Induction Heater Works but Bridge Rectifier burnes up in 3 minutes.

[killivolt]

If the amperage is not going to go to high; you could make a power supply off an old computer. You can derive 2.5v, 5v, 12v, or 24v depending on your needs. There are a few people who have tutorials on the subject and can explain how it's done. If you can find a duff computer somewhere it's possible the voltages would be better as the supply would shut down in overload conditions.

A supply of this kind does regulate power; current control because it is a SMPS.

Watching this with interest.

Good luck,

kv

 

[mvs sarma]

This is a related link i could locate. i simply wonder whether 12V is needed
or we could go lower with say logic gate mosfets.

**broken link removed**

 

[ronv]

You should be ok without the regulator. The gate to source voltage for the FETs is +/- 20 volts. The voltage divider won't work because the circuit needs quite a bit of current to discharge the gate capacitance and to turn them on this comes thru the 240 ohm resistors - so to much drop across the high values for the divider.
The observation about the power supply voltage going up when the circuit is running is troubling. Makes me wonder about the 2 Mh inductor. If it has 2 windings in may be a common mode choke in which case you might want to use only one winding not both. If you have two of those you can put one winding from each one in series.
You might want to check the FETs to make sure they aren't shorted. You can also measure the gate voltage to make sure it is oscillating. If it reads 0 or 15 it's not running.
Maybe a ceramic cap .1ufd in parallel with the big one would help filter what gets thru the 2 Mh inductor.

Happy New Year everyone!

 

[Mosaic]

Can the OP scope the Gates of the FETs and share the image?
Also scope the supply line please.

 

[gary350]

I rebuild the industion heater so it is exactly like the circuit drawing. I used 8 AA batteries to get 12vdc on the gate. There was a big spark that came off 1 of the Mostets then it sounded like a jet engine and a stream of black smoke appeared to come out of the bottom of the mosfet for about 4 seconds, see mosfet top of the photo. I removed the mosfet and I can see, no damage, no burns, no bad places, no cracks. I replaced the mosfet tried circuit again there was a big spark, big flash of light, loud snap, but no smoke from the same mosfet. The circuit does not work. I took it apart all the parts test good. Something must be bad. I compared a new mosfet to the used mostfet ohm meter gives me the same readings, no shorts, no open circuits. Diodes test good, resistors test good, cap tests good, choke tests good. Power supply is 15.3v. I do not have a scope. Something is wrong???

**broken link removed**

**broken link removed**

 

[ronv]

It's one of those circuits that if it doesn't work it kills itself.
You might put a 1 ohm in series with the inductor until you get it sorted out. (cheaper than FETs)

 

[Mosaic]

I'd imagine that the 3300uF should be a low ESR, 105C temp. rated unit, or it might overheat during operation, select a cap with the largest physical body. The .47uF snubber cap could be a WIMA self healing unit.

 

[killivolt]

**broken link removed**

Looking at your schematic; I'm noticing the Zeners placement? I don't know how this circuit works; but my "Plasma Speaker had them across the Mosfets. Not where yours are; if they are placed correctly placed in the schematic; are you sure you have them right way round.

My "Techs" when we built boards would always get them backward. I would check them.

kv

 

[alec_t]

If either heatsink is grounded and there's no insulation between the FET tab and heatsink, that could acccount for the fireworks.

 

[killivolt]

If either heatsink is grounded and there's no insulation between the FET tab and heatsink, that could acccount for the fireworks.

Never even gave it a second thought? Good one.

kv

 

[gary350]

All new parts. Heat sinks and pin #2 of each mosfet are soldered directly to the induction coil. Diodes connected from pin 1 to pin 2 of each mosfet. 240 ohm resistors connected to pin 1 and to the 12 v battery packs with a on/off switch to - side of bridge rectifier. Choke connected directly to center tap of induction coil and + side of bridge rectifier. .47uf cap in parallel with induction coil. 3300uf cap soldered across + and - of bridge rectifier. It is almost time for the SMOKE TEST. LOL. I'm still not sure the 12v battery is a good idea?

Any suggestions before I plug in the transformer and flip the ON switch?

**broken link removed**

 

[kubeek]

Put in the 1ohm resistor ronv suggested in series with the inductor, and use a current meter to watch the consumption and turn the thing it of before things get out of hand.
Also are you really sure about the value of that inductor?

 

[gary350]

Put in the 1ohm resistor ronv suggested in series with the inductor, and use a current meter to watch the consumption and turn the thing it of before things get out of hand.
Also are you really sure about the value of that inductor?

