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Old 15th April 2013, 12:48 AM   #1
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Default basic power supply questions

i have some cisco 6000 power supplies i am trying to convert into a dual-rail power supplies (chaining them in parallel, already made sure the neg is not connected to chassis ground).. it outputs various levels of potentials, 3.3v 5v, 12v and 42v.. and i got them for practically free and they're 1300w so should be plenty for all sorts of projects

problem is they output a raw 42v and i wanted to create a 1.25 (or 0) to 30v variable output power supply using something like a lm317 or lt3080 or something along these lines.. Well, none of these regulators would really like 42v being fed into them so i need to lower them to around 36v somehow. I was going to initially resort to a few series diodes (i have a few 12a diodes with a Vf max of 1.5v which i thought would be good enough if i used like 4-5 of them on each side), but after looking at the datasheet of the diode closer, i think the Vf varies a bit that at low loads the Vf would not lower the voltage enough to still not exceed the Voltage Regs' max volt input levels..

So question is basically, how do i get 36V from 42V? The 42V outputs about 27amp and i dont need that much @ 36v but say 6amps would be plenty (i can then chain a few lm7815 and lm7905 to get a few fixed levels as well.
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Old 15th April 2013, 03:06 AM   #2
DUG is offline DUG  Canada
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4.5V zener and power darlington(s) and a 499R resistor

Can you figure out the circuit just from the parts list?

Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 15th April 2013, 06:35 AM   #3
RJM1 is offline RJM1  United States
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You could use the LM317HV which has the input/output differential voltage raised to 60V allowing you to regulate from 1.2V to 57V.

Last edited by RJM1; 15th April 2013 at 06:50 AM.
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Old 15th April 2013, 02:01 PM   #4
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i actually considered the HV version but it looks not that widely available (newark has them) and also if the neg version would be the lm337hv, that's pretty much impossible to find :-\

Hmm, zener darlington and a resistor.. interesting! Would that zener have to be rated at 6amps tho (or the related 42 x 6 watts maybe)? Might get expensive quickly that way tho no? Hmm, circuit.. i might be able to figure it out but if you have a quick overview i wouldn't be opposed to a hint
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Old 15th April 2013, 11:18 PM   #5
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Alright did some more research around DUG's proposed arrangement, sorry for the silly question about the zener needing to be high power rating, since seems most of the current would going through the series pass darlington, and the resistor would be used to current limit what went through the zener.. Still one thing that's confusing me though is how did you come up with 4.5v zener, if you're doing 42-4.5 then that makes sense, resulting in 37.5 being something i think the lm317 or lm350 for example could handle, but i'm unclear about that arrangement since the reading i've done to get a better understanding usually has the zener @ the desired output level, e.g. 36v zener in this case.. wouldn't mind using a 4.5v zd instead, just always looking to learn more about how this would work.. thanks for the pointers thus far though!
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Old 16th April 2013, 06:11 AM   #6
RJM1 is offline RJM1  United States
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With what you are doing you do not actually need the LM337HV.
The LM317HV are available at DigiKey see my link in previous post.
Attached Images
File Type: jpg LM317HV.JPG (64.2 KB, 185 views)

Last edited by RJM1; 16th April 2013 at 06:19 AM.
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Old 16th April 2013, 08:03 AM   #7
wwenze is online now wwenze  Singapore
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Assuming you're going to use a pass transistor since LM317's max current is only 1.5A -

Use some resistor divider to make a 36V supply for the LM317. If you can do zener regulated, even better. But not much current will be drawn from this supply. (You'll need to calculate the optimal resistor values.) Output of the LM317 will be regulated anyway.

Output of LM317 feeds the base of the NPN pass transistor (but this concept probably works for configuration with PNP transistor too). The pass transistor will eat 42V on the collector and Vout on the emitter, but now you can easily find a part that can handle that. LM317 will eat max 36V on the Vin pin. Find a transistor with sufficient current gain and select the right resistor values, and variation of Vin (at LM317) due to load current will be negligible.
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Old 16th April 2013, 02:58 PM   #8
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RJM1, i forgot to mention, i want to do this at both sides, e.g. on the +V and on the -V (which would be a second supply hooked up in parallel to the first). Hence my need for the LM337HV if i went with an HV part on the +V side..

wwenze, interesting.. i hadn't considered that but it is starting to make sense. I was playing with (after just discovering) falstad's analog circuit simulator last night (man that thing is awesome), and the zener approach i couldn't get to be stable w/ varying load currents, so i'll give this approach a try to see if i can get it working right (except his simulator doesn't have discrete IC's like the LM317 so i'll probably just mock up the basic representation of it)
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Old 17th April 2013, 12:10 AM   #9
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So with DUG's hints.. i came up with this.. the 499 ohm resistor didn't give me that great results so i replaced it with a trimpot for fine adjustments..

it hopefully can help someone else who is looking to do something similar and folks can learn as much as i have from this design Thanks again DUG!
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Old 17th April 2013, 12:25 AM   #10
mlise is offline mlise  United States
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Default Dropping voltage

If you use a linear regulator to drop that voltage, you'll dissipate a great deal of heat in the regulator. See if you can get the service manual for the CAT 6K router power supplies, or just trace out the last few components near the 12 volt or 42 volt outputs. Somewhere, there's a resistor divider from one of the outputs back to a TL431. It could be a TO-92 or SO-8 package. There may even be a small potentiometer to tweak the output voltage. The divider will be set to send 2.5 volts (or possibly 1.25 volts) to the control pin of the TL431. you can adjust the bottom resistor of the divider to drop the voltage down.
The math is simpler to adjust the top resistor, but the power supply will probably blow up, so you need to adjust the bottom resistor of the divider because that doesn't affect eh stability of the supply.

Where did you pick up the supplies? I need something like that!

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