Negative voltage from positive voltage - Page 1
Forum section: electronics / beginners

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 11, 2015, 04:22:06 pm
content: First of all, I believe this question is a long shot, but it doesn't hurt to ask anyway.I am building myself a very basic bench power supply, which will be, in short, a notebook SMPS (HP, 19.5V, 4.7A, it was being dumped because its connector was broken, but other than that is in perfectly working condition) with a voltage regulator, a display for voltage + current measurement, a few knobs, a 5cm fan, and all in a nice enclosure.One of the features I want on it is adjustable negative voltage output. Since the SMPS I have only outputs positive voltage, I was wondering how I can have negative voltage from it, and still be able to output a couple amps on the negative "rail".I've read about using resistors do create a virtual ground and split the positive voltage output, but this approach won't allow me to use more than a few milliamps and will effectively reduce my output. Using ICs like TL7660 or MAX1044, is possible, but also has a very low current limitations.Is there any possible way to do this, other than resorting to a center tap transformer instead of the notebook SMPS?Thanks in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 11, 2015, 04:44:50 pm
content: One can have the virtuell ground with an amplifier, to make it more powerfull. This way you have something like +-9.5 V. However there will be significant power loss. If only one voltage is needed at a time, just define the more positive side as Ground.There are switched mode converters to make a negative voltage from the positive. Alternative one could step up the positive voltage, and use the positive from the notebook supply as ground. This case one might have something like +-19.5V.There is also the option to convert the DC to higher Frequency (e.g. 20 kHz) AC and drive a relatively small ferrite core transformer with that.This way the output might be isolated from the existing supply and you are relativly free to choose the voltage. This is similar to the ready made DCDC convertes you can buy (especially for low powers like 1-5 W).

author: mich41
author_rank: Regular Contributor
timestamp: November 11, 2015, 09:52:54 pm
content: Lots of things produce some kind of negative voltage, but for powering things only an inverting dc/dc converter fits the bill. ICs are available which do that (add PCB, inductor, few caps and you are done) and probably fully assembled modules as well.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 12, 2015, 02:51:52 am
content: Thank you for all the reply, fellows.Quote from: mich41 on November 11, 2015, 09:52:54 pmI did some digging in my components drawer and found aMC34063removed from a car phone charger. I think that the schematics on Fig. 7, Page 9 might be what I am looking for: whatever positive voltage I adjusted on the output will be "negativelly mirrored" through the MC34063. I'll be limited to 1.5A on the negative rail, but that is more than enough for the cases when I'll be using negative voltages (mostly OpAmps).Do you think that if I use the circuit from Fig. 7, Page 9 (w/o using any resistors between pins 4 and 5), I'll get Vout = -Vin ?Quote from: blueskull on November 11, 2015, 10:53:48 pmI though of that too, and I do happen to have a couple notebook PSU around, that I fixed (broken cables, broken connectors, etc), but none of them is identical to the one I am using for this project. Even although my cost to add a second PSU is zero, it would take valuable space in the enclosure I already have (it is already a tight fit as it is).Sorry for the "noobiness", but I don't get what you mean by "their leakage current is guaranteed to be safe without grounding".

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 12, 2015, 04:45:19 pm
content: Quote from: blueskull on November 12, 2015, 02:55:05 amThanks for that. I had to google what are Y (and X) capacitors, and things are starting to make sense. Thanks again.