What is a Constant Current Power Supply and where is it used for?

[josr]

Hi,

I bought a cheap Chinese Lab Power Supply 0-30V/10A, which has constant voltage and constant current mode.

The constant voltage is clear.

However, I can e.g. put this meter on e.g. 5V with one knob, then I can short circuit the supply and then I can set the power supply to e.g. 3A with another knob.

What does this now mean? Does this mean it maxes the output to 3A on 5V?

What is the use case of this constant current?

Thanks, Jos

[Gyro]

The current limit knob sets the maximum current that the PSU can supply, regardless of voltage. Say, for example, your circuit draws 200mA at 5V. You would set the current limit to a bit over 200mA. If you then accidentally short something, the available current will be limited to just over 200mA and minimise the damage that you do to your circuit.

Because you are trying to draw more that the current limit setting, the output voltage will collapse to whatever voltage is needed to maintain that current. If it is a short circuit, then the voltage will drop to very nearly zero, just the voltage of the resistance of the leads and your short circuit - the PSU is now in constant current mode (rather than constant voltage).

Constant current mode can be used for other things, such as charging a NiMh battery. You would adjust the current knob to give the desired charge current, and set the voltage knob to the battery fully charged voltage, so that the current will drop at full charge (that's not a perfect science by the way, just an illustration).

Current limiting to prevent or reduce accidental damage is the main use though.


P.S. Be careful trying to use constant current mode for driving LEDs and similar. Most PSUs have an output capacitor - in constant current mode, with no load, this will charge up to the voltage knob setting and then when you connect the LED it will discharge into the LED, causing a nasty current spike and probably killing it. To use constant current mode for things like this, set the voltage to zero, connect the load, adjust the current knob to the desired current and then slowly bring up the voltage until the PSU goes into constant current mode (the voltage stops rising as you turn the voltage knob further).

[IanB]

The constant voltage is clear.

Maybe not quite clear?

The voltage you set with the voltage knob is the maximum voltage that should appear at the output terminals. It is not constant, as you can see if you put a heavy load on the output. For example, if you short the output the actual voltage will drop below the voltage setting. On a proper laboratory supply, doing this should not damage it.

In a similar way, the current you set with the current control is the maximum current that should appear at the output. It, also, is not constant as you can see if you put a high resistance across the output. If you put a 1 megohm resistor on the output terminals the actual current will drop below the current setting.

On such a power supply you can plot voltage and current as two axes on a graph. There is a maximum current line, a maximum voltage line, and maybe a corner where they meet. But sometimes there is a maximum power line and then it won't be possible to have the maximum current and maximum voltage at the same time.

[retrolefty]

I think that there is some confusion for many on power supplies that offer an adjustable max current limit (most common) Vs a supply running as an adjustable constant current mode. Are these not two different animals? Am I wrong?

[IanB]

I think that there is some confusion for many on power supplies that offer an adjustable max current limit (most common) Vs a supply running as an adjustable constant current mode. Are these not two different animals? Am I wrong?

I think it is a matter of terminology and application. There is in each case a control loop that regulates the current to keep it at the set value. If the current limit is mainly for safety, then the control loop may need to be fast, but it doesn't necessarily need to be very precise. On the other hand, if the current limit is for regulation, then the control loop may be tuned for tight control and give a precise result.

Does the control loop increase the voltage until the current reaches the desired value, or does it decrease the voltage to stop the current going above the desired value? Does it make a difference, or is it just two sides of the same coin?

[retrolefty]

I think that there is some confusion for many on power supplies that offer an adjustable max current limit (most common) Vs a supply running as an adjustable constant current mode. Are these not two different animals? Am I wrong?

I think it is a matter of terminology and application. There is in each case a control loop that regulates the current to keep it at the set value. If the current limit is mainly for safety, then the control loop may need to be fast, but it doesn't necessarily need to be very precise. On the other hand, if the current limit is for regulation, then the control loop may be tuned for tight control and give a precise result.

Does the control loop increase the voltage until the current reaches the desired value, or does it decrease the voltage to stop the current going above the desired value? Does it make a difference, or is it just two sides of the same coin?

 I still think it's not the same coin. An adjustable current limit function would not raise the output voltage above the supply's set value if the load decreased it's current requirement (increased it's resistance value). It would only decrease the output voltage if the load increased it's current draw (decreased it's resistance) above the current limit setting. A true constant current mode would/could  both increase or decrease it's output voltage to maintain the current draw at a desired adjustable constant current setting.

 One mode both can increase and decrease that output voltage as required the other only can decrease the output voltage when required.

 So I still think there are two different modes often confused. I would think they would be more accurately described as constant current mode and maximum current limit mode. Most lab/bench supplies only offer a current limit mode not a true constant current mode.

Am I wrong? 

[IanB]

Well, consider a power supply when first switched on. It must start from zero, because previously it was switched off. So when switched on, it will increase the output voltage until one of two things happens: either (1) the voltage hits the upper voltage limit, or (2) the current hits the upper current limit. So whether it is a "constant current" supply or a "current limited" supply, it must always have the ability to increase voltage as well as decrease voltage.

A so-called "constant current" supply does actually have a voltage limit (the "compliance voltage"), and if the resistance of the load is too high it will hit this voltage and start operating in voltage limited mode.

The main difference is that with a constant current supply the upper voltage limit is not typically adjustable and is fixed by the design, whereas in a lab supply it is finely adjustable and regulated.

However, if you have a high quality lab power supply and you operate it at the current limit, then it really does become a constant current supply. This depends on the quality of the regulation and the control loop. It may in fact regulate better than a dedicated constant current module, which may be engineered to perform just well enough and no more.

[TimFox]

Some “CV/CC” lab supplies have two indicator lights to show which control loop (voltage or current) is limiting the output at the present time. 

[MosherIV]

I agree with retrolefty. A true CC psu is different to a bench PSU with current limit.
I have said that labeling bench psus with 'cc' is wrong and confusing for beginners.

[Gyro]

Given that the OP has said that the PSU is a "cheap Chinese Lab Power Supply 0-30V/10A", therefore SMPS, I think it is safe to assume that it is a simple current limit rather than having a precision current control loop. As I mentioned, it probably has a sizeable output capacitor for stability.