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Why does this DC transformer have three output wires?

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ihaveanewname

Electrical
Feb 6, 2007
6
Hi!

I'm making electronics for the film industry, and I'm working on making cables to power camera equipment.

I've made a lot of cables, all of which I'm terminating with 4 pin XLR jacks. If you aren't familiar with those, no need to look them up, because they aren't the problem.

The problem is, I'm on my last cable that I need to modify, and it's an AC/DC adapter from the 70s. It converts 120v to 20v to run the camera. I want to be able to run the camera off of the 12v battery pack, so I got a 12v to 20v transformer for the job.

I need to connect this transformer to the camera, and this can only be done by splicing into the proprietary cable coming out of the camera. It's got a red, black, and white wire in this cable, and it's supposed to be DC.

I don't want to ruin the camera, so I don't want to experiment to see what works.

Anyone know??

thanks!

Josh
 
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unfortunately, I haven't got much equipment with me currently...I guess I was just hoping someone had heard of this three wire DC configuration.

I did find this post:
It says:

Positive of 3-wire d.c. RED
Middle wire of 3-wire d.c. BLACK
Negative of 3-wire d.c. BLUE

This doesn't help so much because I have red, black, and white. Also, middle wire isn't much a of a description.

Let me know what you think.

thanks!

Josh
 
If it is a battery charging arrangement then one of the leads could be a temperature element. That's not uncommon on rapid chargers. They usually use a thermistor bead in the battery pack to prevent overcharging - one leg of the thermistor is tied to a battery pole and the other is on the white wire.


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Sometimes I only open my mouth to swap feet...
 
No, it's just an AC power cord, like a wall wart...you know? A transformer which attaches to a dummy battery on the camera, in place of a real battery, so one can run the battery off of AC power.

I'm converting the same cord to have the ability to run off of a battery pack.

thanks,
josh
 
There are a lot of odd possibilities, you need to measure to be sure.

One wire might be a ground wire, but it should be green.

If a dummy battery must be inserted in order to run from AC, there might be some kind of detection scheme to assure that there isn't a real battery installed.

If the internal battery consists of a number of cells or a battery with three terminals, more than one DC voltage level might be involved.
 
Please keep in mind this thing is from the 70's.

I'm sure the camera treats the dummy battery the same as it would a real battery, it just ensures it's the proper voltage.

The internal batteries total up to 20v, which is what the DV transformers supply....

i dono....

josh
 
In the 70s I think there were a lot of analog circuits using operational amplifers that required both plus DC and negative DC with respect to common. Your series string of batteries may have a connection between two batteries in the middle of the string providing neg 10V, pos 10V and a common in between.
 
I'm not familiar with that...

So the question is...

If this common cord is involved, and I am converting it to splice in with a modern two wire DC transformer...what do I do with the common cable?

Will it work without it?

I also need to find out which is which (red, white, and black)

thanks!

Josh
 
As stated previously, you REALLY should find yourself a DC-capable multimeter ($12.99 at Walmart, but please find a better one) and measure the actual voltages involved here. Anything we tell you will be based on conjecture.
 
If +10 & -10 are connected, that is probably what is needed to make it work. If the old adapter still works, you need to plug it in and measure the voltages between the wires.

You may be able to tell by carefully looking at the battery holders where the three wires are connected.

I wouldn't connect anything new without measuring to see what you have. If you need +10V and -10V, you will need two external 12V batteries and two 12V to 10V converters.
 
If anyone is curious I figured it out...

I opened up the dummy battery and I was able to see the backside of the connections. Turns out I should have been posting in a camera forum instead of an electronics forum because the extra cable (the white one) seems to be a sync cable (it goes to another small jack on the dummy battery so the sound recorder can tap in and keep the audio in sync with the video).

thanks for all your input!

-Josh
 
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