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SSTC for School

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CT2

Fri Apr 27 2007, 04:52AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

Hey everyone, I've decided to build a SSTC for my physics department at school. I currently go to high school and am in grade 12. Last year I brought in one of my coils as a demonstration and everyone loved it. My physics teacher this semester saw the demo I did last year and wants me to make one for the department (and it also gets me community service hours).

Now I'm not sure exactly how to go about this. The coil has to be very reliable because I'll leaving it there and no one will know how to fix it should anything break. So far I have two drive circuits built; Steve's PLL, and Steve W.'s mini SSTC. I've tested both of them and they both work great, I'm just not sure which one would be the most reliable/idiot proof.

The PLL circuit is fairly simple to use, I'll set it all up so all the user will have to do is adjust the phase, but if (well when) someone changes the phase pot and if the coil is turned on at full power there is a risk of something exploding. Now I could just make it so that no one can touch the phase adjust pot but then it might not be in tune properly...

Steve's mini SSTC circuit is even simpler in that it should always be in tune as long as I have it set up correctly so I'm thinking maybe this is the way to go, but also when turned on at full power something funny (like exploding) may happen.

I'm also thinking I'll add an interupter to the circuit (already built), but while I was testing some funny things happened while twidling the Int. knobs, like like bridge dieing... I'm not sure what has caused this and I definately can't add an interupter unless I have that figured out, so any help would be appreciated.

So I was also thinking I'd add some sort of voltage adjuster so the coil can be started from 0-full power, but this is getting more complex and I don't know if I want to do that. If it's easy then I'll for sure add that feature but I don't know which circuit to use. I've read Steve Conners page for his OLTC power supply and I could possibly build that Link but its alot of parts and I'd prefer something simpler. I also read ST's application notes for control of inductive loads by triac Link , but again I don't know exactly which circuit to use. I was thinking either circuit C on page 11 or the one on page 10... the one on page 11 is much simpler and I would prefer it if possible, also the one on page 10 is for 220V and this coil will be run off 120V so I don't know how the values should change.

Now on to the power section, I have built both a half bridge and a fullbridge, so I can use either. I would think that the full bridge would be more robust so I'll probably go that route, but if anyone thinks different I'd like to know your opinion. Also since it needs to be very reliable I was wondering what you guys think I should add to beef it up (if necessary); such as TVS across FETs, diodes across FETs, I already have zeners across GS.

All of the electronics will be housed in a wooden box with plexy windows to see in, so that people know kinda whats going on inside, the actual secondary and primary will most likely be located on top of the box. With the front of the box containing the power switches and leds and what not. I'm hoping interference won't be a problem (hasn't been in the past) but once again going for reliability so if anyone thinks a wooden box is bad then please let me know. The dimensions for the secondary are yet to be determined, but probably 3-4" x 9-12", if anyone would like to let me know what size they think is best please do so.

And I think thats about all the questions I have at the moment... Thanks in advance to any replies as they'll greatly help and are much appreciated.

ragnar

Fri Apr 27 2007, 05:36AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Public demonstration of coils is a little tricky. My few recommendations for you:

- If your coil has a low enough resonant frequency, go with a topology like one of Steve Ward's feedback circuits. That way users can't twiddle the input and interfere. The fewer controls, the better. For example, you don't want someone turning the pot all the way to one end when the coil is turned off, then forgetting where it was prior to plugging in the coil.

- Add a fuse (not a circuit-breaker) to help minimise collateral damage in the event of a coil failure. If the bridge blows, all you want to fail are the MOSFETs. If you have zener diodes or transzorbs and schottky barrier diodies and ultrafasts and all kinds of other crap in there, then a bridge failure could take this stuff with it. The repair is now an order of magnitude harder. See if you can design your GDT to only run +/-12V, and have the coil always running in tune. Do you know what I mean?

- Use a fullbridge design/layout that allows user servicing of the MOSFETs. That means no soldering!

Use tapped heatsinks and silicone thermal pads. Use those neat screwterminal blocks to fasten the MOSFETs to the board. I say this, because, if someone is drawing groundarcs and (say) shorts the stick to the topload and blows a set of MOSFETs, you don't really need to diagnose that as a reliability problem. Just send the school some more MOSFETs and advise them not to do [whatever they were doing when it asploded] in future.

- Try and make the primary and secondary as solidly mounted as possible. If someone has to "assemble" the coil, and e.g. place the secondary inside the primary, how do you know they won't set it up for a flashover? Bolt everything down.

- A killswitch or dead-man's switch is pretty important. You don't want the OH&S guys to confiscate your coil! Use an IEC socket on the case to connect to power. That way you can use an tagged IEC cord and pretend you had the coil approved. And it also means they can't cut the plug off it.

Tom540

Fri Apr 27 2007, 05:28PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

"- Add a fuse (not a circuit-breaker) to help minimize collateral damage in the event of a coil failure."

Just curious BP, why not a circuit breaker? I'm guessing maybe they'd keep flipping it back on and trying it again causing more damage? One of my coils uses a circuit breaker. Fuses were getting expensive.

Oh yeah you should add an internal interrupter thats already set so they cant change the break rate or pulse width.

-Tom

Steve Ward

Fri Apr 27 2007, 08:39PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Fuses are faster.

Tom540

Fri Apr 27 2007, 10:56PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

That would explain why my igbt's still died. Even though the breaker was set at 1 amp which is right above normal operating current.

Steve Conner

Sat Apr 28 2007, 01:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't think you can rely on either a fuse or a breaker to protect IGBTs or MOSFETs. They do protect SCRs and diodes fine though.

CT2

Mon Apr 30 2007, 01:31AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

Thanks for the replies, I'll go with Steve W's driver for the school project and save the PLL for myself. I'll definately be adding a fuse to the system, and I'll go with the full bridge. What I still don't know, and was wondering if anyone here could let me know, is what to do with the voltage controller. Should I add one (it would need to be solid state because I don't have a variac to give away)? And if I should what design would you guys suggest? As for the interupter, I don't wan't it to be internal because I wan't them to be able to turn it on and off and adjust it to get all the different effects, should I just change the component values, and if so then what ones would ensure it doesn't blow the bridge?

Thanks.

...

Mon Apr 30 2007, 01:57AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You can get dimmer switches that can run a standard 60hz transformer, they are sold as dimmers for 'magnetic ballast' lights, and can be had for up to 600va for a reasonable cost. Might be something to consider....

ragnar

Mon Apr 30 2007, 03:44AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

CT2, I forgot to recommend to you a "soft turn-on".

If you use a soft-on circuit, you shouldn't have problems with stuff blasting after you turn it on mid-cycle. Don't forget to minimise the number of components that can fail in your fullbridge. You don't want to have to "send the school" new zeners, diodes AND mosfets.

Matt

CT2

Mon Apr 30 2007, 04:56AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

Thanks for the dimmer switch idea "...", I didn't even know they had those. BlackPlasma when you say soft start would you mean something like Link ? I'll definately add that on, I remember I was going to befor with my other coil but never really had a problem starting it at full power, but reliability on this one so it'll be on the coil. Have anyone of you guys ever had the problem I had with the interupter? Or know what values to stay away from?

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