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Random 555 smoke-down

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LAN_403

Killa-X

Sat Jul 04 2009, 11:11PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

I'm not sure what happened. I finally got my 2X10 inch coil pumping sparks but...the second I put a ton more speed into the RSG (3V to 17V) it sparked loud, then shot dead. As I went to get the lights I noticed a huge puff of 555 smoke.

Now that I vented my room and it has a smell of Cinnamon, I come to ask, Why? Why would a 555 go Pff all because my RSG was a lot faster? I did it because i was getting thin crisp streamers, so I speeded it up, and saw more together ones.
Note:
555 = NE555P. 16V recommended, 18V max.
Power to 555: 17.1V max

Could it be when the RSG was more in-tune, it somehow triggered the 555 to fault at 17V?

* Should I dim it down to 12V instead of 17?
* Is it possible that no RF shielding on a 555 can cause it to die?

Proud Mary

Sat Jul 04 2009, 11:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You can be completely certain that the fault is associated with very particular circuit conditions which it's your job to identify - quite the opposite to 'random' in fact!

Killa-X

Sat Jul 04 2009, 11:35PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

Harry wrote ...

You can be completely certain that the fault is associated with very particular circuit conditions which it's your job to identify - quite the opposite to 'random' in fact!

Ya but it doesn't happen all that often. The last time It happend, the 555 lived, and I was getting frequency from the mosfet. My only guesses is either No rf protection, or too much power for the frequency :(

So.. ya...If it happened a lot..I mean I can run it for 10 mins on 3V, but once I kick it to 16V it tends to be a bit faulty!

Proud Mary

Sat Jul 04 2009, 11:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The big fat input capacitance of Hexfets is such that I don't think a little 555 is really up to switching them very efficiently.

Put in a fast buffer, lad. That will sort it out. In any case, it is never good practice to connect an oscillator directly to a power amplifier without an intermediate buffer. Keep the power supply for the 555 and the power amplifier as separate as you can, with generous decoupling at all nodes.

(Circuit diagrams often give the impression that electricity flows from left to right , and will obligingly follow sign posts directing a signal on to the next stage. How many a man has been amazed to discover that his amplifier also works backwards!

)

aonomus

Sun Jul 05 2009, 12:02AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

A 555 is not meant to drive a big N-enh. mosfet well, Harry is right, if you don't have a gate drive IC handy, you can use a PNP and NPN transistor (eg: BD139/140) to take the load off the 555.

Proud Mary

Sun Jul 05 2009, 12:20AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Some pundits like to add a sharpening diode in the gate circuit, but I don't think this is at all necessary if there be drive vigorous enough to overcome the sluggish humours of the gate for which a buffer be required - a few shillings' worth of parts at most.

Killa-X

Sun Jul 05 2009, 04:22AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

But does it mean its more getting in-tune when this happens? Like I said I can run it for a LONG time on a slower speed, but when I kick it to high gear, I loose a 555.

But no, I'm using a 16V 400ma transformer (from plasma ball) to power it. And I do NOT own a fet driver. Should I try to obtain some UCC37322P or UCC37321P chips to help the 555 out then? I do have a resistor on pin 3, it's rated 39 ohm, but my PCB clad is set for 10ohm.

But ya, I am just direct connecting the fet, no driver :P

LithiumLord

Sun Jul 05 2009, 10:59AM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Well, there are two failure causes to be a point here. First of all, you can get a kickback through the switch, unless you clamp it with TVS diodes, preferably on both low and high sides. Also remember, driving a 555 unregulated is really very, very bad as any mains-driven interference through your transformer may fry it. Use some Vreg for sure. For the future projects btw, be carefull with the datasheet ratings - it's a good point in running ICs on the max recommended voltage as many run as fast as they can in that mode, but getting in between the max recommanded and max critical ratings isn't good at all. As for the driver, it's really a good point to add it there.

Killa-X

Sun Jul 05 2009, 07:36PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Will have to buy some, Sadly all I have is a few UF4007s, a 5V and 10V regulator, and that's it. I can use the 10V regulator to even the power but..

Also, I know i am over the limits using 17V, but I know the transformer is very filtered. When unpluged, the LED dims very slowly, and I have no filter caps on the 555 circuit, just the transformer built-in one. I do own a 16V 150,000UF cap that I can use to help filter if needed. But I suppose I will look into the possible fixes you listed be, Thank you.

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