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Registered Member #1517
Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304
Thanks a bunch guys.
I did notice uneven heating between the mosfets while running the coil, but I blame bad heatsink contacts. I am not actually using the through holes on the fets to clamp them down to the heatsink. One of the fets was sort of angled away from the sink, so it made sense that it was heating up.
If I had to guess, I'd say they were running at fairly even power. And, the one that blew up, was the cooler one, believe it or not!
I will double check my gate resistors. I currently have 1.2 ohm resistors. Blown resistors would mean... they shorted? or they opened? I could just replace them, that would eliminate all doubt.
Yes, a current transformer sounds like an excellent idea, although it makes me wish I had designed it into my board... I guess that's another lesson. Use a prototyping board, then order your nice PCB.
Ok I will replace the resistors, cross my fingers and report back.
Registered Member #1517
Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304
When powering from the wall, drivers first, then bridge.
I believe you should power on your driver circuits first, to check that they are functional. Then once you know that the driver and GDT are working properly, you can apply power to the entire system.
In order to set up oscillations in the coil, you would have to turn on the bridge and driver at nearly the same time because it is the secondary coil that drives the feedback signal.
Another update: Swapped out the gate resistors, no change, circuit is still dead... I will have to wait for new mosfets.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I believe you should power on your driver circuits first, to check that they are functional. Then once you know that the driver and GDT are working properly, you can apply power to the entire system.
Well, you will not now your GDT is working if you turn the driver on with no bridge power. Remember, with the mini sstc driver there are no oscillations at all if the bridge is not on, it runs solely off its own noise, and where there is not bridge voltage there is no noise being produced. but you should ALWAYS power your driver up first. I turned both one at the exact same time using the master switch on my switchbox, and the sparks where a bit smaller, acted weird, and then boom. Not sure why, but only 1 second or so of sparks and then i guess one of the mosfets died. Either that or two.
Unless you use a 555 to generate oscillations to start it up, which might be the source of your problems. You should only have one 555 in the circuit, that is connected to absolutely nothing other than pin 3 of the ucc's.
Only 1mm sparks from a 40 watt bulb does not sound right to me. Maybe you could check your UCC's, one could be malfunctioning and only one is driving the GDT and you are getting half the gate voltage as normal, which could result in only partially on, AKA more resistance. this means more heating, less power throughput. Maybe...
Maybe you should try 10ohm gate resistors, no diode. This would give a bit more deadtime, and would further help control ringing on the gates. Always make sure they resistors are exactly the same, once again.
Do you have a video of operation, and do you have a scope or freq meter? To measure the freq of my coils, i take my radioshack meter and set it around 2-5 feet from the operating coil. Sometimes i ground the negative lead to a hunk of ungrounded metal, sometimes i leave it just sitting there, and i just throw the positive lead anywhere. Picks up the frequency perfectly, as well as duty cycle. If you can check and post waveforms, that would be great, and frequency might be helpful if it is not the correct freq.
What is the AC voltage on the drivers power transformer when loaded (when the coil is running)? If it is lower than something like 15, the driver might not be getting the full 12 volts needed? The voltage regulators drop some voltage even if the supply voltage is the same as the specified output voltage, and then the rectifier diodes drop some voltage.
And oh my, is your GDT dc blocking capacitor a tantalum? That is a no no, they do not like reverse voltages and it could be shorted, if this is the case. That would likely kill a ucc or both. Not only this, it would likely not allow for correct operation. This capacitor should be something like .1uf or so (can be much more, but this is what i used and i had low ucc heating, plenty of drive voltage, and it worked great with like 5-8 inch sparks at 80vac input and at 125 volts incredibly thick 10 inch sparks in CW) and it needs to be film. Other kinds like ceramic might work, but film is perfect for the job. Also, the caps can be found from TV circuit boards, monitors, and even stereo or audio kind of stuff. Almost anything.
Good luck, and if you try a CT you can ask me, but it is very simple and straight-forward after you see a schematic or know the circuit.
