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Low Voltage on Mosfets

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gary350

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Is the +15 vdc in the original circuit suppose to be, 15 vdc before the Load is in the LC coil or is it 15 vdc after the Load is in the LC coil?

My transformer was 10.8 vac. After the bridge rectifier and capacitor bank voltage is 19.1 vdc with the induction circuit OFF. With the induction circuit ON but NO load in the induction coil voltage was 15.3 vdc. With a 1/4" steel rod in the induction coil voltage was 11 vdc on the circuit.

I added 3 more turns to my transformer voltage is now 13.6 vac. After the bridge rectifier and capacitor bank voltage is 21.1 vdc with the induction circuit OFF.

With the induction circuit ON but no load in the induction coil voltage is 17.8 vdc before the choke, 17.62 vdc after the choke. With a 1/4" steel rod in the induction coil voltage drops to 13.5 vdc. As the metal part heats up voltage drops to 13.18 vdc, then as the parts turns red hot voltage increases to 14.3 vdc.

With 1 choke removed but the original choke still in the circuit, induction circuit ON but no load in the induction coil voltage is 17.8 vdc. With a 1/4" steel rod in the induction coil voltage drops to 14.2vdc. As the metal part heats up voltage drops to 13.3 vdc, then as the parts turns red hot voltage increases to 14.5 vdc.

A 1/4" steel rod is Red Hot in 17 seconds now.

A 3/8" rod is Red Hot in 19 seconds.

Current is now over 20 amps with the small 1/4" steel rod load.

Should I add more turns to the transformer secondary coil to get 15vdc on the mosfets with a large load?

Mosfet STP55NF06L = 60v 55a .015 ohms.

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**broken link removed**
 
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My transformer was 10.8 vac. After the bridge rectifier and capacitor bank voltage is 19.1 vdc with the induction circuit OFF. With the induction circuit ON but NO load in the induction coil voltage was 15.3 vdc. With a 1/4" steel rod in the induction coil voltage was 11 vdc on the circuit.

The only reason I can see for the voltage drop under load is the power supply inability to deliver the current it needs to deliver when under the load. Have you measured the current through any of this?

How hot did the steel rod get?

Ron
 
The only reason I can see for the voltage drop under load is the power supply inability to deliver the current it needs to deliver when under the load. Have you measured the current through any of this?

How hot did the steel rod get?

Ron

I have measured current and voltage on every part of this circuit. The circuit only pulls 5 amps with no load in the LC coil. With a small load current is about 24 amps and the larger load about 30 amps. Transformer is 1050 watts. Secondary coil has 65 amp wire. Secondary coil is 13.6 volts 77.2 amps AC. I have 5 bridge rectifiers rated 1000v 50a each in parallel and 17 filter capacitors = 20,000. uf. When the circuit goes from 5 amps to 24 amps or 30 amps voltage drops.
 
I don't think placing bridge rectifiers in parallel is a good idea as the load will not be shared equally even if they share the same heatsink, I am also not sure how the voltage drops will be shared. Hopefully someone with more on that subject will come along. The only reason I can see for the voltage to fold over is the supply can't deliver the current it needs to deliver. This circuit doesn't seem to be acting like the one in the article it was derived from. Really strange.

Ron
 
The voltage drops because the capacitor after the rectifier can't hold its voltage with high current.
The maximum drop during half of the 60Hz cycle is little less than:
dU=I×t/C ~ 20×8^10-3/20^10-3 ~ 8V
You are measuring the average or rms of this which is about 4V drop.
You can raise the secondary voltage or put more capacitance after the rectifier.
Higher capacitance means higher peak current in the rectifier, which might be too much for the rectifier you are using.
 
The voltage drops because the capacitor after the rectifier can't hold its voltage with high current.
The maximum drop during half of the 60Hz cycle is little less than:
dU=I×t/C ~ 20×8^10-3/20^10-3 ~ 8V
You are measuring the average or rms of this which is about 4V drop.
You can raise the secondary voltage or put more capacitance after the rectifier.
Higher capacitance means higher peak current in the rectifier, which might be too much for the rectifier you are using.

The 1000v 50a bridge rectifiers I bought SUCK. The originally circuit had 1 rectifier it went up in smoke at 12 amps with a heat sink. I replaced it and it went up in flames at 15 amps with a larger better heat sink. I put 2 bridge rectifiers in parallel they both went off like firecrackers above 20 amps with a large heat sink. Now I have 5 bridge rectifiers in parallel on a very large heat sink the temperature never goes above 70 degrees F, I am hoping they are good for 60 amps. So far so good. I need to buy replacement rectifiers before these go up in smoke.

I added another 10 capacitors for a total of 4700 mf. The 1/4" steel rod heated red hot 2 seconds faster. I am very surprised it got almost 100 degrees hotter. Voltage increased .3 volts. Very interesting. I am on the correct path. Thanks
 
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Be careful you don't get the voltage to high because the gate to source voltage of the FETs are only good for 20 volts.
 
Be careful you don't get the voltage to high because the gate to source voltage of the FETs are only good for 20 volts.

I have a 7812 voltage regulator on the gates. Gates are +12 voltage all the time. The voltage difference between the power supply and 12v will make the regulater get hotter the higher as the PS voltage goes up. So far the 7812 heat sink does not seem to be getting warm.

It is time to add another 4700 uf to the power supply and see what happens.

How is the total watts of this circuit determined? Is it amps times 21.1v PS voltage or amps times 17.8v circuit voltage or amps times 14v Load circuit voltage?

