clarification - Page 1
Forum section: electronics / beginners

author: pete g
author_rank: Regular Contributor
timestamp: June 04, 2023, 11:57:17 pm
content: what is meant by constant current/constant voltage? what are there uses?

author: Shock
author_rank: Super Contributor
timestamp: June 05, 2023, 05:17:52 am
content: In terms of a power supply it means it regulates (maintains) either current or voltage at the same level regardless of the load. Here is very simplified led example.

author: pete g
author_rank: Regular Contributor
timestamp: June 05, 2023, 07:02:52 pm
content: thank you.

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2023, 07:19:17 pm
content: In circuit analysis, it refers to an ideal generator whose voltage is independent of the current through it, or whose current is independent of the voltage across it.In simple linear circuit analysis, a more practical two-terminal circuit can be reduced to either of these two cases, which are equivalent.1. Thévenin-equivalent circuit: an ideal voltage generator in series with a resistor to the output. The voltage of the internal generator equals the value measured with an open circuit across the output.2. Norton-equivalent circuit: an ideal current generator in parallel with a resistor across the output. The current of the internal generator equals the value measured through a short circuit across the output.The equivalent resistors in the two cases for an individual circuit are equal.In power supplies, compare the output resistanceROUTto the actual load resistanceRLOADconnected to the output terminals.IfROUT<<RLOAD, that is a good constant-voltage source.IfROUT>>RLOAD, that is a good constant-current source.For amplifiers and similar circuits, a good rule-of-thumb is to connect a constant-voltage source to a high-resistance load, and a constant-current source to a low-resistance load (or input of the following amplifier stage).

author: pete g
author_rank: Regular Contributor
timestamp: June 06, 2023, 05:17:41 pm
content: thank you for your response. i really wish i understood your explanation.

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2023, 05:59:30 pm
content: You can't have a power supply that is both constant current and constant voltage at the same time, under any particular set of conditions it will be either the voltage or the current that is the limiting factor. Constant voltage means that it will try to maintain the output voltage under any conditions up until the maximum current it can supply is reached. Constant current means it will maintain the output current up until the point at which the maximum voltage it can supply is reached. If it's configured to do both then it will cap both the voltage and the current at the set values, but one or the other will be the limiting factor at any given time depending on the load.

author: TimFox
author_rank: Super Contributor
timestamp: June 06, 2023, 06:01:16 pm
content: A specific term used with constant-current power supplies is the "compliance voltage", the maximum voltage discussed in james_s' last sentence.

author: pete g
author_rank: Regular Contributor
timestamp: June 07, 2023, 03:41:33 pm
content: hi guys, i have no problem with constant voltage supplies. what does a constant current supply look like?

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2023, 03:52:29 pm
content: Quote from: pete g on June 04, 2023, 11:57:17 pmIf you are referring to a power supply that is both CC and CV, then what they really mean is CurrentLimitedand VoltageLimited. The 'constant' here just means that instead of fusing, shutting off or folding back when the limit is reached, the supply will simply supply constant current or voltage (only one, not both) as selected, by reducing the other value as needed. So if you had a CV/CC supply and you selected 10V and 1A as your limits, the supply would put out 10V into an open circuit and then if you increased the load, it would continue putting out 10V until the load reached 1A. After that it would put out 1A as the load increases and the voltage would fall.If you are referring to a supply or circuit that only advertises CV or CC but not both, then there are additional characteristics that you'd have to know before you could describe its operation.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2023, 04:09:26 pm
content: Quote from: pete g on June 07, 2023, 03:41:33 pmI own one of these, which is a test-equipment quality constant-current supply.https://bmisurplus.com/product/keithley-instruments-inc-225-current-source/Note that the current value is set by the decade dials and the voltage compliance is set by another dial.The range switch (for maximum current value) moves the decimal-point-indicating lights.The output polarity is switched from + to standby to standby to -.The two BNCs are the terminals, isolated from chassis ground, for the current.See the manual for details, including wiring diagrams and specs.https://www.tek.com/en/manual/model-225-current-source-instruction-manual-rev

author: IanB
author_rank: Super Contributor
timestamp: June 07, 2023, 04:25:57 pm
content: Quote from: pete g on June 07, 2023, 03:41:33 pmBut there is no such thing as a truly constant voltage supply. If you try to draw too much current from your "constant voltage" supply, the voltage will sag. A better description is a "maximum voltage" supply. The voltage will increase to some maximum limiting value, unless it can't.Now apply the same concept to a "constant current" supply. The current will increase to some maximum limiting value, unless it can't.

author: Shock
author_rank: Super Contributor
timestamp: June 07, 2023, 04:37:31 pm
content: At this point you might just want to explain what you are trying to do, otherwise people are guessing. Are you buying a bench/lab adjustable power supply or designing one, or picking out a power supply for a device or circuit?If it's a bench/lab adjustable power supply, they often have a led or indicator to show they have crossed into cc mode. The manual and advertising blurb will say as well. Likely a display and controls to show and adjust the current level, but plain current limiting supplies also have them but indicators like "limit" or only an "overload" instead of cv and cc may be a clue.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2023, 04:43:22 pm
content: The fundamental difference between a CV supply and a CC supply is:1. CV supply: the feedback compares the output voltage to a (programmable) reference voltage.2. CC supply: the feedback compares the voltage across a current-sense resistor to a reference voltage.For extremely high currents, where a current-sense resistor is impracticable, a "DC current transformer" or "flux gate" can be used to sense the output current without generating a high temperature in the sense unit.In my pre-retirement laboratory, we had a custom current supply for an electromagnet supplied by Danfysik that supplied 250 A to a roughly 1 ohm load, using their own excellent current transformer.Current website:https://www.danfysik.dk/en/products/power-supplies/

author: pete g
author_rank: Regular Contributor
timestamp: June 07, 2023, 07:24:24 pm
content: thanks guys, i really think i've got a much better understanding now. again, thank you all, for your time.

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2023, 08:13:26 pm
content: Quote from: pete g on June 07, 2023, 03:41:33 pmIt tries to maintain the current you set it to. It will adjust its output voltage until the current through the load equals the set point, within the capabilities of the power supply. An open circuit would cause the voltage to rise to infinity which is obviously not possible in the real world, real power supplies will have some maximum voltage they can produce, and a real world constant current supply will increase the output voltage as far as it can until the current reaches the value you set it to. Driving a power LED is a good example of where you'd want a constant current power supply. The voltage drop across the LED will vary depending on the temperature and specifics of the LED so you regulate the current instead.