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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
I believe older transformer/rectifier based welders used something similar. A rheostat controlling the current though the control coil And a welder would be in the power range you're looking at too.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, like I said in your other thread, I've built SCR controlled DC power supplies for my Tesla coils. My biggest was about 5kW at 600V DC, drawing almost 30A from a 240V line.
I built two other smaller ones of about 1-3kW at 300V DC, using TO-220 SCRs. I never had any serious problems or SCR failures. Fuses and breakers always popped before any of the SCRs did.
In my design the voltage was not regulated, but it could be adjusted from zero to full output.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Steve McConner wrote ...
I built two other smaller ones of about 1-3kW at 300V DC, using TO-220 SCRs. I never had any serious problems or SCR failures. Fuses and breakers always popped before any of the SCRs did.
Any good suppliers for SCRs? I found semitronics.com
Can I just use the scr to chop the ac signal to a filter, using the duty on the gate to control the amount of the ac going to a rectifier/capacitor filter?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My big supply used a Semikron 92 amp dual SCR brick, that I bought on Ebay or somewhere (I can't remember.) The coilgunners and railgunners here can tell you where to get SCRs, and there are usually a selection for sale over in the Sale and Trade forum.
Here's the schematic of the driver board
and here's the schematic of the power cabinet etc.
All of my SCR drivers were designed for our 50Hz mains, you will have to change a resistor for 60Hz.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Steve, thanks for the link.
How come I can't use one scr, turning it on/off, to control how much ac gets to the rectifier/filter? Is it that the voltage on the scr input has to go to zero before it turns off?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
IamSmooth wrote ...
Steve, thanks for the link.
How come I can't use one scr, turning it on/off, to control how much ac gets to the rectifier/filter?
IamSmooth wrote ...
Is it that the voltage on the scr input has to go to zero before it turns off?
yes, you can only turn an SCR off when the current goes to zero and the voltage is low enough to allow the recovery of the blocking charictaristic, this is the "simple" and most common mode of operation, however, depending on how good and/or crazy you are there are other modes of operation that are wickedly cool, but they can fail sparks/flames/explosions.
revese triggering is "special". some individuals, have wild circuits that delay the rise of current, via saturable inductor, then conducting so well that the voltage drop of conduction drops below the junction hold up voltage, so when the gate voltage and current drop, the SCR turns off even while 100 amps are flowing through it, but these revese triggerings are difficult at best. ( there can be huge kickbacks out of the gate while your trigging the gate, thus blowing your gate drive to hell.)
i think i have explained that right, but its been 11 years since ive designed/built/used 4kW+ SCR circuits, anyway once i found the modern generations of IGBT's and MOSFETS, i gave up on the SCR's for two reasons first, the ability to turn on, but not off, becomes annoying. Second, there gate drive requirements for high power, high frequency operation becomes a real engineering challenge, and a pain, compared to IGBT/MOSFETS.
also, i dont mean to bash SCR's or triacs, they are great when used wisely, they can be scaled up to large volumes of silicon, easily for high power, and handle harsh electrical enviroments well, i remember one of the first SSTC's i ever saw in the mid-90's used a single large SCR.
SCR:
you can get these SCR power conversion circuits to work but you should: -really understand SCR topology, physics, quadrants and such... -choose a wise layout for power, lo inductane, resistence, short power tracks... -make sure you have a good gate drive circuit, establish the gate V and I quickily, hold it, then snap the gate shut again.
and everything else i forgot, anyway the above will get you started.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Patrick, lay off the peppermint schnapps man :)
wrote ... How come I can't use one scr, turning it on/off, to control how much ac gets to the rectifier/filter?
SCRs only conduct in one direction, so they can't control AC without turning it into DC. They are controllable diodes. (It stands for Silicon Controlled Rectifier.)
In one of my SSTCs, I used an ordinary power supply, and controlled the AC input with two back-to-back SCRs.
You can get a bidirectional equivalent of a SCR, it's called a triac. But two back-to-back SCRs is more robust, at the cost of needing two isolated gate drives. Gate drive of SCRs at 50/60Hz is really easy, nothing like as challenging as driving IGBTs at 100kHz. I buy little pulse transformers off the shelf for a few bucks each.
Anyway, because SCRs are triggerable diodes, what makes the most sense is sinply to replace two of the diodes in your rectifier with SCRs. (The resulting circuit is also called a "controlled rectifier".) A full-wave doubler only has two diodes, so you replace them both with SCRs. You can get a dual SCR brick already in the right configuration.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yeah ok, if hes operating them for simple phase angle control @ 60Hz... thats pretty simple, 10kHz is more challenging. did i not imply triacs were for AC? and that two SCRs back to back is for AC? well now i have.
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Other means of power control
