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Question about this circuit.

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gary350

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I found this induction heater circuit on YouTube. There are several videos of people that have built this. One person used original parts. One person used a 12 volt regulator to reduce the voltage on the left 15 volt terminal. One person used 12 turn coil did not work well. One person used a 7 turn coil it work ok. One video is techinal data only it claims this circuit is only 100 watts. The Mosfets are rated 100v 35a that = 3500 watts each. Mosfets appear to be in push pull not sure how many watts this circuit is? Technical data says the choke is current limiting for the 9 turn choke if it is sized correctly it should be 15v 35a = 525 watts on each Mosfet. I think the voltage can be increased to 50v 30a = 1500 watts per Mosfet. That will be full load on my 1500 watt transformer. Can I put 50v on both terminals marked 15v? The frequency of the LC circuit is 225KHz.

**broken link removed**
 
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What are the 1uh 9 turn coils?
They will determine your load.
 
That isn't how the wattage works for a Mosfet.
That Mosfet is only rated at 115 Watts.

If it is only 115 watts it makes no sense to use a 1500 watt transformer. I need to heat up 3/4" diameter steel rods don't want to wait 30 minutes to get hot.

How do I find a 1500 watt Mosfet?

Can I put 10 Mosfets in parallel?

The mosfet is rated 100v 35a that = 100 x 35 = 3500 watts. I can tell from the package size it is NOT 3500 watts. There is something about this I don't know?
 
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You're misunderstanding how mosfet power ratings work.
In your calculation you've multiplied the supply voltage by the current and come up with a large number.
But actually when your mosfet is switched on it will only have a forward voltage drop of ~1.4V so its that number you need to multiply with the current. so 35A *1.4V = 49W .... of Power LOSS within the mosfet.

The 'Vf' (forward voltage) of the mosfets will be given in the component datasheet along with the continuous current rating you'll want to be checking that both are suitable
 
You're misunderstanding how mosfet power ratings work.
In your calculation you've multiplied the supply voltage by the current and come up with a large number.
But actually when your mosfet is switched on it will only have a forward voltage drop of ~1.4V so its that number you need to multiply with the current. so 35A *1.4V = 49W .... of Power LOSS within the mosfet.

The 'Vf' (forward voltage) of the mosfets will be given in the component datasheet along with the continuous current rating you'll want to be checking that both are suitable


I have never worked with Mosfets so I am learning. I have some P55NF06 Mosfets rated 60V 50A. I need to check to see if they are N channel. What your saying I should be able to get the full amp rating out of a mosfet so I assume I can get 50A with these? 50a x 1.4v = 70watt each mosfet x 2 = 140 watts for the total circuit. Is that correct?

I have not wound the secondary winding on my 1500 watt transformer yet. If I wind it for 50A at 30v it should work with the other Mosfets. Will this give me more than 100 watts than before?

I looked at several Mosfet data sheets I don't see anything about watt rating.

I could get 100A 15V out of this 1500 watt transformer if that will help get the power output UP.

Am I barking up the wrong tree? How do I get 1500 watts at the induction coil?
 
I could get 100A 15V out of this 1500 watt transformer if that will help get the power output UP.
You can't just change the current of your transformer without changing the wire size of the coils.
How big of a wire do you think you would need to safely carry 100 Amps?
I think you have this hole power thing confused.
Why are you still calling the transformer a "1500 watt transformer" if its not even wound yet?
 
I think your best bet is going to be start small and work from there.
If you give us the details of the parts you are going to use, then maybe someone can sim it for you.
Stop worrying about maximizing your MOT and make something!
 
I think your best bet is going to be start small and work from there.
If you give us the details of the parts you are going to use, then maybe someone can sim it for you.
Stop worrying about maximizing your MOT and make something!

Yes that is the circuit I am building but that is not where I found it.

MOTs have 100 turns on the primary instead of 200 turns like it should. 100 turns makes the transformer run at full load with no load applied. I can put any size wire I want on a transformer secondary as long as it fits in the core opening.

