Anybody know where to find a +/- power source?

[Igglybob]

So I'm currently working on a project that requires both a +5VDC and -5VDC power source.

I've located where I can find +12VDC (which I can regulate down to +5 without a problem) but I can't seem to find any information on a negative voltage source of any sort.

Does anyone know where I might find one of these? I'm hoping I won't have to resort to some really dirty hacks to get myself a negative voltage.

Thanks!

[ATF]

Get your hands on an ETM (Electric Troubleshooting Manual) and maybe just maybe you can find one?

Whatcha making?

You might look into this as I was unfamiliar to -5 VDC. I know hardware have ratings for reverse polarities, I blew my beautiful Noritake VFD because the max rev voltage was 7v and I hooked up 12 to the ground, anyways thats another story
http://www.hardforum.com/showthread.php?t=1102859

[Igglybob]

What I'm making (warning - it's totally useless, but cool) is a digital display that will show me the factor of the speed of light that I'm going. So, if I was going, say, 80mph, the meter would display "0.0000001192c". I think it'll be cool when I finish it. A link to a rudimentary, incomplete page: http://www.igglybob.com/projects/sol_meter/

I need the -5VDC for the analog to digital converter I am using (the ICL7135). It says I can get away without it, but no matter how I try I'm not able to get it working without that -5VDC.

The second poster on the link you gave brings up an interesting point, but I'm not sure it can work. I would explain why, but I'd be diving into theory that I'm not 100% sure of at the moment and that's always a bad idea.

The easiest solution, of course, is if I can find myself a negative voltage source, but I'm not sure one exists.

Thanks for the help!

[larryn]

As a cheating and somewhat easier solution, you could always just use a 12vdc power source so as to power a charger to a 5vdc battery. then use the battery to power the meter.

I'm not sure how much of a hack that would be, but it would work.

[zboot]

Hi. . .umm, not quite sure what your level of electronics expertise is but. . . you are not going to find a -5V source to tap from the vehicle. I'd suggest adding both a positive and negative voltage regulator to your circuit board and power the whole thing off +12V from the car. You won't find negative voltage regulators at radioshack. You'll have to get them from either a real electronics store or online at places like Digikey or Mouser.

[Igglybob]

Well, I'm an Electrical Engineering undergrad at Georgia Tech, so I'd certainly hope I'm able to figure this out.

I haven't been able to find any negative voltage regulators that would accept a positive voltage as input; the only ones I can find (specifically the 7905) require a negative voltage input also. Unless, of course, I'm reading the datasheet incorrectly, but I don't think I am.

One hack (which is working for me at the moment, but it's rather ugly) is to use 4 AAA batteries in series to produce approximately -5V (technically 6) but that takes up more space than I'd hope.

I've been tossing around an idea in my head of using two 7805s to turn a single +12V into +5, GND, and -5 (GND would actually be at +5 volts if you use the +12V input as reference); however, I don't think it would work. The GND output would be the output of a 7805, which, if I'm not mistaken, can't "accept" current as GND needs to in this case. I might just put it together and see if it works, though -- just gotta dig up some 7805s.

Thanks for the help!

[zboot]

sorry, I was not paying attention before. You make a "negative" source by connective the positive voltage terminal to the common and the negative battery terminal to the input (pretty much you reverse the normal battery connections).

[Igglybob]

Yes, but that still wouldn't work since I need both a +5V and a -5V. If I create the -5V in the way you said, its respective ground would actually be +12V. Then, the +5V would be in reference to 0V (the actual ground). Since the grounds aren't the same, it wouldn't work; it all has to line up to the same ground.

[zboot]

That's not how it works. There are many ways to think about it. . .but here is one. . Your common or "ground" is a reference point. You always assign it a zero potential. Everything else is referenced from that. So, your "ground" point is always zero.

I'll create a couple pics with examples of what i'm talking about . . give me a couple mins.

EDIT:

Ok, look at the attachments. In both cases, Load 1 always has 5V across it while in one, Load 2 has +12V and in the other Load 2 has -12V.

Do you see how that works. Or, in the case where the voltages across both loads are positive, the top terminal of load 1 has a potential of 17V while the bottom terminal of load 1 has a potential of 12V. So, the potential difference (what we term voltage) across that load is only 5V. (doing KVL starting from the bottom terminal of the 12V source)

Don't worry if it seems weird. . .it took me a while to accept it conceptually even though I could always prove it to be correct with circuit theory.

[Igglybob]

Originally Posted by zboot

That's not how it works.

Once I have time (i.e. Friday) I will explain that it is how it works, and why your idea (as far as I can interpret it) will short the battery and cause my car to explode (maybe not, but still...). Not a flame, mind you.

[zboot]

Actually yes, there is a mistake in my text.

The first image is the one with +5V across load 1 and -12V across load 2. The center point is a zero potential. The second image is the other case. I reversed the explanations. . .

But in both cases, there is no worry about shorting. The batteries are only connected together at one terminal, not two. (keep in mind for things to be shorted together, they have to be in parallel).

[ATF]

I don't know who is wrong or right, but zboot's resistor squiggles own.

edit: they aren't resistors, but in my mind thats what I think when i see those >.<

[zboot]

Originally Posted by ATF

I don't know who is wrong or right, but zboot's resistor squiggles own.

edit: they aren't resistors, but in my mind thats what I think when i see those >.<

LoL, I always hated when books used rectangular blocks to represent arbitrary loads. . . unless the loads were complex (yeah, in circuits we can have real and imaginary loads [because in the time domain, we represent them with imaginary numbers] . . .but I digress).

[Igglybob]

Oookay... I finally found some time. Let's begin.

I'm not including basic details of how these circuits work; I'm simply sketching out what I saw to be your suggestions and showing you why it won't work.


Your suggestion.

Here we have a successful +12V and -12V, but they must be isolated. In my situation, the grounds must remain the same. Here, if you connect the grounds (which are B and D), you short the battery, and everyone dies. If the grounds did not need to be the same (which unfortunately they do in this project) I could get away with that.

larryn's solution is interesting, and might work, but I've set off on a different path so I'm not going to investigate that one.

I find your attachments explanatory, but not helpful to this discussion as this is elementary to both of us.

My solution in the end is this:



I don't like the fact that I have to use a battery, but it seems there isn't an easy way around it. My priorities here are size and simplicity; it needs to be as small as possible, and with 3 components this isn't too bad.

Thanks to all for the help!

[John Firestone]

A couple standard solutions for your project would be:

1) isolate your readout unit's internal ground and with a high current voltage follower, float it midway between vehicle ground and +10V

2) use a low-cost ($5-10) DC to DC converter module to generate -5V from your regulated +5V supply (or -9V you can then regulate to -5V).

Personally, I would do neither and instead follow BMW's example: keep everything digital, attach a microcontroller to the instrument cluster's vehicle speed output, and have it count the output's +12V digital pulses, and show the speed. I might also build a Mach meter rather than a speed-of-light meter as I think you'll find passing the sound barrier is challenge enough.

Good luck!