EDIT: that is definitely a tant for the GDT dc blocking cap. I would suggest you find a film cap, and if you cannot find one i could even send you one or a few for no cost, i mean they are worth nothing considering i salvage them from junk, and it would cost as much as a postage stamp to ship -.-. Would save the trouble of having to order from companies -.-
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
Well, you will not now your GDT is working if you turn the driver on with no bridge power. Remember, with the mini sstc driver there are no oscillations at all if the bridge is not on, it runs solely off its own noise, and where there is not bridge voltage there is no noise being produced. but you should ALWAYS power your driver up first. I turned both one at the exact same time using the master switch on my switchbox, and the sparks where a bit smaller, acted weird, and then boom. Not sure why, but only 1 second or so of sparks and then i guess one of the mosfets died. Either that or two.
Hi Matthew. The thing that confuses me here is that one can apply power to the bridge with NO power to the control electronics, and the coil will be fine. The MOSFETS just won't draw any power. So if the bridge can be plugged in before the driver, then why can't they just be plugged in at the same time. I really don't know why. Maybe someone could clear this up for us.
I wanted to make my mini sstc have only one extension cord but I am doubting that now, based on your experience of having the thing blow when you plugged them both in at the same time. What about a small relay that triggers power to the bridge just after the driver is plugged in? EDIT: I could be that when you plugged the driver in and the bridge at the same time, the small AC transformer that supplies the electronics just hadn't reached its peak voltage yet, which kept the voltage regulators sagging for a bit, causing screwy waveforms all around. ---still wondering how to power the thing from one extension cord---
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
SteveC wrote ...
Well, you will not now your GDT is working if you turn the driver on with no bridge power. Remember, with the mini sstc driver there are no oscillations at all if the bridge is not on, it runs solely off its own noise, and where there is not bridge voltage there is no noise being produced. but you should ALWAYS power your driver up first. I turned both one at the exact same time using the master switch on my switchbox, and the sparks where a bit smaller, acted weird, and then boom. Not sure why, but only 1 second or so of sparks and then i guess one of the mosfets died. Either that or two.
Hi Matthew. The thing that confuses me here is that one can apply power to the bridge with NO power to the control electronics, and the coil will be fine. The MOSFETS just won't draw any power. So if the bridge can be plugged in before the driver, then why can't they just be plugged in at the same time. I really don't know why. Maybe someone could clear this up for us.
I wanted to make my mini sstc have only one extension cord but I am doubting that now, based on your experience of having the thing blow when you plugged them both in at the same time. What about a small relay that triggers power to the bridge just after the driver is plugged in? EDIT: I could be that when you plugged the driver in and the bridge at the same time, the small AC transformer that supplies the electronics just hadn't reached its peak voltage yet, which kept the voltage regulators sagging for a bit, causing screwy waveforms all around. ---still wondering how to power the thing from one extension cord---
Well, you could use a relay and a simple delay circuit, such as and my favorite .
I might make one, for something like a sstc or vttc. Very simple, part #'s are not critical on the last one, and variable (you could use a pot if you wanted, and you can use different capacitors). Pulls very little current from 12 volts. Hope it helps.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
SteveC wrote ...
EDIT: I could be that when you plugged the driver in and the bridge at the same time, the small AC transformer that supplies the electronics just hadn't reached its peak voltage yet, which kept the voltage regulators sagging for a bit, causing screwy waveforms all around.
Sounds like a reasonable explanation to me. Commercial-grade power electronics tends to have an undervoltage lockout circuit, and if you want to plug everything in at once, you probably need one too. UVLO disables the whole circuit until everything is charged and the voltage regulators are putting out their proper voltages. It also shuts the circuit down gracefulyl when the plug is pulled.
UVLO is tricky to get right. A ghetto alternative is a hand-held "fire button" that disables the driver until pressed. Just don't press it while plugging the thing into the wall.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Or you can put a relay (possibly with some delay network) across your control circuit supply rails, which energizes the power part only after the control circuit voltage has reached its steady state.
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
A ghetto alternative is a hand-held "fire button" that disables the driver until pressed. Just don't press it while plugging the thing into the wall.
That idea crossed through my mind too, but I'd rather do a delay circuit.
This voltage sagging that were talking about can be seen with any spark gap tesla coil too. If you suddenly plug a spark gap coil in, or if you flick the switch on really fast then shut it off, the sparks don't reach full length till half a second or so. When the Neon sign transformers are being loaded down charging the caps, it just takes a bunch of AC cycles before the reach their peak voltage.
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First Light, Full Power? Mini SSTC (Steve Ward)
