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Registered Member #4672
Joined: Tue May 01 2012, 12:18AM
Location: La Habra, California, USA
Posts: 14
I guess it wouldn't be a variac exactly, but is it possible to use a solid state device to perform this function?
I have 3 or 4 light dimmers from an old movie theater. They are rated for a 2.4kw at 120vac. Specifically I was thinking of using it to control my spark gap tesla coil, approx 900 VA.
Here's a couple pictures. What is the big round thing on the inside of the cabinet? Capacitor? The smaller one looks like a big power transistor of some type. Both are bolted to a fairly large heat sink. If I had to guess I'd say it uses PWM to adjust power levels and the cap smooths it out at low duty cycles. I'm probably way off.
Just kicking ideas around. Figured I'd ask before dropping some money on a real variac.
Registered Member #4672
Joined: Tue May 01 2012, 12:18AM
Location: La Habra, California, USA
Posts: 14
Pulled it off, it's a little heavy. No markings at all. Most of the chips on the control board have no markings either, Kelmar sands the markings off so you have to buy parts through them I guess. The smaller device says TAG T4013??? the rest is unreadable.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I have had success in the past with SCR copping supplies. I made one a while back to power my two small DRSSTC twins. It worked well for some time and the controller can be as simple as a couple 555's
Here are some photos of my build:
Steve C's website has some good info on this as well.
Be warned though, this type of supply will have a horrid power factor at anything other than full power and even then it's usually not much better than 0.7 or 0.8
Another option would be a PFC boost feeding a sync buck, although much more complex this would give you a near perfect power factor and near perfect regulation.
Registered Member #4672
Joined: Tue May 01 2012, 12:18AM
Location: La Habra, California, USA
Posts: 14
Very interesting, thank you for the direction. At this point I decided to just go with a good old variac. Lots of nice ones on Ebay, some of them reasonably priced. We'll see what I end up with.
The second option you described is very interesting. That might have to be my next project.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The boost PFC sounds impressive, but the DC output is only good for a SSTC. The original post says this is for controlling a spark gap tesla coil.
A big industrial grade dimmer like this would probably work well, although it might appreciate a bit of ballast inductance in series if driving a low leakage transformer like a PT. MOTs and NSTs should have plenty leakage inductance.
The dimmer probably includes all of the refinements that I put into my circuits.
The only issue I can see is that the power control is accompanied by a phase shift that would mess up the setting of a synchronous rotary gap.
Registered Member #4672
Joined: Tue May 01 2012, 12:18AM
Location: La Habra, California, USA
Posts: 14
Well, I might give it a try, worst case it fails and I can use the enclosure and heatsink for some other project. I have several of them collecting dust. I placed a bid on a dual 20A Variac also. There have been many times over the years when a Variac would have been handy.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Steve Conner wrote ...
The boost PFC sounds impressive, but the DC output is only good for a SSTC. The original post says this is for controlling a spark gap tesla coil.
A big industrial grade dimmer like this would probably work well, although it might appreciate a bit of ballast inductance in series if driving a low leakage transformer like a PT. MOTs and NSTs should have plenty leakage inductance.
The dimmer probably includes all of the refinements that I put into my circuits.
The only issue I can see is that the power control is accompanied by a phase shift that would mess up the setting of a synchronous rotary gap.
Oh this is a good point! I missed the part in the original post about it being a SGTC, oh well silly me! In that case you will have to replace the sync buck stage with a true sine wave inverter! Oh the marvels of making stuff complex.... However you wouldn't have the same phase issues with the sync gap as with the chopper and still maintain a kick ass PF.
But it gets better! Just make the output cap on sync buck small say 10uF ish for arguments sake and feed a 60Hz sine wave into that reference of the sync buck! Would be an interesting experiment. Also because the frequency of the sine could be modulated at high speed (within reason) not sure about the iron NST and such but this is perhaps a way to auto modulate a SGTC with AM or FM or both.
Juts ignore me, I'm just letting my brains ramble. for a SGTC a variac is going to be the way to go for sure. Nothing beats it for reliability and ease of use.
Good luck with the coil. Eric
oh a little off topic but, Eric, as another Eric interested in Tesla coils I find it funny that we live within 50 miles of each other. Just a funny twist of fate right there.
Registered Member #4672
Joined: Tue May 01 2012, 12:18AM
Location: La Habra, California, USA
Posts: 14
Eric (Goodchild), whereabouts do you live? Your profile shows Arizona.
Anyways, I tested the dimmer on a number of different loads. It works reasonable well on everything except induction motors at a low output, this is to be expected.
I did not actually hook my NST up to the dimmer, I won an 18 amp GE variac for $20 on ebay ($35 shipping). It's open frame so I will have to build it into some type of box. The light dimmer will go back on the shelf for a different day, although it is handy for controlling the output of my little space heater.
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Solid state "Variac"
In that case you will have to replace the sync buck stage with a true sine wave inverter! Oh the marvels of making stuff complex.... However you wouldn't have the same phase issues with the sync gap as with the chopper and still maintain a kick ass PF.