My meter says the choke is 2.1mh instead of 2mh. If I remove 1 turn then is it 1.9mh.

My meter keeps changer back and forth .007, .008, .007, .008,.007, .008,.007, .008, finally stopped on .008mh, made with #10 wire. 8 turns, 1" diameter center to center of each wire. 1.375" long.

I have a .75 ohm 15 watts resistor is can solder into the circuit. The only 1 ohm resistors I have are only 5 watt.

 

[killivolt]

All new parts. Heat sinks and pin #2 of each mosfet are soldered directly to the induction coil. Diodes connected from pin 1 to pin 2 of each mosfet. 240 ohm resistors connected to pin 1 and to the 12 v battery packs with a on/off switch to - side of bridge rectifier. Choke connected directly to center tap of induction coil and + side of bridge rectifier. .47uf cap in parallel with induction coil. 3300uf cap soldered across + and - of bridge rectifier. It is almost time for the SMOKE TEST. LOL. I'm still not sure the 12v battery is a good idea?

Any suggestions before I plug in the transformer and flip the ON switch?

**broken link removed**

I only see heat sink compound; do you have an insulator between the heat sink and Mosfets?

kv

 

[gary350]

I only see heat sink compound; do you have an insulator between the heat sink and Mosfets?

kv

I have no insulator between the mosfet and heat sink. The screw that holds the mosfet to the heat sink will be a short circuit with and insulator. Wire #2 and tab are the same.

I soldered a 1 ohm 5 watt resistor into the circuit, power supply ON, with the 12v battery OFF. The 1 ohm resistor got hot enough to smoke in 5 seconds. The mosfets must be in the normally closed position. Resistor is smoken hot but mosfets are cold.

With the 12v battery ON and power supply ON the induction heater works. The 5 watt resistor was smokin hot in 30 seconds and by then the metal piece in the coil was starting to turn red hot. Mosfets are cold. Bridge rectifier is cold too.

That 1 ohm resistor was a great idea.

I tried the .75 ohm resistor. The metal part heated up about 25% faster and got slightly hotter a dull red. The 15 watt resistor was starting to smoke after 45 seconds. Mosfets are not ice cold like they were it is 50 degrees in the shop they have warmed up to about 75 degrees. Bridge rectifier is not hot either.

I tried a 9v battery the circuit runs good.

I tried the voltage divider the circuit runs good.

The circuit needs a current limiting choke of 15 amps. Is there an online calculator to determine the choke value for 15a at this frequency?

I also need online calculator to find the value of the coils so I can check that again my meter?

There needs to be a way to make mosfet OFF time about 95% compared to mostfet ON time of 5% to reduce current through the mosfet. It seems to me heat in the metal part is not caused by current in the mosfet it is caused by eddy currents in the metal part.


I also need online calculator for resonance frequency of an LC circuit?

 

[killivolt]

**broken link removed**

Why are you connecting the Negative rail to the battery and the rectifiers Negative?

I'm not sure you can do that?

kv

 

[MrAl]

Hi,

This does not look like a very good design, at least not at first glance. Here is how it appears to work and why it's not that good.

1. First, one transistor turns on and that turns the other one off if it was on already because the gate drive drops.
2. Next, current starts to flow though the mosfet because the inductor allows it to ramp up. If it were not for the inductor it would shoot up faster.
3. Next, the current reaches the level where the MOSFET can no longer keep it's drain to source voltage low so the voltage across the MOSFET ramps up. Unfortunately, the current stays high.
4. As the mosfet drain source voltage ramps up eventually it reaches the point where the other mosfet turns on, and that kills the gate drive to the first mosfet thus turning it off. This ends one half cycle.
5. The same thing happens with this second mosfet and that's the second half cycle.

The circuit appears to 'work' because the thing oscillates and drives the output. The biggest problem however is that the mosfets have to stay active for the time where the current ramps up (#2 and #3 above) and this will cause an enormous heating effect. Even though this 'linear' period does not last that long in time, in duty cycle it could be a large part of the entire half cycle, thus eating up lots of power and also overheating the mosfets. Fixing this problem will not only mean the mosfets run cool, it will also reduce the input power requirement and allow a smaller heat sink for the rectifiers.

This same kind of circuit could be used with bipolars probably with better success because bipolars will pull out of saturation at a faster rate. In fact, i would not be surprised to find that this design came from an original design almost the same except using bipolars.

