negative dc voltage 1

[Guest_imported]

How is (-5 and -12) VDC used in a PC. In simple terms, (if possible), what is the concept of negative DC voltage. Are negative DC voltage measurements often taken during the troubleshooting and repair process of a computer. thx P.S.

 

[edfair]

Some chips. Serial communications. - voltages are negative with respect to the ground or 0v potential. Haven't measured -voltages on a computer for at least 10 years. But the same thing that measures positive voltages measures negative voltages. Ed Fair
efair@atlnet.com

Any advice I give is my best judgement based on my interpretation of the facts you supply.

Help increase my knowledge by providing some feedback, good or bad, on any advice I have given.

 

[fenix]

Hi Ed, thx for your reply, they are hard to come by on this subject. Your explanation helped out, but can you answer more on this---is negative VDC used only in electronics, and does it exist wherever positive VDC exists. Maybe this isn't the same thing, but my analog (inexpensive) meter goes UP to 12 Volts when I measure a car circuit, and doesn't have a negative voltage scale like some more expensive meters do. Can I ascertain that this is Positive VDC, even though there is a negative ground in a car. Still trying to comprehend it all. I'd really appreciate more help with this, no hurry on the response, I know time is valuable, I'll be watching the forum. Please label your reply NEGATIVE VOLTAGE. Thx again.

 

[ShaithisDanoveur]

You can measure negative VDC with an inexpensive meter. With your leads on your chosen test points, if your meter's needle heads towards the basement and parks there pretty hard, you're safely assuming a negative voltage exists at that test point. Don't hold the leads there too long or you WILL damage your meter.

Reverse your leads, putting your positive lead where your negative lead was and vice-versa, and record the voltage normally. Remember to put the negative sign in front of your measurement.

Or you can go spend $19 for the little digital multimeter at Radio Shack that will handle positive and negative voltages without lead-swapping

 

[edfair]

Analog meters merely indicate the potential between the two points. It comes down to the direction electrons want to flow when the circuit is connected. The source of electrons is the negative point and the destination is the postitive point.
For example:
If point A has more electrons than point B , then point A is negative with respect to point B. At the same time point B is positive with respect to point A.
In multivoltage supplies there can be many voltages available. generally point A or Point B will connect to a reference point called ground. So depending on which one is at ground, the other one is either positive or negative from the ground common. And when you describe a voltage as negative, in a computer for example, It means that the circuitry wants to provide electrons.
Go back to the car example. You put the black lead on the frame and the red lead on a working light terminal.( one post should indicate voltage) If this voltage is positive the meter will deflect to the right and you will measure 12 volts. And you will know that the power supply is 12 v DC negative ground. If the meter deflects to the left and tries to bend the needle because it isn't designed to go that way you know it is a positive ground and you can swap the meter leads to check the voltage.
I have seen analog meters that measured both positive and negative voltages, but they were in a classroom setting, and very,very long ago
. Hope this isn't too much detail. Ed Fair
efair@atlnet.com

Any advice I give is my best judgement based on my interpretation of the facts you supply.

Help increase my knowledge by providing some feedback, good or bad, on any advice I have given.

 

[fenix]

Yottabytes of thx to Ed for a clear and fact filled explanation, and for his time to write it all out. It really completed my understanding of this concept, and I barely had a clue. Also much thx to Shaithis for his tek-tips re; meters. fenix