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        Help! power supply at 15v..need to knock it down

#1 finishman2000 Sep 16, 2007 07:23 PM

power supply at 15v..need to knock it down
 
I just won a pretty nice and large power supply on ebay. it's got a transformer and a large cylinder thing in it with a resister across the pos and neg posts on the output and one on the top of the cylinder. I need to knock the voltage down a bit because some of the chargers don't like the high voltage.
Any help on how to do this?
Thanks, Cas

#2 vintage1 Sep 17, 2007 07:42 AM

A little more detail would help..most chargers will handle 14v as that is sort of 'full on charge car battery' type voltage.

With resepct, 'cylinder thing' is topographically descriptive, but hardly informative electronics wise. Most electronics components are 'cylinder things' ;)...

#3 kit Sep 17, 2007 08:39 AM

Put a few high amp diodes in series with one of the leads. Each diode has a voltage drop of about 0.6 volts.

#4 finishman2000 Sep 17, 2007 02:08 PM

2 Attachment(s)
ok i guess i had that coming.. :D
The resister on top the round can is yellow, purple, brown and gold.
There is something in the out at the back posts that goes acrss the pos and neg that says 35V 220mF ce 02 w

Some of my chargers don't mind the 15V but the multiplex, triton1 and triton 2 don't like it as well as the poly4 charger. If I have the AF109 running then the others will work. I'm assuming the slight voltage lag will allow the others to be happy.
Thanks, Cas

#5 MarkusN Sep 17, 2007 02:57 PM

That's a simple transformer, rectified (you see the rectifier bridge at the bottom of the unit), then smoothed out some with a large capacitor. Easiest way to get this down in voltage some is indeed to connect some diodes with sufficient current carrying capacity in series.

#6 finishman2000 Sep 17, 2007 03:12 PM

any part numbers and a sorce? I'm assuming Radio Shack won't have them.
Should they go on the poss lead right before the output bolt? And in series meaning across the neg and pos (i thought this would be par not series). You don't mean to cut the pos wire and put them in series do you?
Thanks

#7 dougmc Sep 17, 2007 03:29 PM

Quote:

Originally Posted by finishman2000
any part numbers and a sorce? I'm assuming Radio Shack won't have them.

Actually, they might.

It doesn't matter where they go, exactly, except that they go in the output, and they do go in series. Make sure you have the polarity correct.

Actually, this bridge rectifier might do it nicely -- your input would be DC rather than AC, but it would still work, and I would expect a voltage drop of about 1.2 volts. This page shows how a bride rectifier is wired. The polarity going into the device doesn't matter, and it says it's good to 25 amps.

If you only need six amps, Two of these in series ought to do it. Or maybe just one, if the voltage is 14.0 volts. But two if it's 15.0 volts.

#8 finishman2000 Sep 17, 2007 04:54 PM

there is already one in the ps. are you saying add another one?

#9 dougmc Sep 17, 2007 05:15 PM

Quote:

Originally Posted by finishman2000
there is already one in the ps. are you saying add another one?

Yes. Each diode = 0.6 volts drop in output voltage, most of the time. A bridge rectifier has four diodes in it, but if you put a DC current into it it all goes through only two diodes, so that would give you a 1.2 volt drop. (If you then reversed the current into the bridge rectifier, then the current would all go through the other two diodes, but the output would be the same.)

#10 finishman2000 Sep 18, 2007 11:58 AM

thanks for all the info. Dougmc...you the man. I did indeed find the part at radio shack, soldered it in and now a very nice 14.1v. ALL chargers happy now.
Thanks again, Cas

#11 Dylwad Sep 18, 2007 07:43 PM

Thats a nice, beefy Unregulated power supply you got there, Perfect for CNC. The thing between the posts on the big capacitor is a bleeder resistor, it slowly takes the charge out of the capacitor after the power supply is unplugged or shut off, Usually to prevent shocking yourself, or welding a metal tool between the capacitor terminals while working on or around it after it has been unplugged.

The most efficient way of dropping the voltage would be to take a turn or 2 of wire off the secondary coil of the transformer, nearly impossible unless you wound it yourself.

Capacitors in Microwave ovens have been known to kill people months after the thing was unplugged, but they are storing many thousands of times more voltage than yours.

http://www.geckodrive.com/photos/Step_motor_basics.pdf start on page 11.

Dylan

#12 finishman2000 Sep 18, 2007 08:17 PM

yea a charger stays on for awhile after turning the ps off. I got the unit for $30 shipped so I thought it was a great deal and should last a long time. Sometimes simple is better and lasts longer.

#13 Pete P Sep 18, 2007 09:12 PM

I think 12v is enough to really injure, if it has enough current.

9v to open skin (Like poking your thumbs with multimeter probes, enough to pierce the skin) can kill.

#14 vintage1 Sep 19, 2007 03:52 AM

Quote:

Originally Posted by Pete P
I think 12v is enough to really injure, if it has enough current.

9v to open skin (Like poking your thumbs with multimeter probes, enough to pierce the skin) can kill.

Fleas possibly. Not humans.

Electricity kills one of two ways. Burning and interfering with muscles.

12V DC isn't enough to interfere with muscles, and even sticking your tongue across a 12v battery pack terminals gives you no more than a nasty sort of taste. Certainly no blisters.

You need around 40-50V AC to start to be dangerous.

#15 MarkusN Sep 19, 2007 05:01 AM

Quote:

Originally Posted by vintage1
You need around 40-50V AC to start to be dangerous.

Not so sure about that. The way Pete describes it, with a good connection of the power source to body fluids, I can imagine a few volts to drive enough current through the body to become lethal via exciation of muscles; it takes only a few milliamps to cause cramping.

But that's the very worst case sceanario, of course; you would need two electrodes causing the current to flow over your heart to be solidly connected to the body. I can imagine that in a torture scenario, but not as an accident.


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