Whats wrong with this circuit?

Thread Starter

hrpenley

Joined Oct 22, 2022
15
On another topic, would switching to say a bank of 3 25N120's (25A @1200v) and balancing the cap banks give me enough overhead to do what I am shooting for? Maybe add a good choke coil to limit inrush or am I trying to make a silk purse out of a sow's ear?
 

Ian0

Joined Aug 7, 2020
9,520
On another topic, would switching to say a bank of 3 25N120's (25A @1200v) and balancing the cap banks give me enough overhead to do what I am shooting for? Maybe add a good choke coil to limit inrush or am I trying to make a silk purse out of a sow's ear?
What are you trying to do? I've seen mention of a 3.5kW induction heater, but you don't say what power supply it needs?
 

LowQCab

Joined Nov 6, 2012
3,948
Are You trying to Kill your friend and then claim it was an accident ?

There's no advantage to using DC for this project, or any Capacitors or Semi-Conductors.

You need a Variac, a 2500-VA-Step-Down-Transformer, and a few pieces of #6-Gauge wire,
that's the entire setup.

No matter how You do it, it ain't gonna be cheap.
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BobTPH

Joined Jun 5, 2013
8,677
What are you trying to do? I've seen mention of a 3.5kW induction heater, but you don't say what power supply it needs?
He said 60V 50A. Presumably DC, since that is what he is trying to supply with his linear regulator that wastes 6KW of power.
 

Ian0

Joined Aug 7, 2020
9,520
He said 60V 50A. Presumably DC, since that is what he is trying to supply with his linear regulator that wastes 6KW of power.
really? Who make a power device that runs from 60V DC?
He could use the waste power to heat the steel and do something useful with the induction heater like boil a kettle and make a cup of tea.
 

Thread Starter

hrpenley

Joined Oct 22, 2022
15
Are You trying to Kill your friend and then claim it was an accident ?

There's no advantage to using DC for this project, or any Capacitors or Semi-Conductors.

You need a Variac, a 2500-VA-Step-Down-Transformer, and a few pieces of #6-Gauge wire,
that's the entire setup.

No matter how You do it, it ain't gonna be cheap.
.
.
.
2500 not enough, I had a 3.5kva and it left a bit to be desired at full draw, I had to go to a 7.5kva to get it to run properly, you guys have never heard of an induction heating system? Pretty standard in mill and blacksmith operations and yes it boils water or mash quite well but that's for another thread... I'll throw a standard schematic up so you can take a gander at it. No, I'm not trying to kill my friend just save him from using so much propane and speed things up for him a bit. I can turn a 1" steel bar red hot and workable in under 90 seconds - much faster than the kiln. Design recommends 2x 50 amp ham radio power supply's to run the unit @ 150.00 each about as cheap as I could find them on ebay and no guarantee on what you getting. I have a pretty good collection of parts laying around after 40 years in the industry so its not so much actual cost but what I can do with what I have available without having to put too much money out. I'm just looking for something to power this thing up without using a 300lb xfmr, now it may not be available for a reasonable cost but I am sure as hell going to give it a shot and any suggestions are appreciated. I have seen a lot of crazy designs that do some pretty impressive things from you guys and I am pretty sure someone out there has done it be it mosfets, IGBT or a bank of TIP's, I'm not picky, I have a mill and I can make PC boards or custom heat sinks and it doesn't have to be pretty. Requirements are max input of 60v DC @ 50amps, Minimum input is around 24vdc to make it pop, get much lower and it can blow the mosfets. it’s a tuned tank circuit. operational freq is between 24k - 50khz at the top end - much outside of that and again power draw goes way up and the heat sinks start heating up. Depending on what you put in the coil will change the resonate freq (self-tracking) be it graphite crucible full of copper or steel alloy bar stock for a knife.
 
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MisterBill2

Joined Jan 23, 2018
17,855
I can make the TS a good deal on some burned up Variacs that were used for a similar application.
There is a thing called "safe operating area" which is a specification of voltage and current that describes where a device may be used versus where the device will not survive.
For a power semiconductor it relates to power dissipated as heat at the junction.
What we need to know is how well the 60 volts must be regulated, because a single pass transistor is not the way to go. A step-down transformer is the usual approach, a switching mode power supply will also work. A series linear regulator is a very poor choice, both very expensive to build and horribly inefficient. Probably around 20% EFFICIENCY IN THIS APPLICATION. Maybe much less than that.
 

LowQCab

Joined Nov 6, 2012
3,948
Extremely high Current is the Buga-Boo.
Call-up Hammond-Transformers in Canada and they will wind You a custom
240-Volt Primary-Transformer, with your choice of Multiple-Output-Voltage-Taps.
The Output-Taps will be your Coarse-Voltage-Adjustment,
then do all of the switching on the the primary side,
which will be running at roughly 1/4 of the Current of the Secondary, ( around ~15 to ~20-Amps ),
using a heavy-duty Triac-Dimmer-Circuit.

