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CAD - Engineering - Technical > Help from an electrical engineer
 
 
R0XoRiZoR
Senior Heliman
Location: Austin, Texas - US

I really am not sure as to what to ask for...

Here is the scenerio. I have a 13.8v 20 amp power supply. I need to regulate this to 12.0v 20 amps, with clean enough power to supply even the most sensitive lipoly chargers.

I will also be using this supply for various other projects, and require this device.

Not sure if there is anything available pre-made, but i would like to build it myself. So i was wondering if anyone has seen schematics, or could create one that meets the following:

1. Input voltage 13.8v 20 amps (variable from 0-20 depending on load)
2. Output voltage 12.0v 20 amps (variable from 0-20 depending on load)
3. Dual channel, as the power supply has dual outputs, so i would need dual outputs as well.
4. Power LED's
5. Drawing of circuits (schematic)
6. Parts list (including a good supplier of said parts)
7. **optional** overload protection to save charger/supply

I was running a thermovoltaic cell at 13.8v (when it was 12v rated) pulling 18 amps when after around a day it gave up the ghost. I also know that most chargers that have circuits to accept anything from 11-15v, but i would like it to be a consistant 12.0v.
05-19-2007 Over year old.
 
 
Yug
rrProfessor
Location: UK. Herts

I'm confused at what you're trying to do because a thermovoltaic cell generates electricity from radiation energy.

Normally an electronic device rated at 12v will easily handle 13.8v. Anyhow if you really do need absolute 12v, you will be looking at a switching regulator, however this would probably require a much greater input voltage, better still why not use a mains switcher. 12V mains switching supplies are plentiful and nowdays, do provide low noise performance. I could give you some part nombers but these would be from the UK which would probably not be much use to you.
05-19-2007 Over year old.
 
 
R0XoRiZoR
Senior Heliman
Location: Austin, Texas - US

You are correct, however it still was a mistype.

It is a Thermoelectric Peltier Device, which in this models case, also doubles as a thermovoltaic device (producing electricity from waste heat); however only in the low volt, high millivolt ranges.

I was testing out a heatsink design using this device. The goal is to keep the hot side at room tempurature (ambient) or as close as possible, to reach optimal refridgeration on the opposing side. At the time of failure, hot side was 86.4F and the cold side was 4F. I was running it over night to make the thermal compound "set" as directed. I have noticed this to increase the efficiency of the heat transfer by 20 percent. So thats what i was doing.

I was trying to illustrate that battery charging is not all i do with my power supplies. Unfortunately, these devices and others i tinker with WILL NOT handle anything above 12v, as 12v is its maximum operating voltage (atleast for this particular model).

Anyhow... since i spent 75 bucks on this power supply (which it serves its initial purpose very well), i would like to put it to more use... without spending much money.

I am unsure as to what you are describing in your reply... but if your talking about buying another power supply, no thanks.
05-19-2007 Over year old.
 
 
Portblock
Veteran
Location: Van Nuys, CA

How precise do you want the voltage output?

also, just an fyi, if you build a black box regulator, due to internal losses, etc,, being able to supply a voltage of 12v, with a current load of 20amps would require a source current higher that 20 amps. It is actualy simpler, and cheaper to make a precision regulator from the ground up, rather than having an input voltage of 13.8

I do not know your source voltage, yes 13.8v but is that comming from a powersupply thats plugged into the wall? if so, build a new unit from scratch, its cheaper.

Are you drawing from a car battery (13.8)? from a car that is on (14.4)?

Again, depends on your requirments for a precise voltage, and line condition, but you could make a power supply that converts from your 120 AC line, use a half wave rectifier (vs full wave for duty cycle) and filter with some large caps, then use 1x 7812 voltage regular as an internal reference voltage to a parallel bank of power output transistors as voltage followers. I must tell you that sinking 20amps continuos is alot, its not like people think with these little brushless motor controllers where people think they are drawing a true 45amps. (trust me 45amps continous, on a 18awg wire will not work) (ok, I know I am opening myself up to flamers)

Powersupply design is a very diverse beast with theories and designs to suit many applications.

