EEVblog 1688 - Constant Current Sources EXPLAINED + DEMO - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: May 28, 2025, 07:05:13 am
content: 00:00 - Constant Current Sources11:08 - Practical uses of constant current sources14:00 - Circuit examples19:13 - TL431 Example20:25 - LM317 CC circuit21:42 - Low Side Source vs High Side Current Sink24:19 - Bench examples26:24 - Your PSU is a CC generator!28:04 - Keithley 225 Constant Current Source31:09 - LED strip example of Compliance Voltage

author: vcvs
author_rank: Contributor
timestamp: May 29, 2025, 03:30:19 pm
content: How would the top circuit (high-side current source) be realized in practice? In particular I’m thinking about how to make the three terminal device which tracks the reference voltage difference. A differential amplifier followed by an opamp? Suggestions for resources about this would be appreciated

author: thm_w
author_rank: Super Contributor
timestamp: May 29, 2025, 08:41:56 pm
content: Quote from: vcvs on May 29, 2025, 03:30:19 pmYou can use premade part like LT3080:https://electronics.stackexchange.com/questions/473288/how-to-choose-the-series-pass-transistorMaybe relevant:https://electronics.stackexchange.com/questions/381525/how-to-calculate-this-current-limit-circuithttps://electronics.stackexchange.com/questions/692166/how-to-understand-current-limiter-circuithttps://ecstudiosystems.com/discover/textbooks/selected-circuits/ac-to-dc-power-supplies/current-limiting-circuit/Not sure what you are asking though, how to make a three terminal linear regulator?

author: EEVblog
author_rank: Unknown
timestamp: May 29, 2025, 11:52:42 pm
content: Quote from: vcvs on May 29, 2025, 03:30:19 pmLiterally any PSU constant current limiting circuit implementation.As thm_w said, the LT3080 is literally this, and was used in my early uSupply design video series.Then the latest uSupply design did it more discretely, and put the voltage regular after the current regulator.And there is the P DC PSU handbook:https://www.changpuak.ch/electronics/PowerSupply/HP_AN90B.pdfBut there are probably a hundred ways to implement current limiting, which is why I just explained it generically as basically measuring the voltage across the shunt, compare to reference, drive pass transistor with opamp.

author: JoeyG
author_rank: Regular Contributor
timestamp: May 30, 2025, 12:42:45 am
content: Voltage to Current SourceVarying the current on Dave's current sources often require a variable resistor to adjust the current.I'm making a VI curve tracer and I wanted to vary the current from a mcu dac. IE Voltage to current source.I found some app notes from ADI however in the end I found an article in Wireless World that uses some transistorsto make a voltage to current source (Wireless world magazine articles herehttps://www.worldradiohistory.com/Wireless_World_Magazine.htmYou can see my LTspice modeling of the Wireless World article here.The Q4_c (the PNP collector) drives the load (R2_Load)

author: vcvs
author_rank: Contributor
timestamp: May 30, 2025, 06:12:51 am
content: Cool, thanks for all the links!My confusion was about that the amplifier in the middle of the generic figure both measures the voltage drop and tracks the reference voltage I was thinking about how to implement that part discretely with op amps.

author: jh15
author_rank: Frequent Contributor
timestamp: May 30, 2025, 06:21:26 am
content: Wish I could remember my learning in transistor circuit approximations. The teacher would draw a simple xistor current source, then apply it to circuit examples. One could pick any part of time in a dynamic waveform, and go from there. Just as i got the hang of it, digital circuits dominated.I recently got an old book on transistor circuit approximations, and it has become Greek to me. Wish i had my notes (was done on something called paper back then).For EE stuff, that would bring back the original attraction of EEVblog. I have had no problem with the recent years, but seeing EE stuff when you started was exciting..

author: JoeyG
author_rank: Regular Contributor
timestamp: May 30, 2025, 07:07:03 am
content: Just learn LTspice and create some simulations

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2025, 07:18:22 am
content: The linked video from Wireless world has another related circuit element: a current mirror. That is a current controlled current source. It is a common element inside ICs, but can be a bit tricky with discrete transistors where the matching and thermal coupling is not that good.

author: thm_w
author_rank: Super Contributor
timestamp: May 30, 2025, 09:36:15 pm
content: Quote from: vcvs on May 30, 2025, 06:12:51 amYou can have the current read by a first opamp fed into a second opamp, which compares against the reference voltage.https://www.circuitlab.com/circuit/6vnmv5/opamp-current-limiter/

author: tppc
author_rank: Regular Contributor
timestamp: June 01, 2025, 07:41:48 pm
content: Hi Dave,Thanks for this video, very instructive for young players.Correct me if I am wrong, but I think you should reverse the OP amp inputs on your High side regulated current source model.BTW : No calculators that change model under the white board this timeHowever markers change their color and position.