This will not only be much-much safer, it's also simple and easy.
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Thread Starter

hrpenley

Joined Oct 22, 2022
15
I can make the TS a good deal on some burned up Variacs that were used for a similar application.
There is a thing called "safe operating area" which is a specification of voltage and current that describes where a device may be used versus where the device will not survive.
For a power semiconductor it relates to power dissipated as heat at the junction.
What we need to know is how well the 60 volts must be regulated, because a single pass transistor is not the way to go. A step-down transformer is the usual approach, a switching mode power supply will also work. A series linear regulator is a very poor choice, both very expensive to build and horribly inefficient. Probably around 20% EFFICIENCY IN THIS APPLICATION. Maybe much less than that.
well if I use it to harden a tool I run it from 30-45 volts (depending on what I am hardening) if I am melting copper I run it around 48-50v (more for speed than eff - it will melt copper at a lower voltage setting but I add additional cap bank to adjust for the frequency jump) If I am cooking up a pot of southern comfort I run it around 25 volts (I have a special flat coil I use for that -same thing here I add an additional cap bank to it and I want a slow controlled temp so I also throw a pid on it, pot is stainless steel) On my setup I have a 480-120v xfmr but I put the 120 in on the 480 side (actually I think its the 512v input line) and pull @65v or so - xfmr has a bunch of taps on it both on pri and sec, its a 7.5kva I got because the potting was cracked - I cleaned it and poured a layer of potting compound on it - no issues at all and have been using for over a year. I am using a regulator design I found a while back using 4x 25n120's which with a few mods i did runs fine on an input range of 65 - 120vdc (I have used it to power a number of projects up to 100vdc at power draws well over 50Amps) I have a 1500v 150A bridge on the ac side with a bank of 1000uf 400v caps, choke etc - standard stuff. gives me a real nice clean output - but that's mine and I'm not giving that away friend or not. lol.
 

MisterBill2

Joined Jan 23, 2018
17,855
well if I use it to harden a tool I run it from 30-45 volts (depending on what I am hardening) if I am melting copper I run it around 48-50v (more for speed than eff - it will melt copper at a lower voltage setting but I add additional cap bank to adjust for the frequency jump) If I am cooking up a pot of southern comfort I run it around 25 volts (I have a special flat coil I use for that -same thing here I add an additional cap bank to it and I want a slow controlled temp so I also throw a pid on it, pot is stainless steel) On my setup I have a 480-120v xfmr but I put the 120 in on the 480 side (actually I think its the 512v input line) and pull @65v or so - xfmr has a bunch of taps on it both on pri and sec, its a 7.5kva I got because the potting was cracked - I cleaned it and poured a layer of potting compound on it - no issues at all and have been using for over a year. I am using a regulator design I found a while back using 4x 25n120's which with a few mods i did runs fine on an input range of 65 - 120vdc (I have used it to power a number of projects up to 100vdc at power draws well over 50Amps) I have a 1500v 150A bridge on the ac side with a bank of 1000uf 400v caps, choke etc - standard stuff. gives me a real nice clean output - but that's mine and I'm not giving that away friend or not. lol.
If you have that transformer with all of the taps then it might be possible to come up with a tap-changing scheme to provide course regulation, or a convenient voltage setting scheme if automation would not be worth the effort. Using transformers in reverse of the intended application is one of the things that has served me fairly well in the past. If isolation is not required then there is even more flexibility available, operating as a form of auto-transformer. But it is important to be aware of the current handling ability of each winding segment.
 

Ian0

Joined Aug 7, 2020
9,520
I'm guessing the price tage is a bit more than 250-300 bucks for that baby.
7200W (4x1800W) induction hob is £219.
I know it’s not capable of melting metal, and the best it can manage 200°C for frying chips, but the principle is the same and it runs off 230V AC.
 

Thread Starter

hrpenley

Joined Oct 22, 2022
15
7200W (4x1800W) induction hob is £219.
I know it’s not capable of melting metal, and the best it can manage 200°C for frying chips, but the principle is the same and it runs off 230V AC.
This version will melt everything up to copper maybe a little hotter if I push it but haven't had the need to push something that works. I save that for prototypes,, I am still waiting on another board, that system will put out enough to melt steel, that's my goal - I work at a steel mill lol. after that I will work on something else. I just finished the power setup for a small arc furnace, 2 mot cores I should be able to melt small qtys of anything I want. I want to be able to make alloys. If it lives up to what I read it should be able to reach 10k in a couple cubic inch area. also will allow me to synthesize a few gemstones for laser use. I don't know if this is the right forum but I was looking at building a microwave sputter coating machine (I have all the plans and drawings for a small system. ) don't know if anyone here has tried that one yet, would like to discuss pro's and cons of that, currently In the middle of building a TEA CO2 laser, I have it working in normal atmosphere as Nitrogen base but power output leaves a lot to be desired, my diode lasers do a better job but I am setting it up for co2 so that will be a little better I think. I found the CO2 tank first so unless I happen across a N2 tank, co2 it is. but right now the power supply for this induction system is my hang up. I actually have a great deal of hardware to work with as far as parts- about 20 years ago I bought out the inventory of a NTE distributor so other than what I have used which is not a whole lot I have about 5 of almost every NTE part they made up to that point everything from triacs and scr's to the earlier mosfets and such, lot of ttl, cmos, good deal of mainstream tv repair parts at least for a few years back lol. and a lot of other parts I have acquired over the years. Point is if I can find a good design I can probably build it at minimal cash layout. actual cost is not really an issue - if I already have to main high dollar parts. so for those who think I am in the wind with the igbt, what else you got besides the transformer route?? Most of the switching supply's I have used I really haven't been impressed with and from what I understand (I have not confirmed this only read other reports) a number of issues have been reported with frequency interference from the switching supply - I guess due to the resonate tank design the induction heater uses there can be some disastrous interaction with a switching power supplies due to the operating frequency so i just stayed away from them.
 
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