There is also a very likely second potential problem: the inductor. The inductor for this kind fo circuit would have to have a large DC current rating so that it does not saturate with DC current too easily. It has to be able to handle a lot of current and still act like an inductor. If not, it acts like a resistor which presumably isnt as good.

We could look at solutions if you are interested, except of course if you want this circuit to double as a french fry or chicken parts deep fryer

 

[gary350]

Hi,

This does not look like a very good design, at least not at first glance. Here is how it appears to work and why it's not that good.

1. First, one transistor turns on and that turns the other one off if it was on already because the gate drive drops.
2. Next, current starts to flow though the mosfet because the inductor allows it to ramp up. If it were not for the inductor it would shoot up faster.
3. Next, the current reaches the level where the MOSFET can no longer keep it's drain to source voltage low so the voltage across the MOSFET ramps up. Unfortunately, the current stays high.
4. As the mosfet drain source voltage ramps up eventually it reaches the point where the other mosfet turns on, and that kills the gate drive to the first mosfet thus turning it off. This ends one half cycle.
5. The same thing happens with this second mosfet and that's the second half cycle.

The circuit appears to 'work' because the thing oscillates and drives the output. The biggest problem however is that the mosfets have to stay active for the time where the current ramps up (#2 and #3 above) and this will cause an enormous heating effect. Even though this 'linear' period does not last that long in time, in duty cycle it could be a large part of the entire half cycle, thus eating up lots of power and also overheating the mosfets. Fixing this problem will not only mean the mosfets run cool, it will also reduce the input power requirement and allow a smaller heat sink for the rectifiers.

This same kind of circuit could be used with bipolars probably with better success because bipolars will pull out of saturation at a faster rate. In fact, i would not be surprised to find that this design came from an original design almost the same except using bipolars.

There is also a very likely second potential problem: the inductor. The inductor for this kind fo circuit would have to have a large DC current rating so that it does not saturate with DC current too easily. It has to be able to handle a lot of current and still act like an inductor. If not, it acts like a resistor which presumably isnt as good.

We could look at solutions if you are interested, except of course if you want this circuit to double as a french fry or chicken parts deep fryer

I need an induction heater that works good even if I have to build a different circuit. I need to use it to heat a 7/8" diameter steel rod to 1800 degrees F . This tiny thing I built will be lucky to that that rod to 300 degrees.

Mosfets need to be OFF longer than they stay ON. Can that be done by changing the 240 ohm resistor?

 

[killivolt]

I have no insulator between the mosfet and heat sink. The screw that holds the mosfet to the heat sink will be a short circuit with and insulator. Wire #2 and tab are the same.

I soldered a 1 ohm 5 watt resistor into the circuit, power supply ON, with the 12v battery OFF. The 1 ohm resistor got hot enough to smoke in 5 seconds. The mosfets must be in the normally closed position. Resistor is smoken hot but mosfets are cold.

With the 12v battery ON and power supply ON the induction heater works. The 5 watt resistor was smokin hot in 30 seconds and by then the metal piece in the coil was starting to turn red hot. Mosfets are cold. Bridge rectifier is cold too.

That 1 ohm resistor was a great idea.

I tried the .75 ohm resistor. The metal part heated up about 25% faster and got slightly hotter a dull red. The 15 watt resistor was starting to smoke after 45 seconds. Mosfets are not ice cold like they were it is 50 degrees in the shop they have warmed up to about 75 degrees. Bridge rectifier is not hot either.

Should I risk trying my voltage divider and do away with the 12v battery?

I thought the choke or LC circuit would be current limiting?

What is the best way to test run frequency? Can I attach meter aligator clips to the induction coil?

This part could be some of what is happening, you should isolate the Mosfet; might be something about the arcing which causes erratic behavior. Since you can control that with an isolator do so and see what happens.

However, not sure about the rest. Mr, Al and others will need to lead on the part of the circuit beyond my scope. I had many problems all of these folks helped me with and the circuit did work in the end. It was my "Plasma Speaker" and I thought it was not going to work and did in the end. I don't think it was the best of designs but after these guys helped it did work; it didn't surpass my expectation and fried my power supply after a six hrs test.

Short run times may have been ok.


Good luck,

kv

 

[gary350]

I checked the current on the AC side of the bridge rectifier, 4.3 amps x 10.8 volts AC = 46.44 watts with the .75 ohm current limiting resistor between the transformer and bridge rectifier.

One place online says this induction heater is 100 watts, another place says 120 watts.

Mosfets are 100v 35a.

I need a smaller current limiting resistor to give me 9.26 or 11.1 amps. Math is not working out???

Probably close to .3 ohms resistor?