another powersupply question

Hi all,
I am working on a project .i want to build a power supply using some 3.3kva transformers i have kicking around. these are single phase, 240 in 110v out. and i need adjustable voltage and constant current controls.
i have posted this on other forums and got the usual "are you mad " and "you're gonna die" type responses so time to find somewhere that will actually give some helpful advice , hence me finding here..
I have a basic understanding of electronics, but there are a few things i have never had dealings with , one is transformers, the other is mosfets and triacs. so some help regarding these would be appreciated. all of the linear circuits i have found basically just become heaters when high current is concerned.
so can mosfets be used like this ??


I've also been reading up on pwm controls, again and require some advice on them as well, but 1 question at a time.
thanks

If it were DC it could work, but not for AC, and for high current & voltage it would just become an unmanageable heat source as you say - since it's linear.

thanks, yes its dc control, my idea was if this worked, possibly run several mosfets in parallel with cooling. This is where my lack of understanding comes in, i was under the assumption that when switched on, mosfets have very low resistance thus dont soak as much heat as bipolar transistors. and 60A mosfets are relativly cheap

When switched hard on yes, but that's not what happens when you dim down that circuit. To cool 3kW's worth you'd have to resort to water cooling.
PWM could be the way to go. It's a simple form of switchmode. The gate drive circuit would have to be quite different then.
MOSFET's and Bipolar transistors will heat up just as much given the same (linear) working conditions - as will IGBT's.
They simply have each their own area of "expertise" (voltage/ current/ speed - wise) where in a switchmode supply they will perform better or worse.

excellent, thanks,
can you recommend any links for ideas ? i have been googling this for days with little success with regards to cc-cv adjstable control on pwm. data sheets are a little confusing for us amateurs and most of the schematics ive found are for custom wound transformers and/or the dedicted ic's used are obsolete

At that power level and voltage it's no small task, no matter what technology you use (except maybe thyristor control, that's easy).
So is the 3.3kVA transformer delivering 110V AC, rectified 110V DC, or rectified and smoothed 155V DC?
What is the intended use, and how much power do you want out of it (and at what voltages)?

I have looked into triac control of the primary, again controlled via opto-diac with pwm. but again theres very little info out there. all references ive found with "dimmer" type circuits require the switching to follow the input sine wave, not from an external signal source
As to power requirements, realistically 6-30v more most applications with as much current as possible, 10-110v on the odd occasion with lower currents required.
Its basically a power supply for electroplating, my wife and my self are keen amateurs bordering semi professional when the need arises . most things are done in the lower voltage range but require the high current. my wife would like to experiment with titanium colouring which requires the higher voltages.
Smoothing of the output can be as crude and unfiltered as you like.
I didnt expect this to be easy, but didnt expect it to be this difficult either.
so maybe a re-think. I have 6 of these transformers, so lets just concentrate on the 6-30v spectum. the higher voltage will be simple with just a variac type of control. cc/cv isnt required at that end of the scale so i would make more sense to make 2 separate supplies.

Ok. Now, 3.3kVA at 110V is 30A. Smoothed to 153V you would get 21.5A max. Are those currents in the range you'll need?
I had believed chrome plating to be a quite current density sensitive process and would not produce good results if fed by an unsmoothed DC source. Isn't that so?
You could have gone linear if the transformers had a 24VAC (=32VDC) output, but with 153V DC you'll have to burn off kiloWatts of power.
I'm not quite sure which regulating tecnology would be the easiest to apply though still being sufficiently good for the purpose. I'll think about it.

well ive had constructive day as my previous post says, i'm leaving the high voltage alone for a while, as i need to get on with the lower voltage idea as this all seems to be more complicated than i expected. So i have taken apart a xmfr , which was no mean feat as full on potting compound , ready to rewind the secondary with 4awg . so now i should easily have 100a to work from at 24v . this should make life easier.
I'm not into chrome, but anodic , nickel and electropolishing actually work better with a ripple. But even with 100a, electropolishing only allows for 72 square inches of surface area !!!! it requires minimum 200A per sq Foot of surface area

Wow, that's big job too, but it'll be a lot easier to work with electrically - giving good results I'm sure.
Make several identical and separate windings that you can connect in series or parallel as required.