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LAN_403

Wilson

Sat Jun 03 2006, 09:15AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Without a pull down resistor, the mosfet will act like a thyristor until the voltage on the gate capacitance falls down to ~4V, upon which its in its linear region, and dissipates alot of heat.
Try it yourself, connect a 12V to a mini halogen light, then to the D on the fet, and connect the source to -ve. tap the gate with 12V, and the lamp will stay lit!

Billybobjoe

Sat Jun 03 2006, 10:10PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

So what should I do here?

I tried the test here

and it doesn't seem to be working. I have another MOSFET so i can use that one (only one though so I don't want to blow it).

Are MOSFETs easy to kill soldering?

Steve Conner

Sat Jun 03 2006, 10:32PM

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

I designed the power part of that circuit, and another 4hv member (who's no longer here) added the TL494. I agree that it ought to have the pull-down resistor that another poster recommended. (or maybe it's a pull up, can't remember)

Big power MOSFETs seem pretty tough. I've never managed to kill one either from static discharge or overheating with a soldering iron. Tiny MOSFETs like the VN10LP are a lot easier to kill with static.

Billybobjoe

Tue Jun 27 2006, 06:51PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Ok, I finally got around to putting in the 220 ohm resistor, and I realized how much of an idiot I am. I think I may have had the TL494 in upside down. What should be at the top, the circle or semi cut out?

I switched the circle to the tope (as pin 1 on the right) and current draw went way down but now the TC4429 gets hot? ARGGH.

...

Tue Jun 27 2006, 07:56PM

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

The notch is the top...

Is there any chance you could scope the output of the tl494 to see what is going on?

ragnar

Tue Jun 27 2006, 09:00PM

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

Notch or no notch, I've always found the circle to be at the pin1 end. =P

I've got a bucket of 4049 chips that have text on the top, text underneath, a notch near pin8/9 and the dot near pin1/16.

Haw. =P

Steve Ward

Tue Jun 27 2006, 10:43PM

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

I thought it was standard to have the notch near pin 1 and the highest pin number. The circle is usually near pin 1 for 8DIP's, but almost always near pins 7/8 or 8/9 on 14 and 16 DIPs.

Anyway, Nick, from the pic you posted, pin 1 starts at the lower left corner, and pin 16 is the upper left corner. Also, if you can properly read the text on the top of the chip, the bottom left corner is pin 1.

ragnar

Tue Jun 27 2006, 11:58PM

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

Without meaning to confuse you, I've encountered chips where the text is 'upside down' and there's a notch at the arse (pin8/9) end of a DIP16.

Good luck =)

Billybobjoe

Fri Jun 30 2006, 01:38AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

... - no I don't have a scope but that would be really helpful.

I guess I did have it the right way around in the first place. The datasheet isn't very clear but it seems like it puts the notch at the top. Well, I probably just ruined it now by connecting it the wrong way and the TC4429 was really hot so it probably blew too. I only had one extra but so I'll have to get a few new ones.

Important question: is it necessary for me to be putting audio from a CD player or the like into this for me to get anything out?

...

Fri Jun 30 2006, 02:31AM

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

No, just jumping the input should give you full output... Might need to up the cap in series with it to a 100nf, but you should be ok as is for testing.

I would try this:
-pull out the tl494 and tc4422
-Jump pins 5 and 6 in the tc4422 socket.
-jump pins 7 and 9 in the tl494 socket
-put an ammeter in series with the +12v of the circuit
-plug in the circuit.
*if you short out your supply pull out the transistors. If that solves it replace the shorted one. If it doesnt recheck you wiring.
-pull out the jumper from 5 and 6 in the tl494 and put it across 8/7
*you should see pretty much a dead short.
*if not then check the fets/diodes for being open and then for wiring problems
-Disconnect the primary of the transformer
*the short should disappear. If not check wiring errors...
-reattach the primary
-put the jumper on the tc4422 back to 5/6
-remove the jumper on the tl494 and put it across 8/9
*you should see a short
-remove the primary from the transformer
*the short should disappear.
-move the jumper back to pins 8/7 in the tc4422
-plug it in again
*you should see a dead short
-unplug the primary of the transformer
*there should still be a dead short.
-unplug the circuit
-put a neon bulb on the output of the transformer
-Plug the tc4422 back in.
-Plug the supply in
*it should show up as a short
-move the jumper from the tl494 back to 7/9
*it should still be a short, you should also see the neon bulb flash.

If you got that far the circuit is about 90% working. Just the 494 part is broken. If you just want sparks hook up a 555 running at about 15khz at a 50% duty cycle to pin 9 of the tl494. You should get hv out on the flyback

From there is just the tl494... triple double check you wiring, and try a new chip...

Good luck!

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