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Steve Ward's drsstc interrupter. Pulse width control problems.

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Arcstarter

Wed Jul 15 2009, 07:02AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Hello all, i recently decided to construct a drsstc. I decided upon Steve Ward's interrupter on the solid state page, under drsstc's:

I like it because of the burst mode and pulse width control. I could not get the darn burst mode circuit to work (it seems it is always that particular schematic that gives me a hard time) so i made my own, on a .75 by 1.25 inch PC board. It works well, though instead of a 50k pot (which i have ATM) i would prefer a 40k resistor in series with a 10k pot, i will change that.

Well, the pulse width control is not working. My freq meter claims the duty cycle is 99.8%, but i disagree. It is quieter than the output of the BPS control using a speaker, so i am thinking it is lower duty cycle than the BPS control. The output of the BPS control 555 is around 60-70% and variable from 100-700 hertz, which i am fairly satisfied with at this moment. I might use a 100k or so pot for lower BPS if i ever feel the need.

If i adjust the pulse width controllers pot, nothing changes. The duty cycle says the same thing, it sounds the same, and nothing heats more or anything. I have looked at my circuit, and for the life of me i cannot find an error. It is a very simple schematic, too. I have checked the pot, changed the 555 some 10 times (heh, ya never know, right?). It has a decoupling capacitor, for the record. The diode is a fast diode, just in case. It is rated for 1.5 amps and something like 100-200ns and 600 volts.

Thanks for any replies in advance

Bennem

Wed Jul 15 2009, 08:08AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Hi Arcstarter,

I really think you need to scope the signals,
as the pulse width part is visible by zooming into the scope shot.

Mel

StevenCaton

Wed Jul 15 2009, 05:53PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

You could always just build Dan's Interruptor for DRSSTC's.

Just built the part where it says PW adjust and PRF adjust. You've got some connections to + 9 volts, some connections to a ground, and pin 3 (labeled out) is of course the output. It doesn't get any simpler. This circuit, is of course, specifically built for drsstc's. It gives like 30Hz to 300Hz, and I forgot the pulsewidths, but give it a try.

Someone else on a thread a long time ago said they built Steve's interruptor and they were having problems with it for whatever reason, so you might want to just build Dan's circuit.

Arcstarter

Wed Jul 15 2009, 06:26PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Bennem wrote ...

Hi Arcstarter,

I really think you need to scope the signals,
as the pulse width part is visible by zooming into the scope shot.

Mel

That is what i was thinking, but my duty cycle meter claims 99.8%. Perhaps the duty cycle was too low? The meter is made for 20-80% but i have used down to 9%. I should have a scope soon, so i will scope it when i get the money for a probe.

Also I'd think you would hear the difference in duty cycle.

Something to note is when i turn the pot it sometime will change the sound just a bit. IT sounds like the potentiometer is bad, but i changed it out with another one that works perfectly and it did the same thing, at the same resistance.

teravolt

Wed Jul 15 2009, 09:21PM

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

I built the circuit and it works. I changed the output stage with the fets using a hi side and lo side driver.you have to have a scope to trubleshoot it

Arcstarter

Thu Jul 16 2009, 03:40AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

teravolt wrote ...

I built the circuit and it works. I changed the output stage with the fets using a hi side and lo side driver.you have to have a scope to trubleshoot it

Hrmm, have you ever hooked it up to a speaker? If so, did it sound different when you change the pulse width? Apparently that is my only way to tell at this moment, because my duty cycle meter still insists it is 99.8% :P.

teravolt

Thu Jul 16 2009, 07:33PM

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

oops sorry I was wrong and was refeing to Sreve Wards universal driver witch seems to work. that circuit at the top of yuor page should work with no problem if you have a logic probe you can get to oscillate no problem but you will still need a scope to set the pulse duration

Arcstarter

Thu Jul 23 2009, 02:25AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, i finally received my scope today (Thank you Phillip!) and i scoped the interrupter.

The first picture is the output from the pulse width controller. As you can see, my multimeter was clearly correct about ~99.7% duty cycle. I am not sure why it does this

Note the potentiometer did not change anything.

The second shot is the output of the BPS control oscillator. A nice clean ~50% square wave.

1248315920 1225 FT72947 Interrupter Scope Shot1 001

1248315920 1225 FT72947 Interrupter Scope Shot1 002

StevenCaton

Thu Jul 23 2009, 04:12AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

How funny, I was just about to ask you whether or not you got Steve's interruptor to work.

EDIT: But wait, 50% duty cycle!! What??

hvguy

Thu Jul 23 2009, 06:01AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Second waveform looks great, the first looks like the timer does not have a capacitor connected to it. This would also be collaborated by the fact that the pot does nothing. Check your wiring...

Congrats on the scope, they are sooo useful with these things.

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