voltage regulator +/-

hello everyone, I am making a power supply, and I would like it to have a variable voltage, both positive and negative. however, it is challenging since I am taking circuits next semester.

I have a transformer, building a rectifier is no problem (I am going with a bridge rectifier, I think), and filtering is no problem. but I was having trouble designing a +/- voltage regulator. so, if anyone has any design ideas, I would love to hear them.

I would like it to be controlled by a potentiometer, which would be turned one direction for positive voltage, and the other for negative.

thanks

 

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What sort of voltage range and current levels are we talking about? A single supply that will swing from +v to -v is a bit of a challenge, but one that will go from +v to gnd or from -v to gnd is a snap with an LM series IC.

 

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I am shooting for about +/- 15v. and yes, it is a bit of a challenge =], thats why I asked for help =].

I made a little power supply a while back, but I wanted a nice one. for example, I plan on putting a circuit interrupter instead of a fuse, and perhaps a voltmeter.

 

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Hi cato,
A few minutes surfing indicates that your best bet is to use an IC adjustable regulator. The LM317 seems to be most common. It has an output range of 1.2V to 37V. The LM337 is a similar product that produces a negative output, but is again limited to -1.2V. There seems to be simplish ways of dropping the minimum output to 0V, and it seems simple enough to duplicate the circuit to get negative and positive outputs on different terminals, but I don't know how you'd go about getting a full positive-negative range from a single output terminal.

Both the LM317 and LM337 are readily available from electronics stores. They should be mounted on a heatsink (depending on your current requirements). Your transformer output should be a couple of volts more than your maximum required output.

Have a browse for circuits here:
DicoverCircuits.com

Pete (electronics hobbyist in a past life)

 

thanks, I was looking there before, but could not find anything. there are some that are almost what I want, but I am having trouble figuring out how to make it controlled by a single pot.

 

Can't you just make the output 0-30v and bias the output with a couple of resistors? How much current do you need to draw?


___30v__________
|............................|
|............................R
|............................|
|<-----\-- X ----15v|
|............................|
|............................R
|___0v..................|
|............................|
V..........................V


when your pot -\- is at 30v, the PD across X is +15v, when the pot is at 15, there's 0v across X and when the pot is at 0v, X is at -15v.

If you're drawing minimal current, this will work fine.

(please ignore dots, ascii art formatting doesn't work with spaces! WHAT DOTS? I TURNED 'EM WHITE!!! ARF!)

 

uhhh, I am having trouble visualizing your diagram. just email me a pic

 

I may have misunderstood what you have in mind. Is something like this what you want?

http://michaelgellis.tripod.com/power3.html

 

not exactly, perhaps a backstory would help.

I was screwed on an engineering project last semester because they told us that we were going to be using one knob to adjust the voltage to a motor, and were were given such a device to test our equipment with. however, when competition day came, all we had was a power supply we could switch positive or negative but had to pre adjust the magnitude of the voltage we were using. this caused major problems for my teams machine, and I decided that I should figure out how to wire something like that in case I ever run into the same problem.

there were rumors that they were not going to have the same equipment for competition as they had for testing, and if I had known how to wire it before, I could have brought my own power supply and whooped tail in the competition regardless of equipment supplied to me.

its kinda a making lemonade out of lemons thing.

anyway, what I am going for is to have a potentiometer set so that I can control both magnitude and direction of the voltage with a single knob (for ease of use). I was planning on making a new power supply, as the old dinky one i have is not very good.

 

Ok, so my original take on your project was right. To do that is kind of a pain. Probably need to use a +v/gnd/-v basic supply and a dual transistor totem pole output strung between the +v and -v rails. Connect the motor between the gnd and the totem output.
That setup can work but design is a headache because if you don't have perfect control at zero crossing both output transistors can be on at the same time and you blow the transistors.

 

hmm, I am not sure how to do that. =]

moreover, I am not sure how I would prevent said problem. =[

 

Let me think on it a bit. (What kind of time constraint are you under?) Like most etechs, I like to pretend to be an EE once in a while.

I actually did mess around with something along these lines many years back, for a push pull audio amp I was trying to design and build for myself. Fried a lot of power transistors as I recall.

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It was more difficult than your project because to avoid harmonic distortion I had to let both transistors conduct a little at output = 0v . Never could get the current levels controlled during zero crossing. I'm guessing you could tollerate a small "dead spot" on the control pot near zero. I'll try to work out a rough diagram to show you what I'm talking about.

>>edit I gather you're not under any deadline on this. Maybe I should learn to read posts more carefully.

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Cato dude, sounds like you need a controller like the ones that model train ethusiasts use, and that are widely available. They either have a single potentiometer to control forward/reverse motion, or a simple switch, to reverse the polarity across the pot.

Go google, I'm sure there will be lots of circuits.

 

An H-bridge setup avoids the problem of two transistors in one leg being on at the same time. Also, when putting together a motor controller, you want a dead spot in between and you want anything but the smallest motors to come to a complete stop before switching polarities. Reversing a motor that is moving creates back EMF that can easily destroy your circuitry.

 

its all good =]

I would rather learn how to make it, than know where to buy it

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and the switch would be easy. there are many ways it could be switched (dpdt for example), but I would rather have it automatically switch as I pass a certain point on the POT.

that sounds promising, I have limited time right now, but I will google H-bridges later (unless you know of one to show me)

yes, I was thinking about that, but it would not have made much difference last semester. however, if I am learning how to make such a thing for an unknown later application, I had better cover all bases.

thanks guys, I will be back later to google =]

 

go to your library and look for:
"electronics" vol. 32 issue 25 page 76

the circuit discription is:
transister rectifier gives dc of either polarity

drawback is, it only sinks 7 ma. as designed
it could probably be increased with bigger transisters

i have a copy of the circuit but i do not know how to get it to you.

 

hmm, I am not sure they will have that. perhaps you can scan the page of the book (at home or the library), or take a digital photo.

I googled h-bridges, and I am not sure how I could control it with a POT. they look more useful as a digital type supply. I will look over them more later to see if I can somehow do it.

man, I didn't think this was going to be such a challenge. I like trying to solve problems like this though. =].

 

yeah, I don't think I can manage it with an H-bridge. there must be a way! =]

 

i was thinking cato ( oh no)
maybe you could build a ramp generator and connect it to a hight power dc coupled audio amp.

you might have to think outside the box on this one.

about my schematic
how would i get it into the computer?
what format must it be in to post it on the board?
i have it in hand the problem is getting it to you.

i have lots of schematics i'll keep looking.

 

well, if you either have a scanner, or your library does, you can scan it as whatever format the scanner wants to make, then simply email it to me (clemensc @gmail)