Coming to terms with electrical terms - Constant Current. - Page 1
Forum section: electronics / beginners

author: tifkat
author_rank: Regular Contributor
timestamp: April 09, 2021, 01:48:58 am
content: Hi all. In my understanding, when a power supply specifies a current 'capability', it means it can safely supply UP TO that amount of current. The powered device however can draw any PORTION of that, and things will be dandy.With respect to Constant Current, I've seen a couple of different uses of the term, and I'm not 100% confident I am fully educated on the uses of this term.I have seen circuits which will 'pull' a constant current or voltage, for example to keep a regulator active. And I've seen power supplies with CV and CC modes which means that the voltage you set will be maintained, until you reach the "current cap", and then it stops current flow from exceeding that cap by dropping the voltage.But I see things like LED strip power supplies being sold as constant current. Is this a similar use of the term? The driver on board will supply UP TO, but will not exceed a certain current? Will it sacrifice the voltage level in order to maintain a maximum current flow?My question arises from the need for a 220/230/240v AC to 8v-18v (~12v) DC source to drive a panel meter, and I was wondering if one of these cheap LED constant current power supplies would be suitable.Example LED constant current power supply(1-3W MR16 Low Voltage Power Supply LED Driver Convertor Transformer Constant Current 300mA DC 12V.)Example panel meter(YB5142DM AC 0-500V Digital Voltmeter Ammeter 0-20A Voltage Current Wattmeter Energy Frequency Power Meter)

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2021, 02:36:03 am
content: "Constant current" means that the power supply will automatically adjust its output voltage as needed to maintain a set current being pushed through the load. Obviously this comes with limits, delivering constant current into an open circuit requires infinite voltage which is obviously not possible. In practice most of these supplies will deliver a constant current into anything from a short circuit all the way up to the maximum voltage the supply can produce. This is useful to prevent damage due to a short circuit or miswired component in something you are testing, and it is also useful with current driven loads like LEDs. If you connect an LED to a constant voltage, it will draw nothing until the voltage rises to the forward drop of the LED and then if the voltage rises further the current will dramatically increase until something fails. It's also useful for charging batteries, you set the current to a safe level that the battery can accept and then the voltage will rise as the battery charges up. A constant current will keep the voltage right where it needs to be in order to keep the current through the load constant.

author: TimFox
author_rank: Super Contributor
timestamp: April 09, 2021, 03:10:06 am
content: Terminology:For a constant-current source (supply), the maximum output voltage is called the “compliance”. As stated above, the source should supply a constant current into an external load. Increasing the external resistance will eventually result in the voltage hitting the compliance limit, after which the voltage is stuck at that level and the current will fall as the resistance increases further.

author: tifkat
author_rank: Regular Contributor
timestamp: April 09, 2021, 06:07:04 am
content: Quote from: james_s on April 09, 2021, 02:36:03 amAnd this current rate is fixed by the circuitry in the supply is that correct? If the powered device doesn't need that much current, will it push it through anyway? Will it raise the voltage higher (possibly to a dangerous level) to try to get that current to flow through?

author: Ranayna
author_rank: Super Contributor
timestamp: April 09, 2021, 07:14:36 am
content: As far as i have understood it, the supply will increase the voltage until either the desired current is reached, possibly to a level dangerous to the circuit, or up to the set compliance voltage, or up to it's capabilities.Therefore it is important to properly set the compliance voltage correctly.

author: Dave
author_rank: Super Contributor
timestamp: April 09, 2021, 09:07:22 am
content: Quote from: Ranayna on April 09, 2021, 07:14:36 amThat's correct. It is worth noting that you're not gonna be able to adjust the compliance voltage on a bog standard LED supply, you get what you get.Quote from: tifkat on April 09, 2021, 06:07:04 amSince we're talking about exact terminology here, it is worth noting that we're talking about the current value, not rate. The rate defines how quickly something is changing with respect to time, but we're talking about a fixed quantity here.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 09, 2021, 05:38:03 pm
content: A picture may help here

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2021, 05:58:00 pm
content: Quote from: tifkat on April 09, 2021, 06:07:04 amYes, if it's a true constant-current power supply. Most bench power supplies with a constant current mode also allow you to set the voltage, which is the maximum voltage it will output under any conditions. Once the voltage hits that maximum, the supply goes into constant voltage mode and stops trying to maintain the set current by further increasing the voltage. It depends on the supply.

author: tooki
author_rank: Super Contributor
timestamp: April 12, 2021, 07:27:20 pm
content: In a bench power supply, both the set voltage and current limits are limits, but only one is the limiting factor at any given time: most of the time, the set voltage limits the output voltage. But if the current exceeds the limit, then it takes over and becomes the limiting factorfor the voltage.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2021, 09:32:08 pm
content: It’s important to remember that you are not allowed to specify both the voltage across the load and the current through it simultaneously. As tooki points out, you can set the limits on each, but only one will govern (depending on the load). Typical bench supplies “stabilize” the voltage accurately, but “limit” the current (for safety, etc.) less accurately. A good lab supply can have similar accuracy and stability on both channels, often with indicator lights to tell you which channel is governing (CV or CC). Specialized lab supplies are designed for use as “constant current” units, with a less-accurate setting on the compliance voltage (again, needed for safety of the load or operator). LEDs need a constant-current drive, since if you apply a constant voltage, the current will increase as the LED heats up, and it may “run away” to destruction. With a constant current, the voltage across an LED will decrease, so there is no thermal runaway.