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Pistnbroke
16th January 2015, 09:44 AM
http://e-group.uk.net/forum/<a href=http://e-group.uk.net/gallery/showphoto.php/photo/80254 target=_blank>[img]http://e-group.uk.net/gallery/data/500/flash66.jpghttp://<a href=http://e-group.uk.net/gallery/showphoto.php/photo/80254 target=_blank>[img]http://e-group.uk.net/gallery/data/500/flash66.jpghttp://e-group.uk.net/gallery/showphoto.php/photo/80254]http://e-group.uk.net/gallery/data/500/flash66.jpgI have spent a lot of time on this and its no problem to do with only 3 components a resistor a capacitor and an SCR for about 3
But in the end its not worth the candle as a Meikie Mk300 is about 30 and does everything ...
The problem with a conversion is where do you pick up the power for your components (basically to charge the capacitor in your adaptor) .
Now if you dont open up the flash gun and use the foot to pick up power the chances are you will disturb the firing circuit for the gun. An internet circuit by "craig" does it this way using low leakage components but the small print say may be unreliable /not work with some guns.WEIN can of course source and construct a circuit in China for peanuts but that unit is as much as a MK 300
If you use an external battery say a G23 or PP3 no problem but do you want a battery /switch?
Using the flash internal 4 xAA seems the answer but there are at least 3 variations of flash gun trigger circuit which becomes 6 as the unit can be +ve or -ve earth ( to use vehicle term).The 4xAA are also usually isolated from the trigger circuit . So without the exact internal diagram for the flash you are modifying you are stuck.
Not bothering to post any diagrams etc as this is dead end unless you have a loved Metz CT 1 etc and if thats the case get in touch.
Put a photo of the parts needed in my gallery ..it called flash66

Ralph Harwood
16th January 2015, 10:31 AM
Hi there Pistnbroke!

Why not use the 400V from the hotshoe as the source, using a zener diode and a dropping resistor? All the components would then fit in the hotshoe foot and the current required to trigger the SCR would be miniscule so the dropping resistor needn't be very big either.

I have to agree though that the MK300 is a fantastic price - I really ought to get round to ordering a couple.

Cheers,

Ralph.

Pistnbroke
16th January 2015, 10:41 AM
Does not work or is unreliabe ..what you forget is that the internal resistor for the gun trigger circuit is anything up to 5 meg ohms and any tiny load causes the circuit to mal function. You need to charge a 25uf capacitor to about 6-9v to trigger the SCR. Even another 5 meg ohm + a 6v zener upsets it and the charge time is way too long.
Like I said you need very low leakage components.
The WEIN unit would have and SCR to do the triggering and a capacitor to fire the SCR BUT the unit would take the charge current for the cap and once charged disconnect from the flash gun imposing no load. Now you cannot do this without suitable microcircuitry..like I said peanuts in china but not for construction at the component level.

Ralph Harwood
16th January 2015, 11:12 AM
Hi there Pistnbroke!

Do you need such a large capacitor? My back of the envelope calculations say that your 25uF capacitor could supply 0.2 mA (gate current for a C106D Thyristor) for over half a second - surely it would only need to supply it for a maximum of 1/60th of a second - that's the slowest sync time I have come across. I haven't any bits at home to play with so this is all theoretical, but if your capacitor is say a 2.2uf to give a bit of surplus, charging the capacitor over 1 second would need 13uA from the flash - I would have thought that this should be ok without causing the flash to misfire. The resistor you would need would then be a 27Mohm.

As I say, without the bits in front of me this is all theoretical, but all the parts mentioned could be cheaply bought from Ebay.

Cheers,

Ralph.

Pistnbroke
16th January 2015, 11:38 AM
You are welcome to try it but look here
http://caves.org.uk/flash/isolator/60_21.isolator.pdf
and here
http://caves.org.uk/flash/isolator/61_16.isolator.pdf

Ralph Harwood
16th January 2015, 02:26 PM
Hi there Pistnbroke!

Looking at the first circuit he is using a 22uF capacitor, but seems to be driving the Triac very hard, putting 30mA through the gate. I think if I were to use a smaller capacitor and a lower drive current then it should be possible to use the 400V from the flash. The author of the article has designed his to use with both high and low trigger voltage flashes, so has had to make compromises, whereas if I build mine into the flash then it should be possible to use lower currents and hence a smaller capacitor. The only issue I can foresee is that the higher gate current might be used to increase switching speed, but even syncing at 1/125s doesn't need considerably faster switching than if it were mains being switched. I'm getting tempted to get the bread-board and some components out and having a play now - pity they are all in work!

Thanks for the help,

Cheers,

Ralph.

Pistnbroke
16th January 2015, 02:33 PM
http://e-group.uk.net/forum/<a href=http://e-group.uk.net/gallery/showphoto.php/photo/80256 target=_blank>http://e-group.uk.net/gallery/data/500/HIL_0008.JPGhttp://e-group.uk.net/gallery/data/500/HIL_0008.JPG (http://e-group.uk.net/gallery/showphoto.php/photo/80256)I dont think the sync speed has anything to do with it ..once the SCR has fired it will shut down when the current thrugh it drops ..under 1/1000 sec.

I think the trick is to put your 270mohm resitor across the flash gun contacts and then push the test button ..if it fires you should be ok ..dont forget the zener across the cap

Ralph Harwood
16th January 2015, 02:53 PM
http://e-group.uk.net/forum/<a href=http://e-group.uk.net/gallery/showphoto.php/photo/80256 target=_blank>http://e-group.uk.net/gallery/data/500/HIL_0008.JPGhttp://e-group.uk.net/gallery/data/500/HIL_0008.JPG (http://e-group.uk.net/gallery/showphoto.php/photo/80256)I dont think the sync speed has anything to do with it ..once the SCR has fired it will shut down when the current thrugh it drops ..under 1/1000 sec.

I think the trick is to put your 270mohm resitor across the flash gun contacts and then push the test button ..if it fires you should be ok ..dont forget the zener across the cap

Remembering not to be holding both sides of the resistor at the same time :D.

My thoughts with the sync speed were just related to how fast the SCR will react with the lowest quoted gate current - if I can dig out the datasheet there should be a graph on it with switching speed versus gate current (It's probably been 20 years since I last looked at it:rolleyes:).

Cheers,

Ralph.

PS I've just succumbed to an MK300 so I will see if I have the bits in work - if I have to order them I might not bother!