# Dual power supply

why do we use a dual power supply? Why, for instance, do we use +12 and -12 volt instead of just a 12 volt power supply.

ZapperZ
Staff Emeritus
why do we use a dual power supply? Why, for instance, do we use +12 and -12 volt instead of just a 12 volt power supply.

Er... there are tons of applications where this is needed. For example, when you want to reverse the direction of the magnetic field of a solenoid, for example.

Zz.

In my circuit, i have connected one end of my solenoid to +12 volt and the other end to a ground (where +12 and -12 are connected together). So, how is that any different from connecting it to a 12 V battery?

ZapperZ
Staff Emeritus
In my circuit, i have connected one end of my solenoid to +12 volt and the other end to a ground (where +12 and -12 are connected together). So, how is that any different from connecting it to a 12 V battery?

Try reversing the direction of the magnetic field!

Zz.

Try reversing the direction of the magnetic field!

Zz.

I guess that it is AC that has resulted in the reversing magnetic field. If so, how does Dual power supply provide AC ?

ZapperZ
Staff Emeritus
I guess that it is AC that has resulted in the reversing magnetic field. If so, how does Dual power supply provide AC ?

You just don't answer any questions, do you?

I asked you to use your setup and reserve the magnetic field. You never even bother to answer. If you did, you would have realized the NEED to have such power supply. And now you're changing the game by somehow including an AC supply, as IF that means anything IF you want a constant magnetic field in one direction, and a constant magnetic field in the opposite direction.

Y'know, those of us who work with particle accelerators do need such a thing when we try to maneuver charged particle beams. AC supply connected to coils that produces magnetic field for such purposes are utterly useless! Besides the fact that you can't work that fast, the INDUCTANCE alone in such a coil would render it unusable.

I think I've given you all the answers that you need as one reason why one needs such power supply. The rest is up to you.

Zz.

oh, i will do that as soon as i get to my science lab which i can't do now because it is pretty late night right now. But i will do that first thing in the morning when i enter my lab.

With my AC question what i meant was: "Does the split power supply provide an Alternating current?'

Pranav,Jha,

The simple explanation is di/dt, part of the definition of inductance. With a single supply you have to "wait" for the magnetic field to collapse. With a dual supply you can control it. If it were as easy as you suggest, then everyone here who studied physics/mathematics would have wasted their time. As it turns out, no one here has done that. Please review mathematics with respect to electromagnetic fields before asking such a question. Your question is akin to asking, "What if 2 + 1 were to equal 4?" Not really the proper venue for such questions. Your lack of insight into the mathematical relationship of inductors with a "dual supply" has no bearing on the importance of such. With a single supply you can only excite an inductor and wait for a response, with a dual supply you can drive it. What is the difference? Control versus response.

"AC that has resulted in reversing magnetic field", NO, that is a "natural response", while that can be useful, it has severe limitations. In general, one would use a dual supply to DRIVE a desired response that is within the physical limitations of the passive components....

ok, I surrender....

Fish

Rather than divining the meaning of reversing magnets, try this-

First you need a permanent magnet. Mark one of the poles.

1)Connect one end of your solenoid to the ground wire. Connect the other end to +12V.
2)See which end of the permanent magnet is attracted most strongly to one end of the solenoid.
3)Now remove the +12V and connect to -12V.

The other end of the permanent magnet should now be more strongly attracted.

However most dual DC supplies find use their use in analog circuit biasing.

davenn
Gold Member
With my AC question what i meant was: "Does the split power supply provide an Alternating current?'

No it doesnt.... it supplies a +DC voltage and a -DC voltage relative to the 0V line from the supply.

Dave

(where +12 and -12 are connected together).

As a fully paid up member of the PSPS (Power Supply Protection Society) I am very concerned about this.

why do we use a dual power supply? Why, for instance, do we use +12 and -12 volt instead of just a 12 volt power supply.

Many electronics require specific voltages and polarities, else they can not work properly.
The nature of the beast.

davenn
Gold Member
(where +12 and -12 are connected together).

As a fully paid up member of the PSPS (Power Supply Protection Society) I am very concerned about this.

hahaha a good laugh for the morning :) it can only end badly huh ;)
It makes you wonder.... are some science teachers in some labs, not doing their job properly?

Pranav Jha.....

One direct example of a split rail supply is used in many Op-amp and power amplifier circuit situations :)

Dave

HallsofIvy
Homework Helper
Are you simply asking why we use AC rather than DC?

Edison's original concept was that every one would run there household off batteries, bringing the batteries to a central source to be recharged regularly- and he was really thinking of just one or two light bulbs per room run off the battery. It was Tesla and Westinghouse who developed the idea of running wires from the generators to individual houses. Since just about everything in our houses runs on electricity (even heaters and stoves although I am a strong supporter of oil furnaces and gas stoves), trying to use batteries would be arduous today.

The point is that A.C. can be "stepped up" to high voltage and sent over wires for long distances, then "stepped down" for use in homes. D.C. loses too much strength over long distances to be used that way.

Seriously now.

+12 & -12, referred to a zero terminal is an example of a balanced supply. This has many advantages.

If the supply wires extend over any significant distance, any interference will be equal and opposite in the + and - wires, thus cancelling out. This applies to Direct supplies (DC) and also applies to AC supplies although the terminals are not called + or - any more.

For DC supplies only, using balanced positive and negative supplies (often called rails) allows designers to avoid including a very bulky, expensive and unreliable component called a blocking capacitor.

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rcgldr
Homework Helper
I had the perhaps mistaken impression that the orignal post may have been asking why PC power supplies need to output -5 and -12 voltages (why the components require negative voltages).