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

[mvs sarma]

Multiplying AC V and AC A gives VA but not watts, as the whole device is inductive, the power factor could be low as 0.5 or less.
the wattage would be (V*A)/PF in your case might become double or more.

 

[kubeek]

When you get the inductor right you should not need the resistor anymore. I suggest you find out the specification of the core you are using and then calculate the saturation current and check the inductance dependency with frequency.

 

[gary350]

It is 40 degrees in the shop this morning. I notice my .75 ohm resistor is reading 1 ohm. I have two 1 ohm resistors connected in parallel they are reading .8 ohms. As I hold the resistor in my hand resistance is going down as it warms up.

I have the .75 ohm 15 watt resistor between the transformer and the bridge rectifier.

I have the amp meter connected on the DC side between the choke and bridge rectifier.

Voltage is 15.3 VDC

Current is 3.7 amps with a 3/16" diameter metal rod inside the inductor coil = 56.6 watts.

Current is 4.2 amps with a 3/8" diameter rod inside the inductor coil = 64.3 watts.



I changed the current limiting resistor to .5 ohms.

Current is 3.8 amps going up to 4.6 amps as the resistor gets hot using the 3/16" rod = 70 watts.

Current is 4.9 amps going up to 5.5 amps as the resistors get hot using the 3/8" rod = 84 watts.



I changed the current limiting resistor, 3 resistors in parallel = .333 ohms.

Current is 4.5 amps going up to 4.8 amps as the resistor gets hot using the 3/16" rod = 73 watts.

Current is 5.7 amps going up to 6.3 amps as the resistors get hot using the 3/8" rod = 96 watts.



Now i have 4 resistors 1 ohm each in parallel = 20 watts resistors heat up slower. = .25 ohms.

Current is 4.8 amps going up to 5.7 amps as the resistor gets hot using the 3/16" rod = 87 watts.

Current is 6.3 amps going up to 7. amps as the resistors get hot using the 3/8" rod = 107 watts.

Mosfets are about 80 degrees F. Current limiting resistors are about 150 degrees.



Now I have all 5 power resistors in parallel, 1 ohm, 1 ohm, 1 ohm, 1 ohm, .75 ohms = .187 ohms.

Current starts at 5 amps goes up to 6 amps, 3/16" rod turns RED hot current drops to 4 amps.

Current starts at 6.6 amps up to 7.7 amps. 3/8" rod is not very hot. Mosfets are hot 130 degrees. 118 watts.


This is not a very good accurate graph but in the ball park. A good accutate test should be a straight line. This circuit needs a safety built into it in several places. It would be good to have a 0 to .2 ohm Thermistor on each mosfet connected between pin 3 and the power supply, resistance needs to start going up when mosfet temperature reaches about 140 degrees F.

I made an interesting discovery.
I changed the 240 ohm resistor to 370 ohms the mosfets heat up very fast.
I changed the resistors to 220 ohms mosfets heat up much slower, current finally reached 7.7 amps and the 3/8" rod is getting hot enough to smoke, probably about 450 degrees F. I think all I am doing is buying time for the 3/8" rod to heat up before the mosfets heat up. I wish I had a scope. Heat sinks are ok below 100 watts but too small above 100 watts.

FREQUENCY.
With the 3/16" diameter rod in the induction coil frequency is 186KHz.
With the 3/8" diameter rod in the induction coil frequency is 124KHz.
If I touch the mosfet or the heat sink frequency drops to 50KHz.
If I put my finger close to the LC coil frequency goes low then zero meter reading.
With no rod in the induction coil I get no frequency reading on the meter. I think max freq on my meter is 200KHz.

I found an 7812 voltage regulator in my stock pile of used TV parts. 12v is working perfect on the gates.

**broken link removed**

**broken link removed**

 

[gary350]

I removed the .25 ohm current limiting resistor and to my surprise the run current of this induction heater went DOWN not UP.

With no current limiting resistor the induction heater runs at 2.7 amps = 41 watts. This is crazy, WHY does the circuit run at 118 watts with the current limiting resistor with the same metal part in the induction coil?

I added 3 more turns to my transformer voltage is up to about 19 volts now the current has dropped to 1.8 amps = 34 watts.

The gate resistor is still 220 ohms I changed it to 200 ohms current dropped again. I changed resistor back to 240 ohms current is up. I removed the 3 turns of wire from the transformer voltage is 15.3 and current is up to 4.7 amps. Current will not go higher than 4.7 amps.

I changed the gate resistor to 300 ohms current is up to 7 amps but the metal part in the induction coil does not heat up any faster than it did when the circuit was running at 4.7 amps with NO current limiting resistor. All this does is make the mosfets heat up too faster.

With NO current limiting resistor this circuit runs best with 15 volts with a 240 ohm gate resistor the metal part heats up to a very dull red in just over 1 minute and max current is 4.7 amps until the metal part turns red then current drops to 3.8 amps and the metal part never gets any hotter.

With a .250 ohm current limiting resistor at the bridge rectifier on the AC side and a 220 ohm resistor on the gate current is up to 7.7 amps and the metal part is red hot is 30 seconds.

With a .250 ohm current limiting resistor at the bridge rectifier on the AC side and a 240 ohm resistor on the gate current is up to 7.81 amps and the metal part is red hot is 25 seconds.

I tested some D size alkaline batteries my meter shows they put out 8 amps each. 10 batteries in series is 15 vdc and 12vdc for the gate between battery 8 and 9. The 7812 voltage regulator is not needed with the D batteries. I left in the .250 current limiting resistor just because the circuit runs best that way for some reason and this induction heater works perfect running on 10 D size batteries.

I am not finished experementing yet. I am going to try Litz wire on the induction coil.

**broken link removed**

 

[ronv]

If the current in the power supply is that high you might want a bigger filter cap than the 3300 Ufd. Try 2 and see if it helps.