# Another power supply question, bi-polar

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#### Cobalt60

##### New Member
Well, Im just trying to figure out how every little piece of a power supply works exactly. Right now, I am curious about the term "bi-polar power supply". What little understanding I have of these is they they have both a positive polarity and a negative polarity, in addition to a ground, as compared to what I suppose would be called a "uni-polar power supply", which would presumably have only a ground and either a pos or neg.

So, both linear bench power supplies I use have both pos and neg outputs, in addition to ground, so would I be correct in assuming that those are both bi-polar?

As far as design, would, for example, my 0-36V variable power supply actually in some sense be a -18V and +18V power supply? I also have to ask at what design stage this comes about, I am thinking it begins with the transformer, but basically Id like to understand the entire design.

If my assumptions are correct, this means that on, for example, my Power Designs 3650 (0-36V), I could use the pos and ground to achieve a max output of 18V, or neg and ground for -18V. I am not entirely sure of the motivation to do such a thing but by all means Id love to learn some appropriate uses.

Thanks for teaching me

-Chris P

##### New Member
A 0-36 variable suppy mans you get a max. supply of +36V.
Your bench power supply probably has three knobs. Red, Black and Green(?)
Red for positive voltage, green for negative and black for ground. In this case the voltage varies from the negative maximum voltage to 0 to positive maximum voltage.

#### Cobalt60

##### New Member
My one I have here at home is a Power Designs 3650. The pos is red, and both the neg and gnd are black. Its voltage is adjustable between 0 and +36V. My one at work may have green for negative, I cant remember at the moment, but that one too adjusts between 0 and about +31V.

On both, their advertised voltage range is achieved by using pos and neg, and leaving ground unused. Actually none of us at work really know what the ground terminal is used for... So far every use I have had for a power supply is the equivalent of using it in place of a battery.

##### New Member
Oh is it? Well, its always safer to use the ground. There is a meter(indicator for voltage) right? Does the scale vary from 0 to the max. value or does it include negative scale?

#### Cobalt60

##### New Member
Well I just physically tested a DC fan using the pos and gnd, no power. So...

#### crutschow

##### Well-Known Member
The ground on a single output (unipolar) supply is connected to the case and the ground prong on the power cord. It is mainly for safety and, in some cases, to provide a high frequency ground path. It is normally isolated from any of the supply circuitry (you can check that with an ohmmeter). Thus you can not get +18V and a -18V from a single 36V supply.

##### New Member
Well I just physically tested a DC fan using the pos and gnd, no power. So...
U mean the fan didn't run? Funny! Well here(in India) we've always used Red and Black. Infact the lab attender threatened us and asked us not to touch the green knob

#### Cobalt60

##### New Member
The one I have here has an analog gauge, 0 through 36V, as well as 0 through 5A. My one at work has a digital display, I dont think Ive ever seen it show a negative reading, and at work we all use the neg and not ground. So far I have not noticed much of a functionality difference between the two power supplies.

##### New Member
Check out the meter range. It'll give you an idea if your supply is 0-36V or -18V to +18V

##### Banned
Is the bench supply isolated? The + and - leads would be for the power supply relative voltages, the GND connection is just there if you need a refrence ground, probably goes to saftey ground on the wall outlet, and the case itself as well.

##### New Member
The one I have here has an analog gauge, 0 through 36V, as well as 0 through 5A. My one at work has a digital display, I dont think Ive ever seen it show a negative reading, and at work we all use the neg and not ground. So far I have not noticed much of a functionality difference between the two power supplies.
Hmph.. well you sure the two knobs are black? My friends and I were getting confused with the colours in school... sometimes they use a very dark bottle green-blackish colour for the green knob

As far as I know.. when the black and the green knobs are used then your supply provides you with negative voltage...

#### Cobalt60

##### New Member
My bench supply has ground, negative, and positive terminals, but is only advertised as a 0 through +36V supply, is this a bi-polar supply?

And yes, the fan did not run. To get power on this supply I must use the pos and neg terminals. However, the power supply is old and I am not 100% sure that it is fully functional.

##### New Member
However, the power supply is old and I am not 100% sure that it is fully functional.
Now you didn't mention that one!

#### Cobalt60

##### New Member
So are my bench supplies bi-polar or uni-polar? Somewhere internal to the design is there indeed a -18V and +18V?

And indeed the fan did not work when connected to pos and gnd. I suppose if the grounds are isolated that would make sense. And my supply here is definitely a red and 2 black, at work I just cant remember.

So how exactly would I use a "reference ground"?

##### New Member
I really can't work out the range!!! (I've very poor practical knowledge, sorry). I really hope someone helps you Try this... (I hope its safe) Turn your voltage varying knob to its maximum, use a DMM and check its voltage... repeat this with red-black, green-black combinations.

##### Banned
Do you understand the concept of a floating voltage source? It's just like a battery. A battery's negative terminal obviously isn't electrically connected to your houses ground but it still provides a voltage relative to it's - side from it's + one. This does NOT have to be the same as true ground, which is what the GND connector on your bench supply is for, to provide this ground reference if you need it. In typical bench supplies the primary step down transformer provides the isolation, so in theory if you wanting a 'bipolar' supply you could connect two of your 36volt supplies together + to - of the other. You'd get a 72 volt supply and the point between the two could be grounded. This is assuming you bench supplies are isolated that way.

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#### MikeMl

##### Well-Known Member
...
On both, their advertised voltage range is achieved by using pos and neg, and leaving ground unused. Actually none of us at work really know what the ground terminal is used for... So far every use I have had for a power supply is the equivalent of using it in place of a battery.
The supply delivers its regulated output between POS and NEG. The supply is built such that there is no connection (only slight leakage) between either POS/NEG and GND. The GND terminal is connected to the center prong on the three prong 120Vac line cord (earth ground).

There are three posibilities:

Tie NEG to GND. Supply produces positive set voltage relative to earth ground on the POS terminal.

Tie POS to GND. Supply produces negative set voltage relative to earth ground on the NEG terminal.

Don't tie GND to anything. Supply produces set voltage between POS and NEG. The supply floats relative to GND (earth ground). The actual voltage measured between GND and either POS/NEG will be determined by whatever the supply is tied to...

Your supply is NOT a Bipolar supply!

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##### Banned
+36-xxx+36-

If you GND the xxx connection you get +36 GND and -36 supply. If they're not isolated, you'll get a lot of smoke.

#### Cobalt60

##### New Member
The 3650 I have here uses a variac (presumably instead of a regular transformer). I wouldnt mind doing some crazy things like tapping directly to the variac to have the option of a variable AC output. But basically is the idea of making this a bi-polar supply just not going to work?

Thanks for all the help.