Precision voltage controlled current source? - Page 1
Forum section: electronics / general

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 08:07:18 am
content: Hi,I'm looking for a (preferably off-the-shelf) precision voltage controlled current source, preferably with switchable gain, for precision driving of LED current from a lock-in amplifier. I've found the SRS CS570 / CS580 instruments which look great - but does anyone know of a more economical solution? I don't have a noise / linearity spec yet, except for 'precision'. Max current needs to be 20mA and driven from a 0-5V source.TIA, Alan

author: kridri
author_rank: Contributor
timestamp: August 14, 2018, 08:21:20 am
content: I recently had the same problem.I needed to drive a DUT with a sinusoidal current(0.1 Hz to 5kHz, amplitude 40mA max) while locking onto a nth harmonic.Due to limited budget I needed to fabricate something by myself. I used an AD524 (instrumentation amplifier) in conjuncture with a AD711 (opamp) and resistor.The LT1010 was used as an buffer to supply the higher currents (tests showed a current untill 100mA without noticeable distortions.).The AD524 can be programmed to a gain of 10,100 and 1000 by shorting a couple of pins thus an easy way of programmable gain.The transfer function depend of the chosen value of R1 and the programmed gain. (Iout/Vin = G/R1)RegardsKris

author: Zucca
author_rank: Unknown
timestamp: August 14, 2018, 08:41:15 am
content: alanambroseMaybe you want to consider this:https://www.eevblog.com/forum/buysellwanted/zucca-sales-(d)-keithley-220/overkill?

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 09:02:16 am
content: @kridri - thanks, that looks great. We would rather buy rather than build if we can, but I see that with a bit of care the build wouldn't be so hard, at least for an uncharacterised first cut.@Zucca - looks a v. nice instrument but it's not voltage controlled?Alan

author: Zucca
author_rank: Unknown
timestamp: August 14, 2018, 10:18:18 am
content: No,but I can sell you a 34401ahttps://www.eevblog.com/forum/buysellwanted/zucca-sales-(d)-two-venerable-34401a-(calibrated-in-2014-with-papers)/and then you can talk to the K220 over the GPIBhttps://forums.ni.com/t5/Instrument-Control-GPIB-Serial/Setting-up-a-GPIB-communication-with-Keithley-220-and-HP-34401A/td-p/3808237http://physics.oregonstate.edu/~tatej/TateLabWiki/lib/exe/fetch.php?media=manuals:220_900_01c.pdfLike a Pro, super precision!Moreover you can be very flexible in future developments, anyway don't want to make your homework.The downside is the latency/delay over the chain..... don't know the lock-in amplifier voltage output bandwidth...I make you a good price!

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 10:48:40 am
content: I think I'm looking for an 'analog' solution here (I appreciate the SRS CS580 is digital domain).{edit - duh, this is wrong, the SRS 830 lock-in is digital domain, the CS 580 is analogue with some digital UI which switches off when actually running}In case anyone else is looking at this in the future, here's a quick summary of my research including kridri's design:http://anagram.net/nuts/VCCS/Voltage Controlled Current Source review.pdfI have not found anything off-the-shelf yet except for the SRS unit.A.

author: Zucca
author_rank: Unknown
timestamp: August 14, 2018, 11:22:59 am
content: I understand,well since we have the schematics you could revense eng. the K200 and build your interface...My suggestion is to try first with your analog circuits and if you are not happy then jump in the hyperspace with some other proper equipment.the SRS CS5xx are so expensive... WOW.

author: kridri
author_rank: Contributor
timestamp: August 14, 2018, 01:38:27 pm
content: Quote from: alanambrose on August 14, 2018, 10:48:40 amAlan just to become clear my design was based upon a datasheet example (p18, fig 46). I only added the current buffer to increase the amount of current that could be supplied. I also want to make clear that the schematic I posted is a conceptual block diagram. If you will build it same changes need to be done.Kris

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 14, 2018, 02:34:51 pm
content: Hi,Here is an idea for you, not off the shelf, but simple and uses readily available parts. You can use better parts to get more 'precision'.The first amplifier inverts the reference voltage. The 250 Ohm resistor converts voltage to current. The second op-amp forces the current in the LED, equal to the current in the resistor. BJTs are used to increase the buffer the op-amps.Model is attached.Regards,Jay_Diddy_B

author: mzzj
author_rank: Super Contributor
timestamp: August 14, 2018, 03:05:04 pm
content: Industrial mA loop transmitters may or may not fit your bill, ie:http://www.prelectronics.com/products/multifunctional

author: texaspyro
author_rank: Super Contributor
timestamp: August 15, 2018, 12:23:17 am
content: Valhalla makes some current calibrators that are just V-I converters. I have three different models (up to 20A output). I'm away from the lab and don't remember the model numbers.

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 15, 2018, 04:25:52 pm
content: @mzzj - Thanks for the thought about the 4-20mA loop transmitters, and I see the PR Electronics 4114 / 4501 are actually quite cheap.@texaspyro - The Valhalla 2555A Transconductance amp is fantastic, 100A, nice. Although I can see it is meant to handle low currents too, it may be a but too brutish for our LED drivingI can't that their more gentle calibrators have transconductance - maybe I missed something.@Jay_Diddy_B- thanks for that alternative design, v nice.We're coming round to the idea that we'll have to build - I'll keep you posted for anyone else looking at this in future.Alan