Constant Current vs Current Limit - Page 1
Forum section: electronics / beginners

author: rp58
author_rank: Newbie
timestamp: March 01, 2022, 04:06:05 am
content: I have a nice Sorenson power supply that will deliver a steady voltage within it's 20A current capability.It also will let me set a Current Limit, where it throttles the voltage to prevent the current from exceeding the limit.But sometimes what I want is Constant Current, not Current Limit. Are there power supplies that can vary the voltage appropriately to maintain a set current? All of the so-called "constant current" supplies on amazon are really just CL. Or is there an example circuit that might help me understand how to do it myself?

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2022, 04:15:03 am
content: In simple terms there is no difference between constant current and current limit.If you take a "constant current" supply and leave the terminals open circuit, the current will be zero. Therefore the current is not constant. It is variable between zero and the desired set value.In the same way, the power supply is not "constant voltage" either. If you short the output terminals the voltage will also be zero. So in voltage mode, the power supply is a voltage limited supply, where the voltage is variable between zero and the set value.

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2022, 04:24:25 am
content: Quote from: rp58 on March 01, 2022, 04:06:05 amIn other words, it is throttling the current to prevent the voltage exceeding the limit.QuoteThis is the inverse situation to the voltage limited case.QuoteThis is exactly what the Sorensen power supply is doing.To make it a "constant current" supply, you need to set the current limit to what you want and then increase the voltage limit to a suitably high value. After you do that, the supply will vary the output voltage appropriately to maintain the set current.

author: rp58
author_rank: Newbie
timestamp: March 01, 2022, 04:36:45 am
content: I see. Because I've been experimenting with batteries, I didn't set the voltage high enough to see that behavior.

author: TimFox
author_rank: Super Contributor
timestamp: March 01, 2022, 04:41:34 am
content: The two terms are approximately synonymous, but usually a "current limit" is not exact, and only exists to limit the current to a safe value.Similarly, "constant current" is a more precise stabilization to maintain the current at a precise value (within the limit of the available voltage).A "crossover" supply has one setting for voltage, and another for current, and whichever one limits the output voltage often has a light to indicate which limit is active.

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2022, 04:45:02 am
content: Quote from: rp58 on March 01, 2022, 04:36:45 amBe careful connecting power supplies to batteries. If the power supply does not have sufficient output voltage when the battery is connected the battery could "back feed" the supply and might possibly damage some of the internal components. This depends on the level of protection provided by the power supply to guard against this.

author: aheid
author_rank: Regular Contributor
timestamp: March 01, 2022, 04:45:28 am
content: One can think of it like this: it has a voltage limit and a current limit. It won't exceed either.Thus the voltage won't go above the voltage limit and the current won't go above the current limit. It will operate at either limit at any given time, depending on the load at any moment.If it is operating at the voltage limit it acts as a constant voltage supply, conversely if it operates at the current limit it acts as a constant current supply.

author: rp58
author_rank: Newbie
timestamp: March 01, 2022, 06:27:13 am
content: Right. I have a big diode in series. I use an old transformer power supply when I want to apply a reverse charge.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 01, 2022, 08:45:45 am
content: Quote from: IanB on March 01, 2022, 04:15:03 amI disagree.Constant current power supplies have a dedicated error amplifier that keeps the current fairly constant within the compliance voltage.A digital or analog interface allows the user to set both the output voltage and the output current and the device switches between the two modes according to the load value.Current limited power supply are not usually adjustable and current limit is not well defined. In some cases, once the current limit is reached, the current will decrease further (foldback) in other cases the current will increase as the load decreases. In some (rare?) PSU design the limit might even make the PSU output switch off requiring a reset to restore it.Seehttps://xdevs.com/doc/HP_Agilent_Keysight/HP%2090B%20Application%20Note.pdfpage 10 and 11.

author: macboy
author_rank: Super Contributor
timestamp: March 01, 2022, 04:29:28 pm
content: Some argue that lab power supplies with adjustable current may be considered to be constant current, but I prefer to think of it as an adjustable current limit.A true constant current power supply has a very high (ideally infinite) effective output impedance. That allows the current to remain constant even if the load impedance changes. By contrast, a constant voltage power supply has a very low (ideally zero) effective output impedance. This allows the voltage to remain constant even if the load impedance changes.The problem is that lab power supplies are designed to operate well as voltage sources, and have some capacitance connected across the output, to reduce the output impedance. This implicitly compromises their ability to act as good constant current sources. Imagine a lab supply set to 50 V, 1 A and connected to a 25 ohm resistor. It will be operating in "constant current" mode, delivering 1 A and 25 V across that load resistor. Now connect a 1 ohm resistor in parallel to the other one. An ideal current source will not present a current spike to the output, but with a typical lab supply, this will result in a very brief 25 A spike through the 1 ohm resistor, as the output capacitor discharges through it.Some companies did produce true lab "current sources". I have an old HP 6186C Precision Constant Current Source which is awaiting repair. It has variable current and variable "compliance voltage limit" similar to a typical lab supply, but specifically does not have capacitance across the output.

