Hello. For ZVS induction heater I need a 35V 10 or 15 amps power supply. At home I have 223 VAC and 50hz.

My idea is to remove the seocondary of a microwave transformer and rewound it with a thicker wire. What voltage should I get in that secondary to get the 35V after rectifying and filtering? I´ll use a fuuuuuull bridge rectifyer anf after a feew capacitors in parallel to filter. That is my second question, the value of the voltage and capacitance. I have thought in 63V (they are very common and not very exopensive) but no clue about the value of the capacitance. Any suggestion?

Thank you

Best regards

 

The RMS voltage would be about 35V/1.414 = 24.8V RMS.
The amount of capacitance depends entirely upon how much ripple you want to have.
https://instrumentationtools.com/ripple-voltage-rectifiers/

 

Thabk you

Oh, I thought that the capacitance it has to be not only with the voltage but also with the current.

Ant if I get 35 VDC the value of the voltage has to be smaller? I thought it has to be bigger

 

Capacitors do not pass DC current only AC current. In the case of polarized capacitors, you have to be careful you don't reverse the polarity on them. If you do the magic smoke will exit the package, usually with some level of violence. I have seen jets of steam rise 20 feet into the rafters.

 

Yes yes, I´ll use electrolytic capacitors and after the full bridge rectifier

 

If the TXMR has a magnetic shunt, you should remove/punch that out, preferably.
You need to wind on a small over-wind of ~10 turns to determine the turns/volt of the new secondary.

 

If the TXMR has a magnetic shunt, you should remove/punch that out, preferably.
You need to wind on a small over-wind of ~10 turns to determine the turns/volt of the new secondary.

Thank you. yes I did something similar to make a spot welder. Very thick wire ant only 1,5 turns. But I´m not sure about the value of the capacitors for 35VDC and 10-15 amps

 

Thank you. yes I did something similar to make a spot welder. Very thick wire ant only 1,5 turns. But I´m not sure about the value of the capacitors for 35VDC and 10-15 amps

Hello!
It's an easy calculation:
Q = C * dU = I * dt
dt is about 10ms (full bridge rectifier)
And you have to take care the ripple current specification of the selected capacitance, for this current you may have to use some parallel capacitance to provide enough ripple current.
Gabor

 

A practical formula to calculate filter capacitor value if bridge is used and mains frequency is 50KHz:

C= ( I*0.01 )/dU

Where I means load current in amps, dU means ripple voltage in volts. General purpose electrolytic capacitors do not tolerate high ripple, so usually dU = 0.1 * U , or less.

Required transformer voltage is about 30VAC at no load.

Voltage drop at diodes is about 2V. 30-2=28V. Due to capacitor voltage rises up to the peak voltage 28*1.41= 39.5V. Voltage drop due to winding resistance is about 10% , so 39.5/1.1= 35.9V

 

Induction heater systems are rather tolerant of ripple, it mostly affects the actual power. But there is a lot of current into and out of the capacitor, so physically larger capacitors make sense.

 

For an DIY induction heater it is wise to use metalized Polypropylene caps.
e.g. WIMA MKP4 etc.

 

Thank you!!

 

I should have added that the ref to MKP4 caps was for the oscillator tank components.

 

Thank you. Yes yes I have read that these capacitors are the best for a heater

 

I was commenting only about the DC power supply capacitors, not about the oscillator circuit.

 

Thank you. And what special with wima MKP capacitors? I have KEMET MKP X2 SH. These ones are not suitsable for the oscillator?

 

As long as they are the same composition, should be OK.

 

Thank you!