Negative voltage

[ABCD]

I've been looking at the Elenco XP-720K kit to build for the fun and experience of it.  I see that it provides 1.25=>15V and -1.25=>-15V at 1 amp.  What does it mean for a power supply to provide negative voltage?  I've not come across a situation yet that requires negative volts.

[sacherjj]

Voltage is always a measurement of potential relative to a reference.  With a power supply, the reference is "ground".  This type of power supply will typically have three outputs.  Negative, Ground and Positive.

The typical use for dual ended power supplies are for audio amplifies.  Look are op amps, such as the old LM741 and many newer and better op amps.  By using positive and negative power, the signal being amplified can swing up and down in relation to ground.

[vk6zgo]

(1) Some circuits  such as those using Op Amps require two supplies & a zero voltage common reference point.

(2) Some older equipment has a positive earth set up so that the common reference is the chassis of the device,with the supply rail to the equipment as a negative voltage.
Although with these,you can operate them OK if your single output supply has a "floating" earth, if you want to operate one of these along with another more conventional unit with negative earth,you will need both polarities.

 Note:- Even though the set up in (2) is rare, be very careful that the units do not use DC coupling of the inputs & outputs,as you may damage them,or your power supply.

VK6ZGO 

[EEwannabe]

I built the ELENCO XP-620 Kit over a decade ago and its still going strong. The documentation had a decent beginners circuit discription guide that came with it. I also would recommend an old Radio Shack book called 'Building Power Supplies' by David Lines. The SMPS stuff is a bit dated but the basics are explained pretty well.

- Patrick

[Bored@Work]

(2) Some older equipment has a positive earth set up so that the common reference is the chassis of the device,with the supply rail to the equipment as a negative voltage.

Modern telco equipment still has it, the common DC supply voltage being -48V nominally. There is a reason for that, electrolysis.

[vk6zgo]

(2) Some older equipment has a positive earth set up so that the common reference is the chassis of the device,with the supply rail to the equipment as a negative voltage.

Modern telco equipment still has it, the common DC supply voltage being -48V nominally. There is a reason for that, electrolysis.

Yeah,I knew that !

I didn't want to confuse things too much.

When I worked for Telecom Australia years ago, I ran into quite a few of these supplies. -48 volts can give quite a nice little tingle when you get across the right terminals on a tagblock,especially in humid conditions---ouch! 

The TV & Radio Stations I worked on for Telecom Aust had-24volt supplies,mainly for various control stuff,
the Electronics was mostly mains operated.

Quite a few old Brit cars up to the mid '50s had the same setup,but it didn't help them much!

VK6ZGO

[alm]

(1) Some circuits  such as those using Op Amps require two supplies & a zero voltage common reference point.

Where's the ground pin on an op-amp? An op-amp doesn't care about symmetrical power supplies, virtual ground and that kind of nonsense. It has a positive and a negative rail (you may call any of them ground if you like), and both the input and output signals should be within those rails (often with some margin).

[vk6zgo]

 

(1) Some circuits  such as those using Op Amps require two supplies & a zero voltage common reference point.

Where's the ground pin on an op-amp? An op-amp doesn't care about symmetrical power supplies, virtual ground and that kind of nonsense. It has a positive and a negative rail (you may call any of them ground if you like), and both the input and output signals should be within those rails (often with some margin).

Yes,you are right,but common usage,especially in earlier times was to use the setup I described.
Maybe it was for convenience with interfacing to discrete component circuits,but the number of times I have seen an Op Amp floating between a +ve rail & earth could be counted on the fingers of one hand.

Note:- I am referring to classic Op Amps such as the 741. LM358s,etc are single
power supply devices,& can be simply regarded as such.

If you need to work on a piece of equipment which does use a common point & 
+ & - voltage supplies,the  dual polarity supply is useful.

VK6ZGO

[Zero999]

I couldn't find a schematic so here's one of a dual tracking 1.25V to 15V PSU I made awhile ago.
http://www.silicontronics.com/index.php?action=ezportal;sa=page;p=7

[pablo]

an easier way to understand this (IMHO) is this:
think of a battery. a 9 volt battery has two terminals, a positive and a negative one. There is no such a negative or a positive voltage. There is just a voltage. A differential potential between the 2 terminals, just like height. There is no
positive or negative height, there is just a vertical distance between two places.
It all boils down to a reference.
In the height example, if you decide that ground will be your reference, then you may say that a hole in the ground has a negative height.
because that distance (height) between the two points is below your reference point (which is ground).

In a similar fashion, a building has a positive height because is above ground.

For electronics, there is no such a negative voltage. Again it depends where you wanna place your ground.
Going back to the battery example, you may decide to place your ground in the negative terminal or the positive one.
If you take your multimeter and place the black lead (which is the common, ground or reference in your  multimeter) to the negative terminal in the 9 volts battery and the red one in the positive terminal then you have a "positive voltage"

If  you place them in backwards then you have a "negative" voltage and a minus sign will be displayed.
Although there is no positive or negative voltage it is quite valid to express a voltage in those ways because it only tells
that is more positive or more negative compared against your reference.

In op amps it is useful to have a power supply that has a negative voltage because sometimes an application may require
that voltage must swing above or below ground.

If you place two AA batteries in series, one battery above the other, you will have twice the voltage, but each battery
will keep its rated voltage and one of them will be positive and the other will be negative to you reference (which are the terminals where the two batteries make electrical contact.)

I hope this helps.

[Joshua]

You might read through this. Starting with the 3rd or 4th replies it starts talking about negative voltage.

https://www.eevblog.com/forum/index.php?topic=3141.msg42220#msg42220