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2022, 07:04:37 pm
content: That was a test that I did, by setting a lab supply to 10 mA limit and 10 V, and then connecting an LED across the output. Ideally it will not blow up the LED.I think I did this test successfully on my TTi supply in the past, but I don't recall exactly.[Edit: Here is the test that I did:https://www.eevblog.com/forum/testgear/tti-el302p-power-supply/]

author: mawyatt
author_rank: Super Contributor
timestamp: March 01, 2022, 07:17:58 pm
content: Think the separation between CC and CL is if the supply maintains a relative high output impedance for dynamic loads, since most conventional "lab supplies" have a significant output capacitance for low output impedance as designed basically as CV supplies, then they can't maintain a high output impedance under CL conditions for dynamic loads, and thus perform poorly as CC sources with dynamic loads.Best,

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2022, 07:34:51 pm
content: Since lab supplies tend to have a defined "cross-over" between CV mode and CC mode, is it possible for the regulation circuit to have an active low output impedance in CV mode, and an active high output impedance in CC mode?

author: TimFox
author_rank: Super Contributor
timestamp: March 01, 2022, 08:38:46 pm
content: Yes, the DC output resistance in constant-current mode can be quite high, but many lab supplies have a large capacitor across the output terminals that decreases the output impedance at higher frequencies.Many years ago, I used a Kepco linear power supply to power a laboratory electromagnet, using their application note with an external shunt resistor to produce an excellent constant-current supply.

author: mawyatt
author_rank: Super Contributor
timestamp: March 01, 2022, 09:16:37 pm
content: Try this experiment, take a lab supply and set CL to say 5ma, and set the CV limit to say 30V. Then take an LED with a 1K ohm series resistor and connect a scope across the resistor. The current thru the resistor will be I=Vscope/1K. Now connect the LED and resistor to the lab supply and note the scope exponential decay pulse. The peak will likely be ~27ma and decay to ~5ma with the supply output capacitance time constant with a ~1K load impedance, certainly not true Constant Current which would limit the current to 5ma in all cases. Now remove the 1K resistor and repeat, you likely now have a dead LED!!!Point here is that CL can be very misleading for dynamic loads, and should not be considered True CC Mode. This can and will destroy sensitive semiconductors if neglected and assumed a True CC. To mitigate this effect one should always carefully set the CV level to slightly above the expected usage, so the peak current under dynamic conditions doesn't exceed the limits of the devices involved.Best,

author: TimFox
author_rank: Super Contributor
timestamp: March 01, 2022, 09:53:55 pm
content: Yes, in conventional laboratory power supplies that capacitor can be large.For similar applications, such as testing Zener diodes, I use a classic Keithley 225 "Current Source", capable of 100 mA at 100 V maximum compliance.It crosses over to a less-accurate CV mode using single-turn compliance pot.It can source or sink current.There is a 1 uF capacitor that can be switched across the output ("Filter ON"), limiting the output impedance to 1 uF in parallel with > 1010ohms, which Keithley recommends for inductive loads up to 100 mH.Otherwise, there is no discrete capacitor across the output.With "Filter OFF", the load current is constant (to +/- 0.005%) from 0 to full load (except on lowest range, 0.1 uA, for +/- 0.05%).On 100 mA full-scale, the DC output impedance is 20 megohms, up to 2 teraohms on 1 uA and 100 nA full-scales.With "Filter OFF", the output capacitance is not specified but the modulation capability (transformer coupled input) useful from 50 to 500 Hz.

author: jh15
author_rank: Frequent Contributor
timestamp: March 04, 2022, 02:41:44 am
content: Decades ago when repairing and then performance testing (used to be Agilent - used to be HP -now Keysight) cc/cv power supplies, there was a user option to disconnect the large capacitor on the output. It was on the same barrier strip as the sens leads, just for this purpose.