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Registered Member #2466
Joined: Thu Nov 12 2009, 09:49PM
Location:
Posts: 3
Our high school has a solid state Tesla coil. I am told it is a drsstc modeled after Steve Ward's early drsstc driver designs. I have inherited the job of getting it into working condition. It had a bad solder connection in the remote control, probably from dropping it. I have repaired the bad connection. I know the students enjoy the Tesla coil and I want to get it working again. Some of the students even seem to understand its mysteries :^)
At this point, the coil will spark for 1 or 2 seconds and then it trips the 20amp circuit breaker. The power cable gets warm in the couple of seconds it sparks.
I think the burst rate adjustment is working as I can hear changes in the firing, but I don't feel the on-time is quite right.
I've posted images at If you need closeups of specific components, I would be happy to take more photos. I'm hoping to identify and replace bad components, but at this point I only have a multimeter.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
It definitely sound like the coil is running with a much too long on time (ie a very high duty cycle, which is what is causing it to draw excessive amounts of current).
That said, to avoid blowing out the poor IGBT's, do not run the coil until you have remedied the issue with the controller.
I am not sure if you have an oscilloscope available, if you do simply scope across the output from the interrupter and see if the duty cycle of the output changes when you adjust the pulse length knob. If it doesn't, the pot is probably damaged, and you should check/replace it.
If the output does vary, but has a duty cycle more than a few percent investigate the timing capacitor on the interrupter board (the 2x 100nF yellow capacitors) and the IC (I can't read it, but suspect it is a 556).
After you have the output working as a nice low duty cycle pulse train, then go ahead and fire the coil up again to make sure it is working. But don't keep relying on the breaker to shut it down!
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
... wrote ...
It definitely sound like the coil is running with a much too long on time (ie a very high duty cycle, which is what is causing it to draw excessive amounts of current).
That said, to avoid blowing out the poor IGBT's, do not run the coil until you have remedied the issue with the controller.
I am not sure if you have an oscilloscope available, if you do simply scope across the output from the interrupter and see if the duty cycle of the output changes when you adjust the pulse length knob. If it doesn't, the pot is probably damaged, and you should check/replace it.
If the output does vary, but has a duty cycle more than a few percent investigate the timing capacitor on the interrupter board (the 2x 100nF yellow capacitors) and the IC (I can't read it, but suspect it is a 556).
After you have the output working as a nice low duty cycle pulse train, then go ahead and fire the coil up again to make sure it is working. But don't keep relying on the breaker to shut it down!
I would bet that is 100% correct! Bring down the duty cycle, it can also kill components if it is too high! Trust me, i know from experience and replacement part costs. Austin also made a good point about adding a fuse. If you do that, use a class J or class CC fuse in an appropriate holder. They are semi fast acting and will stop current flow in about 1/4 cycle (realistically). That can help prevent damage to the coil and its just plain safe! Good luck, you came to the right place to get help! Steve himself may even chime in to help you!
Registered Member #2466
Joined: Thu Nov 12 2009, 09:49PM
Location:
Posts: 3
Thanks to all. So far, I have replaced the pots and the 556 timer since they were very inexpensive and easy to change. I ordered the capacitors since they were cheap too -- though I did have to spend a bit to get them shipped. I do not have a scope. But I will monitor the output from the remote with a multimeter. Testing it before these changes, I found the on-time pot made no difference. Now it does. The DC milliamps go up and down with the pot.
I will wait until I get a fuse in so I don't burn down the high school. It's an old building. In fact, it was visited by Orville Wright-- in his plane. Mr. Collier, whose wife bequeathed their estate to the school, was quite the futurist. I'm sure he'd want a Tesla coil or two on the premises.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
You would install the fuses inline with the coil power. You could rig up a fuse holder with your power plug on one end (mains) and an outlet to plug your coil into on the other side. (Line side of fuse holder wired to mains, load side wired to coil) Here is a link to a fairly inexpensive holder. Take a look at the class cc single pole. Thats all you would need. Keep in mind that the fuses are a bit pricey, and 100% worth it! I picked this holder as an example for you because it is "finger safe" IE when it is hot there are no exposed surfaces that you can come in contact with. Hope this helps! You could actualy just wire this inline on the coil end (in the base somewhere) with your plug coming off like normal.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
One comment--an ultra fast acting fuse may not be your best choice for a coil like this. Depending on how carefully they designed the inrush limiting (and by the looks of it, there isn't anything at all), when you first flip on the coil the capacitor is going to draw a large amount of current until it gets up to line voltage. Furthermore, after each 'burst' the capacitor is going to draw a fairly large amount of current (certainly more than the ~15a average the coil draws), which when using a slow acting fuse/breaker doesn't cause any issues.
I would recommend either a slow acting fuse or a small ~15a breaker. I have a 15a breaker that I salvaged out of a power strip I use for applications that I suspect may try and draw more juice that is allowed.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Yea, now that i think about it, you may be right with that. Either way, the cc holder is your best bet. You can still get a class cc that has a slower blow curve. Or you could just use a 20 amp fast blow class cc. I actually use a type rk fuse on my minibrute coil. No problems and the rk is an ultra fast acting fuse. 15amp. You can give it a shot and see what works for you. Class CC is actually a fuse type, not necessarily the specs. You can use the 20 amp. I use a 10 amp variac with a 20 amp fuse in it with no problems as well. Just trying to help steer you in the right direction. Don't take it for gospel.
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High school Tesla fans need help
