what is - 12v dc

can someone explain what -12v dc is? i'm converting an old pc psu to a desktop psu and it has 3.3, 5.0, 12.0 and -12.0 voltage. I can't get my head around what -12.0v dc is. thank you

what -12.0v dc is.

Earlier PC electronics devices used both positive and negative power supplies, especially when they had various analog components such as operational amplifiers that used both + and - supplies. The original IBM PC had a floppy disk controller that used one special IBM chip that needed -5V, which is probably why we still have -5V today.

Even today most modern computer power supplies follow the ATX convention: They output +3.3V, +5V, +12V , -5V and -12V on a series of color coded wires.

IF you want lots of detail Read THIS

12v is 12 volts above ground

-12v is 12 volts below ground.

We don't use voltage below ground much in digital electronics (indeed, exposing any arduino pin to a voltage below ground will damage it), but it's quite common with analog stuff. It's not unusual to see opamps, comparators, or digipots that have voltages both above and below ground.

can someone explain what -12v dc is?

No, but I can explain what -12V (negative 12 volts) is... (V is the symbol for volt, and case matters for unit symbols)

Voltage is more technical known as (electrical) potential difference. Its the amount of energy change
when a charge moves from one part of a circuit to another. So a unit charge (+1 coulomb of charge) flowing
from a point of 0V to -12V will liberate 12 joules of energy from the electric circuit
into heat (if through a resistor), or some other form if a some other device perhaps.

-ve energy means energy goes from the circuit to the surroundings, +ve means the opposite (such as inside
a battery, where the charge is forced to flow by the chemistry and gives energy to the rest of the circuit).

Voltage is always a difference, and charge flowing down or up a pontential will consume or liberate energy
according to the product of the amount of charge and the voltage, signs matter for both charge and voltage.

Because its a difference you choose which part of the circuit to call 0V, that's entirely a convention.

DrAzzy:
12v is 12 volts above ground

So what is my car running on when I'm in an underground parking garage?

Voltage is entirely relevant to a particular circuit.
Granted, most battery powered circuits use the (-) terminal as ground, but you can also get negative voltages from batteries as well. It's all in how you connect the circuits to them as to how the difference in potential (voltage) is determined.

Take 2 AA batteries and put them in series. A dvom will show 3V when connecting the leads across both batteries, but if you use the center as relative Ground (connect blk lead), you will get +1.5V and -1.5V.

When you can understand that, you will have a grasp of negative voltages. It's not that they are negative at all, but negative to a arbitrary ground point that you or whomever designed a circuit has named as ground for their particular circuit.

Take 2 AA batteries and put them in series. A dvom will show 3V when connecting the leads across both batteries, but if you use the center as relative Ground (connect blk lead), you will get +1.5V and -1.5V.

Excellent explanation! I may Steal Open Source that...

terryking228:
Excellent explanation! I may Steal Open Source that...

Have at it. It's an easy way for a beginner to see the concept

Take 2 AA batteries and put them in series. A dvom will show 3V when connecting the leads across both batteries, but if you use the center as relative Ground (connect blk lead), you will get +1.5V and -1.5V

Ok, I did your two 1.5v batteries set up and if I have the black lead of my meter in the middle I got a negative reading on one end and positive on the other. So back in the day on the farm we had an old MM “Z” tractor that was positive ground and the negative was hot. Is that what you are saying? It was a 6 volts battery and if I had hooked my meter up the “normal way” red to red and black to black it would read negative 6 volts?

Don-T:
So back in the day on the farm we had an old MM “Z” tractor that was positive ground and the negative was hot. Is that what you are saying? It was a 6 volts battery and if I had hooked my meter up the “normal way” red to red and black to black it would read negative 6 volts?

Exactly.

// Per.

Don-T:
can someone explain what -12v dc is? i'm converting an old pc psu to a desktop psu and it has 3.3, 5.0, 12.0 and -12.0 voltage. I can't get my head around what -12.0v dc is. thank you

3.3, 5, 12, and -12 volt.

They should have also included 0V terminal, which often has black coloured lead(s), which is usually '0 volt'.

If you have a resistor and you connect it between 12V and 0V terminal, then 'conventional' (basic circuit theory) current will flow from the 12V terminal to the 0V terminal (via/through the resistor).

If instead you connect the resistor between 0 V and -12 V terminal, the conventional current will flow from the 0V terminal to the -12V terminal (in that particular direction) via/through the resistor.

The basic reason is ..... the conventional current flows from the higher voltage potential region to the lower voltage potential region. Eg....... zero is higher in level than -12 (negative 12) in terms of a number scale.

Similarly 12 is higher in level than zero in the number scale. And conventional current (which is usually associated with POSITIVE charge) will flow from the higher level to the lower level if a conductive path exists.