author: EEVblog
author_rank: Unknown
timestamp: June 02, 2025, 12:15:29 am
content: Quote from: tppc on June 01, 2025, 07:41:48 pmThat was deliberate. As I mentioned, this is not an exact practical circuit example, and I explained how I was effectively showing a differential amplifier across the shunt resistor, the left side of which would be positive.

author: tppc
author_rank: Regular Contributor
timestamp: June 02, 2025, 08:36:20 pm
content: OK, sorry for this.Thanks for your patience.

author: rvalente
author_rank: Frequent Contributor
timestamp: June 09, 2025, 12:39:00 pm
content: I've got curious on this cute CC Dummy load, could not find any reference on google.

author: temperance
author_rank: Super Contributor
timestamp: June 09, 2025, 01:09:28 pm
content: QuoteDepending on the required accuracy you need thermal coupling between Q3 and Q4. This will be difficult to achieve because they don't dissipate the same power.There are better options if you need better accuracy:https://www.ti.com/lit/an/slyt768/slyt768.pdf?ts=1749420278451&ref_url=https%253A%252F%252Fwww.ti.com%252Fsolution%252Fanalog-output-modulePage 5 for example.

author: thm_w
author_rank: Super Contributor
timestamp: June 09, 2025, 09:10:08 pm
content: Quote from: rvalente on June 09, 2025, 12:39:00 pmIt was from mailbag:https://www.youtube.com/watch?v=Xfs0dglIVOM

author: jirij
author_rank: Contributor
timestamp: June 13, 2025, 12:19:56 am
content: One thing I don't think was mentioned in the video (and could go into one of the "common beginner mistakes" compilations) is that many (most?) CC PSU implementations do the limitingbeforeany output capacitors. This is also the standard for any current-measuring PMICs that typically have a few small MLCCs near the chip (presumably for fast transient response on the feedback pin), then a current sensing resistor, and only then the big electrolytics.In practice, if you want to test a ~3V LED at 20mA, you short the terminals, dial in 20mA, open the circuit, then ramp up the voltage to "good enough" 5V because CC will save it, andthenyou connect the LED, you get a big spark and a dead LED....OTOH youcandrive LEDs from voltage-controlled PSUs because, as you increase the DC voltage, any parasitic effects (like ripple) become less relevant. I've built a custom 17-LED lamp based on careful V/I measurements of the specific LEDs I wanted to use (~2.74V @ 110mA) and am running it on a CV 48V Meanwell PSU with the trimmer pot adjusted down to a voltage low enough where the LED heatsink stays below ~50C.Yes, a LED Vf drops with higher temperatures (and they get brighter / draw more current as they approach 50C), but it's still surprising how little effect +- 1V has. Even if the PSU was wildly inaccurate, the LEDs would not burn up.Is it advisable? No. But I can have a switch near the lamp head and the lamp doesn't self-destruct when I turn it on.(Yes, the best solution is a CC PSU with voltage set to just a few volts above, and the LEDs don't mind an inrush current that's 2-3x higher than the operating one. Most "LED driver" PSUs have TL431 or zeners clamping the output voltage and changing them is easy, as long as the PSU is not in a brick of silicone.Oh well, in-line resistors to the rescue.)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 13, 2025, 06:44:03 am
content: If a LEDs likes voltage drive depends on the LED. Blue and white LEDs are usually OK as they have quite some internal series resistance. Especially older red LEDs tend to show thermal run-away and will likely get too much current.The note about the output capacitor is good. Many of the lab supplies have a surprisingly large capacitor (like 1000 µF range) at the output. In addition (often the lesser problem than the capacitor) that can be a delay in the cross over from the CV to CC mode in the controls. This kind of acts like a simulated extra capacitor.

author: jh15
author_rank: Frequent Contributor
timestamp: June 15, 2025, 04:44:16 am
content: The output capacitor in the HP supplies i use, you could disconnect the output capacitor that was effectively on the output terminals. Set the voltage and current limit, maybe shorting the output terminals, and think you would be limiting the overall current at start. If the capacitor was involved, you could get a large current spike into the load.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2025, 04:53:05 am
content: Many supplies need at least some capacitance at the output to be stable (not oscillate) at all. More capacitance may be needed to reduce the overshoot in the CC to CV transition or with load steps.

author: jh15
author_rank: Frequent Contributor
timestamp: June 15, 2025, 05:01:59 am
content: That is why the default configuration is that way. The option for the engineer is made available easily.