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'[EE] PIC-triggered flash'
2007\10\19@035639 by Jinx

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Can someone please suggest a fix to this problem. In the diagram,
the 220R 0.5W gate resistor keeps showing me how much smoke
it can blow out

I plug this in, wait a couple of seconds, then press a pushbutton,
which makes a 12F675 send a 10us to the opto-triac. The pulse
is OK, checked that on a scope

What will happen is that the flash will go off, then the 220R starts
slowly burning. The power gets turned off, maybe a second later,
before the resistor actually goes open

I've tried a large-value resistor pull-down on the SCR gate, but
that makes no difference. It's presently 2 x 560k (some resistors
in this circuit are series pairs, for the voltage rating)

Tried several values for a pull-down, from 1M to 5M, in the position
shown, but the 220R burns up just the same. I feel like General Haig
sending them over the top, poor little blighters

Must be on the wrong track with a pull-down. Can't see where that
power is coming from. I figured that for all but a very brief time, the
resistor is isolated from any PD across it. Something is obviously not
turning off, and making it dissipate a lot more than it should need to.
The only candidates, AFAICS, are the tube, the SCR or the opto

I don't think anything else is suffering. My logic being that the flash
will go off with a new 220R

Plan A - replace the 220R after every flash, but I'm pretty sure that's
not how the pros do it ;-)

Plan B - ask you guys

TIA



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2007\10\19@065849 by John La Rooy

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Maybe the description here helps
http://sound.westhost.com/project65.htm

John La Rooy

2007\10\19@072930 by Jinx

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> Maybe the description here helps
> http://sound.westhost.com/project65.htm

The differences are

220uF vs 1u8. I thought maybe something that can't hold a
stink-load of energy at this stage - I was wrong ?

180R vs 220R - could try 180R

20us vs 10us - could try reducing pulse time further

2007\10\19@085215 by David VanHorn

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On 10/19/07, Jinx <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:
> Can someone please suggest a fix to this problem. In the diagram,
> the 220R 0.5W gate resistor keeps showing me how much smoke
> it can blow out

As drawn, I don't see how you could cook the 220 resistor.
Perhaps there's a mistake in implementation?

2007\10\19@112951 by Morgan Olsson

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Den 2007-10-19 09:54:07 skrev Jinx <.....joecolquittKILLspamspam@spam@clear.net.nz>:

> Can someone please suggest a fix to this problem. In the diagram,
> the 220R 0.5W gate resistor keeps showing me how much smoke
> it can blow out

Check your wiring.

As drawn it seem OK and there is no way a current gen get strong enough to cook the 220R 0.5W resistor; max it can get is what is coming through thoose two 47k resistors.

--
Morgan Olsson

2007\10\19@121357 by Herbert Graf

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On Fri, 2007-10-19 at 20:54 +1300, Jinx wrote:
{Quote hidden}

If what you've got is exactly what's in that schematic I don't see how
it's possible for that resistor to smoke.

So, either you've got a wiring problem, or perhaps your SCR is shot?

TTYL

2007\10\19@133537 by Peter van Hoof

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This will IMHO only happen if the 47k's are not 47k  (shorted or low)
that can keep the current for the MOC 3023 above the hold current and
keep it conducting (the 1.8uf cap will not be discharged enough by the flashbulb to
reduce current below hold current) thus frying the 22r resistor.

Peter



{Original Message removed}

2007\10\19@142045 by Cedric Chang

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Plan A - replace the 220R after every flash, but I'm pretty sure that's
not how the pros do it ;-)

Plan B - ask you guys

Plan C - put resistor in socket.    pull the resistor out as soon as  
you flash.

TIA

2007\10\19@174737 by Jinx

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> Plan C - put resistor in socket.    pull the resistor out as soon as  
> you flash.

Outstanding ! That's not thinking outside the box. That's throwing
the box away and getting a bigger one !!

Cheers for a morning smile Cedric ;-)

2007\10\19@185712 by Dr Skip

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You forgot the software patch approach applied to hardware...

Put another bigger SCR across the resistor and add another input to fire a few
mS after the first... Automate it! ;)

Jinx wrote:
>> Plan C - put resistor in socket.    pull the resistor out as soon as  
>> you flash.
>

2007\10\20@121016 by Brooke Clarke

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Hi Jinx:

Is there a way you can post a photo of the hardware?

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

2007\10\20@183352 by Jinx

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> Is there a way you can post a photo of the hardware?

http://home.clear.net.nz/pages/joecolquitt/picflash.html

Either it's OK or I'm missing something ridiculously obvious

2007\10\20@185430 by Mark Rages

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On 10/20/07, Jinx <joecolquittspamKILLspamclear.net.nz> wrote:
> > Is there a way you can post a photo of the hardware?
>
> http://home.clear.net.nz/pages/joecolquitt/picflash.html
>
> Either it's OK or I'm missing something ridiculously obvious
>

Layout doesn't match the schematic.  220 ohm resistor is connected to
wrong side of 47k's.

--
Mark Rages, Engineer
Midwest Telecine LLC
.....markragesKILLspamspam.....midwesttelecine.com

2007\10\20@190654 by Kelly Kohls

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Jinx,

>From your schematic and board layout it appears that you have the 220
ohm/0.5 watt resistor on the wrong side of the two 47K ohm resistors.

Hopefully someone will confirm my diagnosis.

Kelly Kohls, N5TLE
Bedford, TX



-----Original Message-----
From: EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu [piclist-bouncesspamspam_OUTmit.edu]On Behalf
Of Jinx
Sent: Saturday, October 20, 2007 5:34 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] PIC-triggered flash


> Is there a way you can post a photo of the hardware?

http://home.clear.net.nz/pages/joecolquitt/picflash.html

Either it's OK or I'm missing something ridiculously obvious

2007\10\20@213018 by Jinx

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>220 ohm resistor is connected to wrong side of 47k's

Oh f....or goodness sake, right in front of me all the time. Thanks
to you and Kelly for spotting it. I should be grateful I don't often
make mistakes like that (and especially one that could have been
very noisy, smelly and shrapnelly), but it's still embarassing

I've changed the PCB, but it doesn't flash. Possibly the MOCs are
dead, have to dig out the isolating transformer and check, so I'll
scrounge up some new parts and let you know

Will also put up the proper layout and leave it there for Googlers

2007\10\20@222905 by Russell McMahon

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> I've changed the PCB, but it doesn't flash. Possibly the
> MOCs are
> dead, have to dig out the isolating transformer and check,
> so I'll
> scrounge up some new parts and let you know

I have some zero crossing MOCs which may work - in 2nd to
deepest dungeon - I can probably lay my hands on them.

May save you a day or 3.


       Russell

2007\10\20@224929 by Jinx

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> I have some zero crossing MOCs which may work - in 2nd to
> deepest dungeon - I can probably lay my hands on them.

Thanks, I have at least one 3041 and some 3021s in a similar
place. Will ferret for those first, thanks again

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