# Power supplies, inject current to its output

#### vortex3

Oct 24, 2011
3
Hello everyone,

I have a general question about a power supplies:

I gonna buy a commercial bipolar power supply to which it is possible to control the voltage/current output and also have monitoring circuits that feedback to you the real voltage/current on its output. In normal operation, when the output voltage is positive, also the output current is positive; and when the output voltage is negative, also the output current is negative. However, in my application I need to be able to read negative current when the output voltage is positive and positive currents when the output voltage is negative (in other words, the circuit will inject current on the power supply instead of the power supply inject current to the circuit).

Does anybody knows is a power supply is capable of this behavior?

Thank you very much for your help.

#### davenn

Moderator
Sep 5, 2009
14,215
ummmm no such thing as negative DC currents that I am aware of

you either have 0A of current or you have a positive amount of current.

give us some links to info on as to how you are coming up with this explanation

cheers
Dave

#### vortex3

Oct 24, 2011
3
Thank you for your fast answaer davenn,

I'm working on a physics experiments that in theory is a little bit complicated but in the principle is very simple, so I would try to explain it:

I'm using a probe call Langmuir probe that basically collects electrons from a plasma (like a cloud of particle with electric charge). If you connect this probe to Ground, when the particle are collected, they will generate a current on the cable that goes to Ground. Specifically, I'm working with electrons, so when on of them arrive to the probe, it "goes" to ground generating a positive current that goes from ground to the probe.

The experiment consist on this measurement, but you need to put the probe to different levels of electrical potential, from more or less +100V to -100V. I'm doing this with a bipolar power supply: negative terminal connected to Ground and positive terminal connected to the probe. So, I was thinking on using the current monitor circuit of the power supply to measure the current. For positive values of the output voltage there is no problem because the current will be positive (the collected electrons go inside the power supply to it positive terminal generation a positive current). The problem is on the negative region: when the output voltage is negative, the electrons go inside the power supply through its now-negative (because the output is negative) terminal, still generating the positive current on the power supply (although its output is negative). So, my question is if the power supply will be able to read this current with invert sign respect its voltage.

I'm not sure if this explanation is clear enough. If not, I will try to explain it better...

Thank you very much...

#### duke37

Jan 9, 2011
5,364
Power supplies are normally made with rectifiers so will not accept reverse current.

I expect that the currents you generate are very low so it is only necessary to load the power supply with a resistance taking more than the generated current to get a working system.

You will need a method of measuring the +/- current to your device.

#### Resqueline

Jul 31, 2009
2,848
I believe such four-quadrant power supplies do exist, but they are far between and surely expensive. I can't remember any good search term for them right now.
For that matter, I believe a DC coupled audio amplifier will do what you ask, you just have to add meters. An A-meter in series with the output will always read right.

Jun 10, 2011
443
One such supply is the HP 6826 bipolar amp/power supply; I remember using one once or twice in the early 80's. Also look for Lambda (are they still in business?) and Kepco (how about them?), as I believe they made operational power supplies.

I agree with Resqueline that the most straightforward thing would be to stick an ammeter in the circuit, as it will measure the current and the direction. Those typical power supplies, IIRC, only had analog meters. I'll let you do the web searching.

#### vortex3

Oct 24, 2011
3

I agree that with an ammeter I can read the value and direction of the current. the problem is that I think a standard power supply will be damage if it sinks current. So, I was thinking that it will be necessary somehow to decouple the voltage power supply from the current path. In this way, the power supply will only put the probe on the desired potential and the current will flow on another path where I can measure it. The problem is that the levels of current are very low (1uA to 100mA), so the measure will have a high impedance, so i'm not sure how to decouple the power supply...

One such supply is the HP 6826 bipolar amp/power supply; I remember using one once or twice in the early 80's. Also look for Lambda (are they still in business?) and Kepco (how about them?), as I believe they made operational power supplies.

Thank you daddles. Yes, the HP can sink/source power so it is something like that that I need, but I think is discontinued. Also Kepco has some models and I will be ask for some quotes. And yes, lambda is still in business and actually I'm in communication with them about this application and they told me that they have this kind of power supplies ("4-quadrant capability"), but they are very expensive and I have the feeling that it will be possible to construct one with much less money. I just have to figure it out how to isolate the voltage generator from the current path. I will think about that, any ideas??

Thank you very much

#### Resqueline

Jul 31, 2009
2,848
Ordinary power supplies won't be damaged by (moderate) reverse current flows, they simply won't keep in regulation (i.e. the voltage will rise with even a small in-current).
This is only due to them not having sink transistors (like audio amplifiers have). I don't quite understand this "isolate the voltage generator from the current path" thing.

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