Negative Voltage - Theory Question

[Mark Cox]

I consider myself versed in electronics as a hobbyist it is not my day job think I understand the concepts and a bit more.

I was working on a small DC power supply for a serial to Ethernet switch it has ground, +5 volts, +12 and -12. It is the neg 12 that got me thinking... Is it just a label for secondary 12 volt supply? Or it is it referring the difference -12 to +12 meaning 24volt?

If someone could explain that to me it would be great, I tried google but you can imagine how many hits you get about everything except what I was looking for. I have built a few power supplies mostly full wave bridge with filters and reg so basic type. I never seen reference to a -x voltage.

Thanks
Mark

-

 

[davenn]

hi Mark,
welcome

I was working on a small DC power supply for a serial to Ethernet switch it has ground, +5 volts, +12 and -12. It is the neg 12 that got me thinking... Is it just a label for secondary 12 volt supply?

it really is a -12V with respect to the 0V rail

Or it is it referring the difference -12 to +12 meaning 24volt?

yes, there would be 24V difference between those 2 rails


Dave

 

[Mark Cox]

Thanks Dave!

I think I understand now in theory, just not practically, thinking of a basic power supply and how you could get -voltage. I think I will have to do some more reading on that. It is hard to get your head around!

Any ideas on what to google to get more information?
Mark

 

[davenn]

thinking of a basic power supply and how you could get -voltage

for a transformered linear supply you use a transformer with a centre tapped secondary .....
here's one I did some years hack




Dave

 

[Mark Cox]

Awesome! thank you I can see it now, depends on you point of reference so to speak!
230v 50hz you must be in UK somewhere?
Many thanks for the reply and the example.

Mark, Nova Scotia, Canada.

 

[davenn]

230v 50hz you must be in UK somewhere?

see the flags below your and my avatars ?

I'm in Australia, ( specifically Sydney area), I could see you were in Canada ...just didn't know what part

 

[Colin Mitchell]

But you have to remember, it is 12v higher than the earth rail and 12v lower than the earth rail.

 

[BobK]

Look at it this way. Your car probably has a negative ground chassis (unless it is very old). There is a 12V battery with the negative terminal connected to the chassis. The positive terminal will be +12V from the chassis. Now add another 12V battery with the positive terminal connected to the chassis. Its negative terminal will be -12V from the chassis. And if you now look at the two batteries, they are in series and it is 24V from the positive terminal of the original battery to the negative terminal of the battery you added.

Bob

 

[elebish]

for a transformered linear supply you use a transformer with a centre tapped secondary .....
here's one I did some years hack

View attachment 24828


Dave

The ground may just be common but could be chassis ground or earth ground (or both). In the case of USB, there would be a +5 source with a common and 2 pins for neg and pos data. As you know, a USB connector has 4 pins. See https://en.wikipedia.org/wiki/USB

 

[HellasTechn]

In general there is no such thing as negative voltage. It just represents the reverse direction of electrons movement relative to the forward movement.
In other words say you have a battery with point A as positive lead and point B as negative lead, current will flow from positive to negative, If for some reason current starts moveing from point B to point A then we have "negative voltage".

By definition voltage is the potential difference between two points, so there can not be a negative potential. Got the point ? we eather have current or not, it is the currents direction that makes it positive or negative.