View Full Version : Negative Power supply

03-02-2007, 05:32 PM
I have a need for a regulated negative power supply to be used in automotive applications. The use will be for op-amps.

I have done my research, and found multiple ways to supply a negative voltage, but I can't seem to find one that is guaranteed to work in an automotive application. Some of these include:

555 timer

I'm no EE, so I'm not sure what the best solution will be. I am using this to drive standard issue (http://injector.com/sensors.php?PHPSESSID=f392ed9648be271f2ff5234f6154 c3b5) temperature sensors in a radiator. This will feed into a 741 op-amp, and read into my Propeller.

Ideally, I will have my vehicle battery hooked into a ~9V voltage regulator, this will then supply all other circuits. This is subject to change, but the point I am trying to make is that this negative power supply will be driven from a (lower than automotive) regualated supply voltage, not directly from the battery. I think this makes it a bit more bulletproof.

I know there are users here that have done this before, and can suggest a bullet proof solution for my problem. Anyone???



03-02-2007, 08:53 PM
Without knowing all the details of your project, have you thought of moving away from an op-amp that needs +/- supplies? There are numerous chips that run fine on +ve only supplies and most designs can be tweaked to not need to drive below the 0 volt level.

If you have a proposed schematic of what you're trying to do, I'm sure you'll get several more suggestions.


Tom Sisk


03-03-2007, 12:49 AM

- Rick

Tracy Allen
03-03-2007, 01:37 AM
There are little modules in dip or soic packages that provide isolated voltages. They are actually little hybrids that have a transformer inside. They cost less than $10. For example, the DCP01 series from TI. The "01" stands for 1 watt.

Like Tom says, though, it might be possible to do this with a different op amp or a change in circuit so as not to require the (-) supply. The '741 is pretty old school!

Tracy Allen
www.emesystems.com (http://www.emesystems.com)

03-03-2007, 04:40 AM
Here is one of those little modules


Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

03-06-2007, 01:08 AM

Thanks for the input. I don't suppose any of you have tried this in an automotive application and can attest to it's reliability?

Re: "the '741". I'm not sampling any more than 10kHz, 8 bits (for now). Frankly, I've spent time trying to figure out what the (billions) of different op-amps are, and I can't seem to figure out, if in my application, it makes a difference. I'm an ME with a new passion. :) Care to suggest a good modern day replacement to the '741?? I want to build my setup around a few core hardware pieces. Right now, the '741 coupled with a MCP3001 are filling the need. I am open to advice.


Jim Richey
03-06-2007, 08:30 PM
Hello Parsko
The 741 will work out fine for most applications.It can easily be configured to
run on a single polarity supply for some purposes,a comparator for instance.There is probably little advantage in using more refined versions for your purpose.
The automotive environment is noisy and will require some filtering,but it's not any more difficult than most other situations.It would help to post a more specific example of what you are trying to accomplish.

Best Regards

Thanks, Parallax!

04-05-2007, 02:01 PM
Well, I finally ordered, recieved, and built the circuit from below. I used a MAX765.

My reaction to this circuit is simply, WOW WOW WOW!!! I'm very impressed at what it offers.

First, the input supply can be nearly anything a hobbiest might have. I plugged it into my 3.3V rail, then into my 5V rail, and there was no difference in the voltage output, VERY convenient!

Second, the output is fully adjustable, regardless of input! I used the adjustable supply setup. Specifically, I used a 1Mohm potentiometer for R2, and a 150Kohm resistor for R1.

I can adjust the voltage from -1V to -16V. I don't actually know if my specific setup will hit -16V due to the 1Mohm pot. I guess my setup will go to -((1M/150K)*1.5V) ~ -10V.

As I stated earlier, I am using this for temperature sensors to eventually be used in my brothers dirt track car. My guess is that this will work perfectly in this environment.

Anyhoo, I wanted to post an update so ya'll know what I know. If, as I continue to develop, I see anything quirky or out of the ordinary, I will reply to this post with updates... Not sure what any quirks would be just yet, as I only plugged it in to my 5V rail last night at midnight to see if I soldered correctly.


EDIT: I scoped the output, it was rock solid. I was under the impression that the output would be a high frequency square wave, but it wasn't, at least not on my scope.


Post Edited (parsko) : 4/8/2007 12:44:29 PM GMT