.....
05-19-2007 Over year old.
HOMEPAGE  
 
 
enyar
New Heliman
Location: Indianapolis, IN

Not a flame but, the fusing current of 18AWG is about 83 Amps. Up until then the wire just keeps getting hotter and the Voltage drop per foot (at 45 Amps) will be only be about 0.3V so it is possible to find someone who'd do it but they'd need much better plastic insulation that wouldn't melt.

I agree with the others. To regulate from 13.8Volts to 12Volts you are unlikely to be able to do it cleanly with a switching regulator since that is such a low drop. To use any kind of linear regulator at 20 Amps you'll need to dissipate 36 Watts of heat in your regulator. That is a lot of heat to get rid of and makes the heat sink very prohibitive.

You'd be better off just getting a different supply.
05-19-2007 Over year old.
 
 
z11355
rrMaster
Location: 10000 is enough time wasted.

and not only that, but I've never seen a decent charger that had
such a narrow input voltage range so that it would not accept 13.8v.
05-19-2007 Over year old.
HOMEPAGE  
 
 
enyar
New Heliman
Location: Indianapolis, IN

Yeah, I have no experience with the chargers so wouldn't know.

To build your own:

AC to 12V like described above:
http://www.zen22142.zen.co.uk/Circu...wer/1230psu.htm

If you really want to go from 13.8 to 12 for the fun challenge you want to use a buck regulator and you probably want to use a multiple phase. Keep in mind most engineers make a lot of smoke when they make their first switching supply design. So be safe and wear eye protection... If you have some training in electronics download Linear Tech's Switchercad and use it to help you design one.

http://www.linear.com/designtools/software/

Have you considered just grabbing a PC Power supply? You can short pin 14 of the ATX connector to any ground (black) pin to turn it on and off. It will then give you a solid 12V within 5-10% and the sky seems to be the limit on current anymore. They had one that could deliver 30Amps at 12V for $10 after rebate at Fry's last week.
05-19-2007 Over year old.
 
 
marked23
Key Veteran
Location: Lynnwood, WA

It sounds like what you want is a cheap circuit to allow you make your power supply do what this one does. Or you could just sell that power supply and get something like this:

http://www.powersupplydepot.com/pro...roduct=15950+PS
05-19-2007 Over year old.
 
 
Portblock
Veteran
Location: Van Nuys, CA

Enyar, you are correct, in my haste, I just summed it up without explanation. The wire would get very hot, and the heat shrink would break off the reg or controller.

.....
05-20-2007 Over year old.
HOMEPAGE  
 
 
Topher
Veteran
Location: Grosse Ile, Michigan

Are you just looking for a cheap power source to charge your batteries? If thats the case then just use a computer power supply. Im building my second one right now. They come with good voltage regulation a variety of voltages can be produced, among other features like short circuit protection.
05-20-2007 Over year old.
HOMEPAGE  
 
 
nivlek
rrProfessor
Location: Norfolk England

It would be easier to adjust the output voltage from 13.8V down to 12V , than to build a new unit , if you really need exactly 12V .

At the end of the day , it gets dark .
05-20-2007 Over year old.
 
 
R0XoRiZoR
Senior Heliman
Location: Austin, Texas - US

nivlek;

and how would that be done? As everyone else seems to think there is not enough overhead.

Topher; did you actually read this thread before your post????


Thanks,
05-22-2007 Over year old.
 
 
nivlek
rrProfessor
Location: Norfolk England

It would probably just require a resistor change in the voltage control circuit . It may even be that the voltage is adjustable on a pot anyway . The voltage overhead would be an issue if you wanted to use 13.8V to run a 12V linear , but that's an entirely different thing .

At the end of the day , it gets dark .
05-22-2007 Over year old.
 
 
R0XoRiZoR
Senior Heliman
Location: Austin, Texas - US

I'll pop the top on the supply tonight, and have a look around. If i need help i'll post the pic and see if you guys notice anything simple like that.

thanks :-)
05-22-2007 Over year old.
 
