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magnet18

Thu Mar 24 2011, 04:02AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
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Posts: 624

Hi everyone,
I'm currently attempting to build a plasma speaker using a 555 timer, an IRFS654 MOSFET, a lead acid battery, and a flyback; from what I gather it's not an uncommon procedure.
My problem is that the frequency coming out of the flyback doesn't seem to match the 555's frequency. I scoped the 555 pin 3 as being 25 kHz, which is well beyond audible, but when I attempt to create an arc it makes a nasty tone approximately that of nails on a chalkboard, and the plasma arc is erratic and jumps about. Also, as I change the distance of the spark gap, the frequency changes audibly, growing lower as the distance is increased and higher as the distance is decreased. I have tried this with both 20 and 10 turns on the primary and got the same result, with the 10 turn's providing more voltage, but the same effect.
Does anyone know where I might be going wrong?

I can post a pic/video of my setup if anyone thinks it will help.

Thanks in advance :)

ZakWolf

Thu Mar 24 2011, 06:22AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

i built the 555 timer plasma speaker with very poor results. The sound quality was like you described "like nails on a chalk board" with very quiet music in the background. Your better off making the tl494 driver

or my favorite so far the sg352a driver

good luck ;)

James

Thu Mar 24 2011, 05:44PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

LittleVenture wrote ...

i built the 555 timer plasma speaker with very poor results. The sound quality was like you described "like nails on a chalk board" with very quiet music in the background. Your better off making the tl494 driver

or my favorite so far the sg352a driver

good luck ;)

What's with the 10,000uF lytics? I've never seen capacitors anywhere near that large used with a half bridge like that.

magnet18

Thu Mar 24 2011, 07:53PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

I tried those, couldn't get either of them to work, but I intended to build grenadiers in the future.
@James, I believe they must be so large to ensure that they can supply, absorb the power required to drive the flyback.

James

Thu Mar 24 2011, 08:50PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

I'll have to try the circuit and see what happens with smaller caps. 1-2uF ought to be enough, and generally using polarized lytics in applications like that is unwise.

ScotchTapeLord

Thu Mar 24 2011, 11:23PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

10,000 uF is overkill for the half-bridge midpoint tap, but the types of capacitors he is using probably aren't rated for enough current, so paralleling them will help. But the proper thing to do would be to use higher current capacitors. Also, they need bleeder resistors.

Also not sure how I feel about the polarized capacitor on the *AC* audio input.

magnet18

Fri Mar 25 2011, 01:11AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

ScotchTapeLord wrote ...

Also not sure how I feel about the polarized capacitor on the *AC* audio input.

He's a member of this forum, and I noticed that he had the cap polarized backwards on the pictorial schematic, generally the negative goes to the AC audio while the positive goes to the chip, which is generally positive. I personally used a nonpolar cap.

In any case, I reduced the turns and doubled the voltage on my 555 circuit and it seems to be doing fine, apparently one pulse was too short to build up enough charge to jump the gap.

ZakWolf

Fri Mar 25 2011, 01:27AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

I used the parts he said to use and it works just fine, the 10,000uf caps never ever get warm either.

**The heat sink in the pictures is totally unneeded, i thought there was going to be a lot of heat from the fets. Maybe the heat is so little because i am using an lm7812 instead of an lm7815 which i will replace when it arrives for better output and sound quality.**

James

Fri Mar 25 2011, 05:37PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

I'm sure it would work, I'm just wondering if some small film caps would work as well as the big relatively expensive lytics. I have loads of smaller caps.

Dr. Dark Current

Fri Mar 25 2011, 08:43PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The divider caps value goes up with inverse square of the supply voltage, so if you use 1uF with 320 volts you need to use over 100uF at 24 volts, and film caps of this value are really expensive and large...

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