Re: potiental divider


John Lonigro
 

Shawn:

Just because it is a 400 Watt transformer doesn't mean you can draw 400 Watts from any of the secondary windings.  Each winding has its own current rating, depending on the gauge of the wire used.  If you can measure the gauge of the various secondary windings, you'll have a better idea what you can do with the transformer.

72,

John AA0VE


On 12/07/2013 04:51 PM, Shawn Reed wrote:
 

On 12/7/2013 2:05 PM, WA0ITP wrote:
 



GA Shawn,  nearly everyone on the list will have better ideas than I do, but heres some food for thought.
 
Does your PS have lower voltages available?  If so use something closer to 20 or 30 volts if you can. With semiconductor projects you'll seldom need more than 20 volts.
 
Barring that:
 
I think the best option is to use a buck-boost regulator and set the output to 30-40 volts, then use your 317.  Or just use the buck/boost as your primary regulator and forget the 317.   The buck boost will have to be capable of handling quite a bit of power so choose it with that in mind.
 
One possibility is to use a series pass transistor (1 or more) to handle the full voltage, while providing more current capacity than the 317.  A regulator or voltage divider would need to be incorporated to supply the 317/337.  This would take somw fiddling to make it work. There are lots of 315 circuits and calculators on the web.
 
Another possibility is to use a resistive divider to make the voltage drop for the 317. They arent very practical however.  It would need to pass about 10 times the current you'd expect to pull from the supply in  order to be stiff enough to work.  I used this approach a long time ago and don't recommend it, except in vy low current applications (ma range).  Even there a 3 termial regulator is as easy and is superior.
 
Another option might be to add a zener in the ground leg of the regulator between the adjusting pot and ground.  The idea would be to prevent the 317 from seeing the full 56 volts.  So the zener would have to be rated at more than 16 volts.  Havent tried this approach so dontknow how effective it would be.
 
In any of these, a heat sink will be needed, e.g dropping 16 volts at 1A = 16 Watts.   Good luck, it's an interesting project.
 
----------------------------------
Back to the bench, Winter's too valuable to waste.
I love this radio stuff !
72   WA0ITP
www.wa0itp.com
www.4sqrp.com
 
 
 
----- Original Message -----
Sent: Saturday, December 07, 2013 2:23 PM
Subject: [4sqrp] potiental divider

Hello 4stategroup.

      I have a LM317adj. TO220 regulator-1.5A, 40v max., I mean to use. --Coming off my  PS transformer I have several voltages, one of them being 56vdc, unused. I would like to drop this to 40v output, and then use a 10-turn pot. to vary the voltage down to the min. 1.5vdc.The 56v has been filtered. What is a preferred method to drop this voltage? I imagine a heat sink may be needed, also?  Thanks for any help. Shawn Reed, kf7yff

    Good to hear from you Terry! Thanks for the different thoughts on an approach. The voltages I do have are; 10, 17.5, neg. 28, and 56vdc. Plus 3.2ac. I have the 17.5vdc going to a 7815 reg., and then to a variable situation. I've never figured if I could also use that voltage towards another, separate  circuit. It's a 400W transformer. I imagine I could, and that would be great! What could I do with the negative 28vdc? Maybe just have a neg. variable voltage on hand. -------cold days up here. 17deg. at the moment. Low down to 5 deg. No ice. Some snow expected in a day or two. I've got every 4-state project to bump back to this season, and take them a step further. Great fun, and learning. Having started at zero, I'm doing 60 now :-) Best regards, Shawn, kf7yff

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