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Topic: what is - 12v dc (Read 378 times) previous topic - next topic

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

aarg

  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

terryking228

#2
Jan 13, 2018, 02:07 am Last Edit: Jan 13, 2018, 02:10 am by terryking228
Quote
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

Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

DrAzzy

#3
Jan 13, 2018, 02:37 am Last Edit: Jan 13, 2018, 02:38 am by DrAzzy
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.
ATtiny core for 841+1634+828 and x313/x4/x5/x61/x7/x8 series Board Manager:
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ATtiny breakouts (some assembled), mosfets and awesome prototyping board in my store http://tindie.com/stores/DrAzzy

MarkT

Quote
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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

aarg

12v is 12 volts above ground
So what is my car running on when I'm in an underground parking garage?
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

tinman13kup

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.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

terryking228

#7
Jan 13, 2018, 05:57 pm Last Edit: Jan 13, 2018, 05:57 pm by terryking228
Quote
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...
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

tinman13kup

Excellent explanation! I may Steal Open Source that...
Have at it. It's an easy way for a beginner to see the concept
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

Don-T


Quote
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?

Zapro

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.

Southpark

#11
Jan 16, 2018, 01:05 am Last Edit: Jan 16, 2018, 12:41 pm by Southpark
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.


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