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Need help with my SSTC

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rice.364

Sun Mar 05 2006, 03:15AM

Registered Member #284 Joined: Sun Mar 05 2006, 02:38AM
Location:
Posts: 7

Hello all, i'm new here and new to solid state tesla coils. So i got all my parts, made the 2ndary and primary, gate drive ect. but when i hook it up nothing happens. I checked all connections to make sure clean, polarity checked, fuses. I'm out of options on what i done wrong or what component could have failed.
The coil is 21in w/ 22g magnet wire (approx 800 turns), approx 10in alum duct torid with steel electrode. mosfets are irfp450 15a 500v mounted on a heatsink and a over head fan. I dont have a exact schematic but it was a kit someone put together so it should all work.
One place the the problem migh tbe is the primary its 1/4in soft copper tube (10 ft) with only 4 turns. I plan on making this bigger (more turns) once the stores open tomorrow.
Is there an easier way of testing it to see where the problem might be? I've tested for continuity and its all good where it should be.
Any help would greatly be appreciated as this is part of a final (senior in college) for a tech show. The shows next week so time is against me. If you would like me to post any pictures just ask. Also i have some HV transformers (NST) 9k 30mA if you think my remaining time would be better put to use on one since they are more simple as far as problems. I just didnt want to give up on this one as i've invested alot of time into it. Or if anyone has a extra driver laying around for the same size coil forsale that would also be an option since i get an I (basically a F) in the class unless its complete and classes cost over 800$ here at ohio state univ plus what i've already spent.
So any options guys, i'll be checking this post at least every hour inbetween trying diffrent things.

Things i've checked so far:
Gate driver
Reversing the primaries wires
Continuity
Soldering connections
Power (although the board does nothing, the fan works) (running a 12v 300mA to the board)

EEYORE

Sun Mar 05 2006, 06:32AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

What kind of driver are you using(self resonant, PLL ect.) In a sstc, many things can go wrong. How long do you have before its due?Id try to stick with solid state rather than a NST, as its just "neater" especially for electrical engineers

Pics would be GREAT!We can see what kind of "kit" this is...
check out steve wards microsstc, its great for beginners, and very easy to get going.

Matt

vasil

Sun Mar 05 2006, 06:34PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

You didnt say what is more important....the driver type..
Soft copper tube turns on primary? Is it flat? Maibe you have to increase a bit the coupling. Use at least 10-12 turns wrapped together at the base of the secondary.

EDY19

Sun Mar 05 2006, 11:11PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

1/4 inch copper tubing is also a bit overkill for a SSTC with mosfets only rated for 15A. Mine are rated 40, and i use 12ga wire, which only gets slightly warm. Also, tight coupling is important, most people wind the primary almost right over the secondary. Also, have you checked the gate drive with an oscilloscope?

rice.364

Sun Mar 05 2006, 11:45PM

Registered Member #284 Joined: Sun Mar 05 2006, 02:38AM
Location:
Posts: 7

Quick Question, is there a simple way to test the mosfets to see if they work?
Pictures are on the way as well, i took the primary off as i think it could of been the problem.
Also the driver is a self resonant

CT2

Mon Mar 06 2006, 01:39AM

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

If you have a multi meter with diode check on it then its easy to check if a mosfet is blown.
Heres what you do:

1.Set the meter to diode check and connect the meter's negative to the mosfets source
2.Touch meter positive to gate
3.Move the meter's positive to the drain and the meter should give a low reading

EDY19

Mon Mar 06 2006, 01:42AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I would actually use the resistance, since techinically the MOSFET doesn't have a constant voltage drop like an IGBT, but either should work fine.

teravolt

Mon Mar 06 2006, 01:58AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Do you have access to a volt meter and have you checked the basics like regulator voltages main dc to fets and power, and ic voltages. Are there any shorts? If there are any ocilators in your setup you may need a basic O-scope or logic probe. A basic block diagram or schematic would shed light on your setup. Also if you have a variac that might be handy when bringing up the main ac voltage on your bridge.

ragnar

Mon Mar 06 2006, 02:10AM

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

Testing mosfets is easy: grab your multimeter, put the *positive* probe on the source, and the *negative* probe on the drain.

You should read a few megohms. Breath on the MOSFET, the reading should go down a bit. Blow on the MOSFET to cool it again, the reading should go up.

In my local climate, my IRFP450s measure 9M-ohm from source-drain, and my lil ST4.4A 500V low-gate-charge FETs read about 13M-ohm from source to drain.

If you get anything <4Meg, it almost guarantees the FET is dead (or at least, with the FETs I work with...)

There should be 'out of range' resistance from gate to source in any direction ever. (at least with the FETs I work with). If you've got conduction from gate to source, something isn't right, I've found.

rice.364

Mon Mar 06 2006, 03:42AM

Registered Member #284 Joined: Sun Mar 05 2006, 02:38AM
Location:
Posts: 7

I got a picture with this post, its crap quality but closer then 5 feet and my camera is crap, mile away crystal clear 5mp. Anyways, dont have a oscilloscope but i'm sure i can go to the electronic shop and pick one up tomorrow. The shows thurs and setup is weds. I'm not quit sure i follow with the multimeter.heres my steps, put the multi meter to the diode test (2kohm, or at least its the one with the symbol it tell me to put it at) ,neg(black) to mosfets source and pos to gate then drain.
_ _ _
|===| <say this is the mosfet laying flat with the 3 prongs on the top first is source, 2nd is gate, and 3rd is drain

i get a read out of .010, this sound about right?

also about the picture the gat driver is on the bottom, i just labeled the bottom for the gtd and hooked it up this way since i couldnt get anything in the top, i made sure to make sure it was exactly the same if on top. This shouldnt matter should it?

btw the variacs are on a box with the on off, the wires are only speaker wire <-- would this be a cause? i think i'm going to just put them directly on the board

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