What is negative voltage? - Page 1
Forum section: electronics / beginners

author: Dave Wave
author_rank: Contributor
timestamp: March 24, 2012, 08:23:03 pm
content: So I have been working on an old power supply that outputs negative voltage. The positive output is a 0 volts. The negative is at -15v.Can sombody explain this concept to me and why one would use such a setup?Thanks,-Dave

author: IanB
author_rank: Super Contributor
timestamp: March 24, 2012, 08:28:01 pm
content: Quote from: Dave Wave on March 24, 2012, 08:23:03 pmAll power supplies and batteries output negative voltage if you want them to. Just put your digital voltmeter on a battery with the red probe on the negative terminal and the black probe on the positive terminal and see what the display says.Negative outputs on a power supply make sense only when they are paired with a positive output. You can then have symmetrical voltages, for example -15 V : Common : +15V. This is useful for various amplifier or op amp circuits that work best with voltage swings around zero.

author: ciccio
author_rank: Frequent Contributor
timestamp: March 24, 2012, 08:55:51 pm
content: I believe that the original poster means that the positive is connected to ground.Maybe the maker of the PSU built two models, one positive ant the other negative....This reminds me of the old days when low noise transistors were only PNP, so you used a positive grounded circuit, and the supply was negative.In these days some cars had a positive ground wiring (and some other used a 6 volts battery).Funny.

author: IanB
author_rank: Super Contributor
timestamp: March 24, 2012, 09:58:46 pm
content: But what is "ground" in this case? Except in special cases, wouldn't the output of most power supplies be floating?An ATX power supply has the negative output connected to ground, but in this case ground is the mains ground and case ground.

author: Psi
author_rank: Super Contributor
timestamp: March 24, 2012, 11:16:47 pm
content: Yep, if the power supply output is floating then it is neither a negative or positive supply until you connect one side to something you decide to call 'ground' (or you connect one side to mains ground).That said though, it always has polarity (a positive and a negative connection) and that doesn't change.It can be a bit tricky to understand that.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 25, 2012, 04:44:06 am
content: Quote from: Dave Wave on March 24, 2012, 08:23:03 pmPower supplies don't really care which side you take as the reference.If you have a supply with,say, two output terminals which are not connected to the case of the supply,("Floating" in other words),you can make either the positive or negative terminal the reference in a circuit you are operating off the supply, by linking that terminal to the common (usually "earth" or "ground")of the circuit.It was fairly common in Communications sites to run the main DC supply with the positive terminal "common" (in fact,earthed),so as to avoid the problems of Electrolysis,where ,with negative earthing,there was a greater risk of corrosion of the earth connection.Equipment designed to run off this DC supply would be designed to operate with a "positive earth".The power supply you have may have been used to test this equipment.Another possibility is that it may have been designed for early solid state equipment.Germanium technology was found to be easier to design in PNP type than in NPN.With PNP,the Collector is connected to the negative supply,so it made sense to make positive "common"(or chassis,or ground,depending on the terminology).Note: the terms "earth"& "ground" are used fairly carelessly in Electronics as a synonym for "common".In a particular circumstance "common" may be connected to the actual "dirt",as in the Comms sites I referred to.In many cases it will not.

author: amyk
author_rank: Super Contributor
timestamp: March 25, 2012, 01:04:27 pm
content: Quote from: IanB on March 24, 2012, 08:28:01 pmThis can be thought of as 30V, 15V, and 0V outputs too, because voltage is relative not absolute. A point with negative voltage just means that its potential is below that of the reference point chosen. However in many cases such as mains circuits the earth literally is the reference point. With isolated/battery-powered equipment the choice is more arbitrary.