Mosfets and zener diodes? Answered
Backstory: I'm in the middle of modifying an ac welder to dc and plan on doing my first ibble using it as the topic. Sorry for writing a book here, I didn't want to be too vague and just confuse everyone.
I have most of the parts for the DC power supply, I'll be using a bridge rectifier, and LC filter consisting of 4x22000uF 63v caps, and a self wrapped torroid inductor. It'll also have a bleeder resistor if some undetermined resistance.
I'll be adding the rectifier to the secondary output, and adding the caps in parallel with the gun and work leads, and adding the inductor to the work lead.
It occurred to me today that by making this mod, the gun tip will be hot for some time until the caps are drained by the resistor. It's not the end of the world, but I'd like to see if I can figure out a way to mitigate it with your help.
In its current state, the trigger on the gun closes a circuit which contains a relay. That relay closes the circuit on the primary coil of the main transformer, and starts producing the stepped down ac voltage for welding..
I want to do 3 things:
1. Initiate a short across the relay supply to the relay drain and closing the transformer circuit until the caps reach a certain voltage. (Nice to have but not critical)
2. Insert a switch between the caps and the gun, it should be closed when the relay is closed (must have)
3. If I can get both of those working, I can have the bleeder resistor on an open circuit while the welder is on, and close it when is turned off to start bleeding the caps.
I think all three of these can be done with some combination of resistors and transistors.
For the cold tip, can I use a mosfet? FDP030N06-ND looks pretty close. The ac supply is 38v 60A(low) or 19v 120a(high), I expect the dc to be close. The caps I have are 63v though. Will the extra few volts burn out the transistor since it's only rated for 60v?
I know nothing about zener diodes, but was reading that they should be used to protect the mosfet from back voltage caused by induction in the circuit. How do I size it?
For charging the caps, I believe I can tap the main power to a transistor which classes a circuit bridging the -/+ of the caps, with a resistor to only allow voltage to pass once it reaches a certain potential. If there is enough voltage it would trigger the gate on a transistor and close the short across the transformer relay. Would this work, am I over simplify ing it?
Last, to drain the caps when power is off, I think I can use a transistor to be open while the main power is on, and close when the power is shut off, letting the bleed resistor is doing is job. Do they work like this? What type would it be that is normally closed, and opened when the gate is powered?
I'll see if I can create a circuit diagram and attach tomorrow, but please bear with me, I'm a total noob to electronics, it may be pretty bad.