 
Portblock
Veteran
Location: Van Nuys, CA

Well, I have another idea, opinion, that is if you are willing to pop open your current power supply, then go for it, add some more circuitry.

Pehaps something like (I know I mentioned it above) like a transistor follower?

Supply 13.8 to the output transistors, and tap off a spot that has a bit greater than 15v and feed that to the reg, so the reg pureley acts as a ref votage to the output transistors.

This is just a lousy mockup schematic,missing a few resistors



.....
05-22-2007 Over year old.
HOMEPAGE  
 
 
MooneyDriver
Veteran
Location: Kent, Ohio

Dudes,

Most switched-mode power supplies have built-in voltage adjustment! Look for a little potentiometer and crank that thang down!

-Neil

Hey man, why does my lipo get bigger every time I charge it?
05-22-2007 Over year old.
 
 
enyar
New Heliman
Location: Indianapolis, IN

No one is mentioning the heat this thing is going to have to give off dropping from 13.8V to 12V at 20 A.

If you are really going to do this. Use these parts (I picked everything from parts stocked at Digikey so you would be able to buy it without shopping all over the place.

A low dropout regulator so you can use 13.8V as an input and it'll still output the 12V you want. Digikey# 296-10998-5-ND Datasheet: http://focus.ti.com/lit/ds/symlink/tps75701.pdf

Don't use parallel transistors if you can avoid it. Transistors have very different betas (gain) even when made in the same batch so one transistor may be doing much more than the others and blow up leading to the rest blowing up. Use this single transistor instead. Digikey# 2N5302OS-ND Datasheet: http://rocky.digikey.com/WebLib/On-...ta/2N5302-D.pdf

And use this heatsink to keep the thing from grenading when you start pulling 20A from it. Digikey# 198540B00000 Datasheet: http://rocky.digikey.com/WebLib/Aav...98540B00000.pdf

Use this part for your output capacitor. Digikey# 565-1787-ND Datasheet: http://rocky.digikey.com/scripts/Pr...Y160ELL472ML35S

You probably won't need an input capacitor since you'll be close to the original power supply. You'll need to provide a few resistors but they won't be high power but try for 1% tolerance parts. Try to use 14 guage wire if possible so you don't drop any Voltage and keep the accuracy high. Hook it all up the way Portblock said but notice this part has a feedback pin and you only have one transistor. Take the emitter side of the transistor and connect it back to the feedback so the final output is regulated. It'll be more accurate. You need to provide your own lugs too.

Maybe someone else can draw it up for you if you need it drawn up, I don't have any cad tools here.

Don't forget heatsink compound. The transistor will burn up without it.
05-23-2007 Over year old.
 
 
enyar
New Heliman
Location: Indianapolis, IN

I doubt the original supply will have a trim pot.

Trim pots drift quite a bit with temperature and that would make for a bad supply design as the output Voltage would change a lot with changes in temperature.
05-23-2007 Over year old.
 
 
Portblock
Veteran
Location: Van Nuys, CA

I like the parts Enyar has chosen, thats a nice output transistor too

I am just so use to doing parallel transistors, mostly in power amplifier output stage. Old habbits die hard, but thats another story.

There is a good chance you can stuff this in your existing power supply. If you do, make sure there is suffecient cooling.

.....
05-23-2007 Over year old.
HOMEPAGE  
 
 
stickyfox
Key Veteran
Location: Troy, NY, US

Trim pots don't drift with temperature if the circuit designer had a little bit of common sense. It's true that the resistivity of the pot changes with extreme temperature swings, but power supply trim pots are almost always wired as voltage dividers. Since both halves of the pot are drifting (assuming that they're the same temperature), the voltage doesn't change because the ratio stays the same. And btw, it's also been my experience that almost every switching power supply has a trim pot in it somewhere.

Another suggestion: Now that switching supplies are popular, there are a lot of cheap linear ham radio supplies floating around. Most output the standard 13.8V and have an adjustment pot. I'm using a 35A Alinco that goes from 0-16V. You can never have too many power supplies.

-fox
05-30-2007 Over year old.
HOMEPAGE  
 
 
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