Power supplies, inject current to its output

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.

Attempting to do such a thing is likely to damage the PSU - why do you want to do something so silly?.

t is a very specific application and a little bit difficult to explain, I will try:

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 one 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 putting the probe to different levels of electrical potential, from more or less +100V to -100V. I will do 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...

It helps - but while you're actually 'collecting' electrons, I would suggest you're confusing your self by thinking of 'electron flow' in various directions (as many people seem to) instead of current.

For the purposes of electronics all you need to do is think of 'current' flowing from one point to another, so for a positive supply it flows from the most positive point to the least positive point. For a negative supply it will flow from the most negative point to the least negative point.

What sort of current values are talking about, is it a lot or just very small?.

Kepco makes several varieties of bipolar output power supplies that may do what you need.

for a positive supply it flows from the most positive point to the least positive point.

Click to expand...

Agreed.

For a negative supply it will flow from the most negative point to the least negative point.

Click to expand...

Disagree. Conventional current flows from the least negative (e.g. -6V) to the most negative (e.g. -15V).

alec_t said:

Disagree. Conventional current flows from the least negative (e.g. -6V) to the most negative (e.g. -15V).

Click to expand...

So you 'disgree', and then post something completely different to what I said?

Where did my post mention 'conventional' current?.

Nigel Goodwin said:

you're confusing your self by thinking of 'electron flow' in various directions (as many people seem to) instead of current.

Click to expand...

Actually, the current IS always a flow of electrons.

Alec_t is right. What he meant to says is that the conventional direction of current is always from the most positive potential point to the most negative potential point (from 5V to 0V, or from -30V to -50V). In other words, the conventional direction of current is opposite to the REAL direction of the flow of electrons.

Nigel Goodwin said:

...for a positive supply it flows from the most positive point to the least positive point. For a negative supply it will flow from the most negative point to the least negative point.

Click to expand...

Here your are not consistent. POSITIVE current (the conventional current) for negative power supplies (and also for positive power supplies, the polarity does really matter anyway) will flow from the least negative (= most positive) point to the more negative point.

In my case, I'm collecting electrons, which will flow to ground generating a Positive current in opposite direction to their flow. So, the electrons will always go inside the power supply through its positive terminal (So, I will always have a "positive" current). For positive output of voltage there is not problem (because the current is also positive). But for negative output of voltage the current will still be positive and the power supply will absorb power instead of deliver it. That's my problem...

Thank you for your answer , it is always interesting to discuss this things...

vortex3 said:

The experiment consist on this measurement, but putting the probe to different levels of electrical potential, from more or less +100V to -100V. I will do this with a bipolar power supply

Click to expand...

How much current must the supply handle? Several amps, several milliamps, several microamps? How rapidly must the supply respond; that is, what will be the maximum rate of change of the current from your experiment?

the range of current does from 1uA (microamp) to more or less 100mA. The velocity is not a big problem, so some milliseconds of response will be sufficient...

vortex3 said:

Actually, the current IS always a flow of electrons.

Alec_t is right. What he meant to says is that the conventional direction of current is always from the most positive potential point to the most negative potential point (from 5V to 0V, or from -30V to -50V). In other words, the conventional direction of current is opposite to the REAL direction of the flow of electrons.

Click to expand...

And as I stated, it's this that is causing you problems - I specifically didn't state 'conventional current' - only 'current', no polarity specified or required.

By considering current in that way all the 'problems' you're imagining disappear, there's no difference between positive or negative values.

Consider a very simple circuit - 9V battery, LED, resistor - draw it out with the positive of the battery at the top of the page, and calculate the LED current using just 9V (not +9V or -9V). Now turn the page upside down and calculate again - it's still exactly the same, so what advantage does confusing yourself over electron flow and conventional current flow give you?.

Both are only 'conventions' - historically everything was 'conventional current' - and in fact the term didn't even exist until electron flow started to be used, it was just 'current'.

when the output voltage is negative, the electrons go inside the power supply through its now-negative (because the output is negative) terminal

Click to expand...

But do they? Won't they be repelled by the negative potential and instead go to the most positive potential, i.e. ground?

vortex3 said:

the range of current does from 1uA (microamp) to more or less 100mA. The velocity is not a big problem, so some milliseconds of response will be sufficient...

Click to expand...

For currents this small, there is an easy way to make sure that the power supply won't be upset by forcing currents back into the supply. All you have to do is connect a resistor to the output of the supply such that the resistor draws a current larger than the current you will be injecting into the supply, a "bleeder" resistor. If you do this, you will probably have to add your own circuit to sense the current from your experiment and not rely on the current sense output from a power supply.

One problem might be that when the desired voltage is near zero, the "bleeder" resistor won't draw much current; perhaps your experiment won't supply as much current when the desired voltage is near zero.

What you want is actually what might be described as a DC coupled audio amplifier. An audio amp is able to supply or absorb current while holding the output voltage to a specific value. It has to be able to do that in order to drive reactive loads such as loudspeakers.