why didnt I think of this current source? - Page 1
Forum section: electronics / projects

author: Terry Bites
author_rank: Super Contributor
timestamp: May 21, 2025, 03:45:56 pm
content: Well, because I'm not Stephen Woodward!Every so often you see a beautifuly simple circuit. This current source/sink just does the business with no pain.www.edn.com/precision-programmable-current-sink/If you don't already, keep an eye on this EDN column. The complete antidote to monochrome digital boredom.

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2025, 03:58:17 pm
content: By the time I retired, I had tired of schematic diagrams comprising only rectangles connected to busses.This article was refreshing.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2025, 08:10:48 pm
content: Quote from: Terry Bites on May 21, 2025, 03:45:56 pmFor some values of 'precision'It relies on thedifferencebetween two 431's for the offset, and has a minimum load requirement.Simpler to just use a single Opamp, which can come with a much lower offset error.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 22, 2025, 08:31:30 am
content: Yes, TL431 is not metrology grade, and under 1mA doesn't work too good, if at all, but it can work well enough for certain applications.For maximum performance of TL431, the application notes will recommend about 10mA through the anode-cathode, and 1mA through the resistive divider of the Vref (but it can work with less, just that the regulation won't be as good as for 10mA). A capacitor in parallel with the anode-cathode may make TL431 unstable for certain values, there is an U-shape stability plot in the datasheets.For battery discharging and such, works OK. I am using TL431 in a similar manner (as a controlled power Zener, as seen in a datasheet, though without PWM).Fromhttps://www.eevblog.com/forum/projects/cc-cv-power-supply-as-a-contolled-load-for-batteries-(i-e-rigol-dp832)/msg5744693/#msg5744693DP832 is a normal CV-CC power supply, that can only source, but can not sink.Using a TL431 as a power Zener is a trick, such that any normal CV-CC power supply can be used to discharge batteries. The advantage is that DP832 can log V and I on 3 individual channels at once, which makes possible to plot the I/V curves, and to calculate the measured mAh (or Wh) for 3 individual batteries at the same time.The PSU is set for constant current (the battery discharging current), and when the battery voltage becomes too low (value programmable from the DP832), the PSU voltage will increase, and this will trigger the PSU overvoltage protection, which will disconnect the battery, so to not overdischarge it.The power transistor (on a radiator) is used not only as a regulator, but also as a load, to dissipate the energy stored in the battery under test.With batteries and discharging currents between 3mA..3A it works OK. I did the power Zener circuit 3 times, one for each DP832 channel. The 3 power Zenner were made galvanic isolated from each other, so they can be tied in series, to discharge batteries up to 15V and 3.2A. No stability problems so far (when used with batteries as a load).Don't know how well the circuit will behave with inductive or capacitive loads (probably not very good, but I didn't try).

author: Terry Bites
author_rank: Super Contributor
timestamp: May 22, 2025, 04:02:48 pm
content: Yes, joyless, artless boxes. Best left to the 01 zombies.