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hacking old flashguns

Hey all.

I have an old cobra 700AF flashgun that I'm going to be using off camera via a PC-hotshoe cord.
Currently the gun runs on batteries - is there any reason not to make some dummy batteries and hook it up to a regulated 5V supply ? i figure 4*1.2V rechargeables = 5V near enough !
Any idea of the current draws on one of these things ?
Is there any way it could blow the cameras internal circuitry, is the PC hookup a relay or a diode ?
I'm using a canon 10D.
Any comments would be cool !

Thu Feb 24 14:25:06 PST 2005

Your idea is ok.

Is there any way it could blow the cameras internal circuitry, is the PC hookup a relay or a diode ?

Very good question, and yes it may!

You have to be very careful when using a flash that is not made for the EOS. The EOS flash circuit can only handle a trigger voltage of 6 volts. Even though the battery only adds up to 5 volts, the flash itself may amplify the voltage to several hundred volts. One example is the old Vivitar 283: Vivitar 283 flash trigger voltage. Use a digital multimeter to measure the trigger voltage of your flash.

Chieh Cheng
Thu Feb 24 23:24:52 PST 2005

It was an EOD dedicated flash - I now want to use it aways from the camera - so it *should* be ok.
I'm still curious as to the current drain on the re-charging part of the flash cycle ?

Fri Feb 25 02:43:43 PST 2005

Based on "Sam's Strobe FAQ", the electronic flash gets charged up to 300 volts. And if you know your batteries supply 5 volts and if you measure the amount of time it takes to charge up your flash, couldn't you calculate the current drain?

I am a bit rusty with electronics equations and don't have my reference books with me, but maybe you can let us know how that is calculated.

Chieh Cheng
Fri Feb 25 11:16:09 PST 2005

Ok, so I just finished making the dummy battery pack to put into the flashgun - probably easier ways to do it, but that was kinda fun !
Wired it up to the PSU on the 5V 20A line, just to make sure it had enough juice :D
Seems to take about 5-6 seconds cycle time, and the current draw is about 2-3 Amps tops.

Sat Feb 26 08:52:41 PST 2005

Excellent! Do you have a picture of your dummy battery pack? Just curious what material you used and how you made it.

Chieh Cheng
Sat Feb 26 20:19:48 PST 2005

No photos as of yet - camera is in recharge mode.
I had some nylon rod knocking around in the workshop - i think it was 20mm diameter, so i turned it down on a lathe to the same size as an AA battery (I think its 12mm?)
I drilled a hole in one end in the centre of the rod, then tapped it to accept a bolt. I then worked out where the end of the bolt would be and drilled another hole ar right-angles to the first. By pushing some wire into this one, the bolt trapped it in the rod, and the bolt head was the contact. I made two of these and glued them together with hot-glue. I also had two more short off-cuts which i was going to glue to the first two to make a proper pack, but as it turned out, these werent needed.
I then cut a small sheet of metal to the correct size for a battery cover replacement, and the job was done !
For reference, you can buy 12mm nylon rod for about �6-7 for 3m from farnell components.
Will post photos at a later date if anyones interested...

Sun Feb 27 05:55:58 PST 2005


Just got a rather nasty electric shock off the case of the PSU - not sure why.
It may have just been the PSU - will do some checks to find out.

Sun Feb 27 12:12:15 PST 2005

If memory serves - i got a bit of a shock off that box before - was only little though - i thought i just touched a daft bit of a switch.
Suffice to say that PSU has been consigned to the bin !
I metered a new PSU output at the flash end, and it remained steady at 5V, so I think the concept is still sound.

Sun Feb 27 12:28:44 PST 2005

That shock sounds nasty. You think there is a short or the case is not grounded properly? Does it have three prongs?

Chieh Cheng
Mon Feb 28 12:38:41 PST 2005

I dont know what the deal was with the PSU - the three pronged IEC was wired correctly, no dodgy leads or anything, and the case was earthed to the correct pin.
Like i said though - i got a little shock off it before, about 2 years ago, but for some reason didnt chuck it then !

Mon Feb 28 14:24:19 PST 2005

G'day Charlie and other interested folks, why not use a 6 volt motorbike battery and use a slave to save any probs with your 10d ? Just a thought !!!! Barry Shrewsbury Uk

Wed Mar 23 05:00:38 PST 2005

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