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Mosfets and zener diodes?

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. 
 

Picture of Mosfets and zener diodes?
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iceng1 year ago

The SCR is a SILICON CONTROLLED RECTIFIER- 25A 400V TO220

Mfr. Part#:NTE5554

Allied Stock#:
70215455 $6.62

http://www.alliedelec.com/nte-electronics-inc-nte5...


First) The turn on load with the relay mosfet was wrong,

then we added a fast SCR discharge using R8 10 OHM 25W resistor.

When the relay is closed M2 is on through R11-R12 raising the gate to 5v +, this keeps the SCR off.

When the relay opens to turn the load off then M2 also turns off, because gate voltage goes to zero. Now the capacitor C2 charges up and fires the SCR which starts to discharge C1 down to the holding current or under ten volts. After which the SCR TURNS OFF.

When you reapply relay power M2 prevents the SCR from getting a C2 charge and in fact guarantees discharge of C2 by the 1N4004 diode..

The 40Vz is needed because the 2N7000 mosfet is less the 60V drain-source and the 15Vz is needed to protect the gate-source voltage of M1.

As usual click the pic to see the whole image....

xxx.jpg
iceng iceng1 year ago

An improved schematic with component group definition...

crowbar.jpg
DonaldF9 (author) 1 year ago

I don't think I explained very well. please excuse the awful diagram, I'm not quite sure how to run a simulation yet to see if it's even valid, but this is what I was trying to get at with the mosfet and zener diode.

You guys are right about the other parts, I'm way over thinking it. I'm not going to mess with turning the transformer on or off, likewise, i'm not going to create any elaborate bleed circuit. I've scaled it back to just turning on/off the tip when the transformer is on/off.

I'm sure a relay would work great for this, but to handle up to 120a at up to 65v, it would cost way more than the welder did ($70 new). Not to mention, I'm trying to learn. I'm sure there is a way to switch it with a mosfet which would be a lot cheaper.

in the diagram s1 is the relay switch that is closed when the trigger is closed and powers the transformer. s2 is a smaller 120v normally closed relay that only needs to handle a momentary load of a couple amps to push the gate. That way Vgs=0 and the fet channel is closed. When s1 is closed, s2 opens, and the gate is pulled to ground and the fet channel opens.

Does that make any sense at all? could this work, or am I just going to fry components?

I don't see the point of the output filter on a welder like this, and designing a filter from parts that can handle the currents in a welder is not for the faint hearted. Unless you have carefully specified the saturation current for the core, and used capacitors rated for VERY high discharge currents, something is going to a least over heat, or go bang.

-max-1 year ago

You can leave the bleeder resistor connected the whole time the thing is on, that is not a problem. The bleeder resistor should probably be sized accordingly so that the capacitor bank can discharge to a safe voltage in a few seconds.

However, if you really want, you could make a circuit that detects if the welder is on, and when power is shut off or it is turned off, then the capacitor is shorted. This could be easily done with a relay or solenoid, as long as it has normally open contacts. I would connect the coil portion to the power input, after the switch to turn on or off the welder, and have either the normally open contacts in series with the welding stick, or the normally closed contacts in series with a bleeder resistor across the capacitor. It should go without saying you you need to make sure the contactor or solonoid is big enough to handle the currents.

iceng -max-1 year ago

I would use an SCR with a suitably low and high power (wattage) resistor to discharge the capacitors fast.

Nice thing about SCRs is they reset once the cap is drained and no arcs...

Transistors, mosfets, zener diodes - did I miss something or was the topic just a DC conversion of an AC welder?
Add a switch to turn the system and transformer on and off.
Use the relay output to switch a SSR - the SSR closes the connection to the gun.
If you have proper capacitors all you need is to add a bleeding resistor of a high value parallel to the cap, they should not mind being switched on with the transformer.
Of course you can also use a low power supply and SSR to switch the transformer on when you press the trigger on the gun.
If you use relay here you must make sure that it is overrated for the job, I guess a SSR would come cheaper and last longer.