MOTs duty cycle is very low they can not run full power more than 10 to 12 minutes they get hot enough to smoke. The thermo over load will shut the power off before it burns up the transformer. So with that in mind I can under size the secondary wire if I need to.

MOT secondary winding is 1.2 volts per turn.

13 turns will give me 15.6 volts. If I take the core apart I can get 5 secondary windings with #12 enamel coated copper wire in parallel that is 100 amps. I can not buy #8 enamel coated copper wire so it will need to be THHN there is only room for 2 secondary windings in parallel that = 40 amps. I might be able to get 13 turns of #4 wire = 70 amps. 70 amps will work for 10 minutes. I have a 5 KW laminate core I have been saving all I need to do is wind a primary and secondary coil.

40 years ago I worked for a company that made transformers so I don't have any trouble winding my own I know how it is done. Learned it in college engineer class too. Never worked with Mosfets before.

This use to be a MOT. Now it is 200 turn, 120 vac primary, 48vac with center tap secondary, power supply transformer for a 380 watt transistor amplifier. I also build a Dynaco 120 tube amp, made 2 audio transformers and 1 power supply transformer. Transformers are easy.

T1.jpg

T4.jpg
 
All right then if you mosfets are rated for 60Volts at 50Amps max, you may want to derate them some and run them at say 50 volts at 40 amps, that way heatsinking them will not be so much of a problem. Thats 2000 watts so you will need to drop your current or voltage to get to your 1500 watt target.
Dont forget you need to make DC so adjust your voltage accordingly.
Your primary will be drawing 12.5 amps if you are at a total of 1500 watts so adjust your primary wire gage accordingly.
That will set you up for max supply power, then you will have to arrange your induction coil to use all that power.
I dont know if an induction coil works better with higher voltage or current do you?
 
The link you posted has the best information of all the other videos.

They used 4 turns on the LC coil then doubled the capacitance to maintain 200 KHz this doubled the current to the LC coil.

They are using 15 volts DC to power the circuit but the RF produces 800 volts AC on the capacitors that is why caps are rated 1000v.

They use 15 Volts DC on the Mosfets and the meter reads 70 volts AC on the Mosfets. That is why the Mosfet is rated 100 volts.

Current flowing through the Mosfets is 5 amp COLD once the metal turns RED HOT inductance changes current goes up to 26.5 amps.

OK ....I understand now.

That is probably why industrial units use only 2 turns on the LC coil so they can use more capacitors to increase the current.

Transformer 9 turns #10 wire = 10.8 volts AC = 15.3 Volts DC x 26.5 amps = 405 watts required from the transformer.

Now I see what I need to do to get what I want. I need a 4 turn LC coil with SIX .033uf caps rated 1000 volts each in parallel using STP30NF10 Mosfets.

Lots of extra power left in the transformer to run 2 more circuits = 1215 watts.
 
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Let's break down what the different parts of a mosfets rating are.

Drain to source voltage.
The maximum voltage that can be across these two terminal when the mosfet is open (not conducting.) Running a mosfet at this voltage is almost guaranteed to fail. Give yourself a good margin.

Drain current.
A calculated value which will produce the maximum junction temperature when the case is held at 25 degrees C. Terms used in this calculation include the drain-source resistance, and the junction to case thermal resistance. This current rating is a totally impractical number unless you are using refrigerated cooling.
Setting aside the guts of the part, look at the size of the pin. Do you really think you can run 50 amps through a wire of that equivalent size. For reference, the smaller part of a TO-220 pin has a cross section a bit less than a 20 AWG wire.

Power dissipation.
Like the current, the maximum calculated power that the mosfet can dissipate that will run the junction at it's max temperature while the case is at 25 C.

-
In a push-pull circuit, the nominal voltage that the drain of the mosfets will see is twice the DC voltage at the center of the winding. Add to that circuit ringing, transient spikes, etc. and you see why the reference circuit used 100V mosfets for a 15VDC input.
 
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