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

 



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 -----

From: neivahul@...

To: 4sqrp@...

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

 

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 -----

From: neivahul@...

To: 4sqrp@...

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

 

Shawn:

I took a quick glance at your power supply picture.  If you have no idea how much current the various windings can supply, you might examine the diodes in each circuit.  Part numbers would be ideal, as you can look up their specs online.  But physical size will give some indication of how much current they might handle.  Also, take a look at the various PCB traces and you'll see some are obviously wider than others and designed to handle more current.  All those clues might give you an idea of what you can do with the transformer.  I noticed one of the windings is center-tapped.  If that's the 56 volt winding, you could use a full-wave rectifier, instead of a bridge, and perhaps the voltages will be more in line with what you need.

If you plan to use the transformer as a bench supply, you really don't care how big it is physically or how much it weighs.  If you want to power a QRP rig portable with it, then building a little wall-wart power supply like Terry WA0ITP has done would be a lot more convenient and less hassle.

Just some random thoughts.

72,

John AA0VE

Hello John. It may be best I deal with something more lightweight. The transformer, (Tamradio),  came out of an nice stereo receiver, so I thought I could make it apply to a versatile bench PS. I think I got a picture of it in my 4state photos. I see what you mean, tho. The secondary windings are relatively small - appear to be 18-20 awg. The wires coming off the rectified and filtered board are 20 awg. Two fuses say 1A -125v at the ac input, and the other, 3A-125v, towards the dc end. Never able to find any data with prior searches. Just for gp's, I will add a photo of the circuit board bottom in my photos. Thanks for answering. I've studied into a bit, but know little, at this stage. Shawn, kf7yff

 

Shawn, another possible way to "skin this cat":

There is a high voltage version of the LM317, try searching Mouser for "LM317AHVT".  You could build a 2-stage regulator with the high voltage 317 stage bringing the input voltage down into the LM317 closer to where you want to be.  Perhaps a few settings on the HV 317 then a pot on the LM317 for exact adjustment.  This will also help divide up the power handling of the regulators.  The heat dissipated by these linear regulators is a factor of the amperage and the amount of dropped voltage.  As Terry pointed out, you will need a heat sink and you will probably need something substantial.  One thing I have used for these types of things is an old computer processor heat sink with a fan.  I find these pretty cheap at hamfests.  This should be a pretty low parts count circuit and pretty cheap.

de KC0PET, Rick

---

From: "WA0ITP"
To: 4sqrp@...
Sent: Saturday, December 7, 2013 4:05:06 PM
Subject: Re: [4sqrp] potiental divider

 



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 -----

From: neivahul@...

To: 4sqrp@...

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