only for help.

Have a good day.

Jac.
 
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gary350

Well-Known Member
Thread starter #7
Hello Garry 350.

In France we have make this "induction heater" some of us in the forum "usinages.com".
This scheme is a scheme of the "De Royer" oscillator .
It's a sort of an astable oscillator which provide power across an LC parallel circuit .
So ,for that I have understand ,...it's a very simple circuit , but as all simple circuit ...it have some conditions for oscillate .
The first condition , a power supply upper than 12 Volts ..if not MOSFET are destroyed .
Our friends on our forum have make fuse aluminium and copper in the "fusion" coil, with a 48 Volts power supply and apropriate MOSFET ...near 1 kWatts consumption.
But for fuse steel and rise in this case the 1300/1500° C ...nobody make a successfull try .
The convenient of this scheme is the efficiency of the machine , many calories goes in the capacitors and make warming them.
It seams that , it don't work with all MOSFET ...to verifie.
Perhap's a solution,with a circuit working , make progressivly , the "ZL" very lower than the "ZC" ...even If the wave are near square .
You can go on this french site the name is " chauffage par induction très simple ", if french is a language you know .
It's only for help.

Have a good day.

Jac.
I watched this video, I have seen this circuit before but not built this one yet. I like the osc coil with no center tap. I can usually watch video in any language and understand it, I see circuit drawing, I see the project, I see the meters, I see demonstration, I understand it. Circuit drawing shows 12v -24v I did not know the mosfet gate could take 24v? Seems like mosfet gate will osc at same KHz as the circuit? I bought different mosfets & different capacitors a month ago I will test this new circuit soon. I ordered more toroids last night when they arrive I can make another choke coil. When my circuit was pulling 35 amps 20a mosfets burned up quick the 60a mosfets I have not work much better. Now that I have a better understanding of this circuit I can keep amps below 20a. I have not cooked any resistors, diodes, capacitors yet. I tested my circuit on a 12v car battery last week for the first time it works good but not as good as my 22vdc power supply. Car battery makes this circuit very simple with no power supply parts. I just realized my circuit drawing above is wrong top of the capacitor bank is not connected directly to the top end of the 10T osc coil it is connected to a U shape wire connection I am not sure why this works better?

 
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#8
Hello Gary.
The gate of each MOSFET is protected ,I think , by a zener diode ( 12 V ) on your scheme...so the power supply can climb at 48 V probably , "full mouth" of limit intensity of the MOSFET.
I think we can put 2 MOSFET in paralel in each branch , but certainly it wil be necessary to adjust the gates résistors ( down) and the zener .
Have you try to add some turns ( 1 or 2 ) to the heat coil ,and remove 1 or 2 capacitors in the LC circuit...this change make a frequency not change very much , but certainly a bade sinsoïdal signal ( quasilly square) .
But the efficiency would be better I think , and this pseudo square wave make more saturation effect in metals ( more induction) .
To verify...of course .
For the U shape problem ,... for me ,...you have a frequency of 150 kHz with a hight power , so a wire curving on left or on right in a branch change the impedance of the circuit, because of HF induction 10, 20,30cm around the heat coil.
So a protection by a metal box of all components exepted the heat coil , seems to me a good approch .
Have a good day.

What is the target of your prototype ...?

Jac.
 

gary350

Well-Known Member
Thread starter #9
Hello Gary.
The gate of each MOSFET is protected ,I think , by a zener diode ( 12 V ) on your scheme...so the power supply can climb at 48 V probably , "full mouth" of limit intensity of the MOSFET.
I think we can put 2 MOSFET in paralel in each branch , but certainly it wil be necessary to adjust the gates résistors ( down) and the zener .
Have you try to add some turns ( 1 or 2 ) to the heat coil ,and remove 1 or 2 capacitors in the LC circuit...this change make a frequency not change very much , but certainly a bade sinsoïdal signal ( quasilly square) .
But the efficiency would be better I think , and this pseudo square wave make more saturation effect in metals ( more induction) .
To verify...of course .
For the U shape problem ,... for me ,...you have a frequency of 150 kHz with a hight power , so a wire curving on left or on right in a branch change the impedance of the circuit, because of HF induction 10, 20,30cm around the heat coil.
So a protection by a metal box of all components exepted the heat coil , seems to me a good approch .
Have a good day.

What is the target of your prototype ...?

Jac.

When I started this project I did a lot of research and learned high frequency is used for skin effect heat treating only the metal surface heats up and the metal parts can be case hardened. Lower frequency is better for deep heating and certain frequencies work better for certain metal parts and certain frequency works best on smaller metal parts. I started out and built the original circuit with 2 capacitors it was very slow to heat anything. 1 by 1 I made the circuit better. The original gate resistors make mosfets over heat very fast so I changed those. When I was building Tesla Coils more capacitors gave it a bigger band so I added 2 more capacitors to the induction heater it heated faster. I added 2 more caps again then 2 more caps after that metal heated up many times faster. I changed the work coil several times and the circuit a few times and the choke several times. 160khz to 200khz is a good place to be to heat 7/8" metal parts all the way through. This project was originally for repairing a broken crow bar tip made of 7/8" steel. I finally got enough heat to repair my crow bar. I am still having fun and learning good things too. I tried 2 parallel mosfets it does not work someone mentions they both can not switch on/off at exactly the same time 1 mosfet takes all the power. I swapped mosfets a few times finally found one that works best. Now I have a new project for this induction I need to heat 1/2 inch diameter steel rods so I can make 90 degree bends. I also have 2 more circuits I want to build one uses a 12v car battery. The other induction heater project operates on 180 vdc 14 amps. When I get project finished it will be in a box, might have air or water cooling too. I also have 4 1GBTs for a project. Now I am retired it seems like I am busier than I was when I worked a full time job. I think the big problem is I am much slower it takes me all day to do 30 minutes work and my brain is on auto delete all the time.
 
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