I might be able to determine watts with an amp clamp on the transformer secondary.
 
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Another caution:
The AC voltage at the FETs is higher than the input voltage by several times because of the resonant circuit. If my simulation is anywhere close to correct you are pretty close to 60 volts now.
The simulation shows about 350 watts in the rod at 22 amps.
I've got to hand it to you Gary, you have squeezed a lot of blood out of this turnip. :woot:
 
My meter shows the RF voltage in the circuit is 112 volts. Last time I checked RF voltage it was 70 volts about a month ago. I am surprised the 60 volts mosfet are still doing good. I wonder is 112 volts is an accurate reading with a digital volt meter?

I just did this test. I have 7 low resistance power resistors in parallel for a total resistance of .1 ohm. I get a reading of 3.3 volts across the .1 ohm resistor with the 1/4" steel rod as a load in the LC coil and I get a reading of 3.9 volts across the .1 ohm resistor with the 3/8" steel rod as a load inside the LC coil. With no load in the LC coil I get a voltage of .892 volts across the .1 ohm resistor. Remove the .1 ohm resistor and replace it with my amp meter I get 5.8 amps with no load in the LC coil.

Voltage reading across the .1 ohm resistor. 3.3 volts divided by .1 ohm = 33 amps.

Voltage reading across the .1 ohm resistor. 3.9v/.1 = 39 amps.

Voltage reading across the .1 ohm resistor. .892v/.1 = 8.92 amps

Amp reading with meter in the circuit .1 ohm resistor removed, 5.8 amp meter reading with no load.

There is a difference of 3.12 amps with no load between the 2 readings of 5.8a and 8.9a? Humm.......I am guessing my .1 ohm resistor is not really .1 ohm, meter is not very accurate this low, the resistor must be closer to about .13 ohms.

If I substract 3 amps from the amps readings I get, 30 amps and 36 amps.

Using the lowest and highest reading that puts the circuit in the 395 to 522 watt range.

I am just trying to get is the ball park with a current estimate.
 
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The Multimeter will be inaccurate for reading RF current due to ESL in wiring. Maximum power will be transferred when the switched RF source impedance matches the load impedance with core steel losses and coil in parallel/
 
Gary, if you want a 50 Amp 50 mV actual .001 Ohm shunt let me know and I will send you one. I don't want anything in return. I have a pile of them I will never use in my lifetime so if you want one just let me know. We went over this shunt stuff and why a shunt should be very low resistance in another thread.

Ron
 
When I was monitoring 10kA on a diffusion welder with a 8" diameter solid copper pipe, I just tapped into the arm with 2 screws to measure a 50mV drop and calibrated that with a high current PSU using the adjustable current limit. THen brought the two leads out in twisted pair at right angles to the current flow into a shielded braided cable and used a balanced differential amp to monitor current. In this case, a 75mV drop to a hot-carrier peak diode detector or Schottky diode cap can be used to monitor peak current in mV and calibrated to RMS with calculations or scope.
 
My meter shows the RF voltage in the circuit is 112 volts. Last time I checked RF voltage it was 70 volts about a month ago. I am surprised the 60 volts mosfet are still doing good. I wonder is 112 volts is an accurate reading with a digital volt meter?


If I substract 3 amps from the amps readings I get, 30 amps and 36 amps.

Using the lowest and highest reading that puts the circuit in the 395 to 522 watt range.

Your induction heater is giving you about 75% efficiency and that is very good. Yes, you are on the right track.
To get the max power out of your supply you have to reduce the ripple to 1.5V or less. For 30A and 1.5V ripple you need total capacitance of 300000uF or 300mF it is about 10 times what you have. Because the total ESR of your caps is high you get too big fall in voltage at heavy currents
 
Gary, if you want a 50 Amp 50 mV actual .001 Ohm shunt let me know and I will send you one. I don't want anything in return. I have a pile of them I will never use in my lifetime so if you want one just let me know. We went over this shunt stuff and why a shunt should be very low resistance in another thread.

Ron

Yes I would love to have that meter and shunt. Thanks. I dont know how to do PM to give you my address. Can you PM me?

I am in the process of ordering a capacitor. I was looking of a 470000 mfd 200 volt capacitor. It is a 4" diameter 6" tall blue color thing. I think this capacitor is over kill.

Which is best 1 large capacitor of 30 smaller capacitors in parallel?
 
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Number of capacitors isn't important. Specs of caps specify the max ripple current, make sure the total currents of your caps is over 30A.
Most designs use more than one cap, they use caps that are widely available to reduce the cost.
Caps with higher voltage have higher ESR so you are better of with caps of 25v or 35V.
I recommend to look at the cost of the caps. A better power supply will give you extra 10-20% power. If you need more than that then you may have to increase the voltage.
 
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Check your forum inbox. :)

Ron
 
I rounded up all my electrolitic capacitors, tested them 1 by 1 then soldered enough of them in parallel to = 34,410 uf.

Power supply voltage with no load in the LC coil is 17.76v. With a load in the LC coil voltage drops to 14.6v at the lowers point then climbs to 16.03 volts.

The metal part is 1400 degrees in 15 seconds.

The most amazing thing is, the Mosfets don't get Hot like they use to. Mosfets use to get 50 degrees F hotter in 45 seconds, now they only get 7 degrees F hotter in 45 seconds and I am pulling 3 times more current than before. Maximum current use to be 8.8 amps now I am close to 35 amps but I am using different mosfets with much lower resistance.

It will be interesting to see what happens when I replace the LC coil with copper tubing that will reduce skin effect 5 times.
 
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