Precsion Voltage Source from EDN article - Page 1
Forum section: electronics / general

author: munorc
author_rank: Regular Contributor
timestamp: January 20, 2025, 03:33:20 pm
content: I have seen various posts referring to this old video that was posted and ended up on YouTube. It was interesting since I have an old Dekapot that I ended up with and was looking for a practical use for it.https://youtu.be/GszypfZJ-UU?si=jAnUpho5HBty1QBbAll the references I found seemed to have aged and were not available but I finally came across a different EDN articlehttps://www.edn.com/recycle-precision-potentiometers-as-useful-voltage-sources/Here is the schematic from that article.Any insights on this approach? I am looking for a voltage source to compare various instruments to. I have many old DMM's and analog meters, but nothing I would view as trustworthy.

author: munorc
author_rank: Regular Contributor
timestamp: January 20, 2025, 03:35:39 pm
content: A few anomalies with the schematic, Q7 is not identified and Vref is not indicated on the output of the LT1236, but is shown as input elsewhere, e.g. input to the voltage divider.

author: tszaboo
author_rank: Super Contributor
timestamp: January 20, 2025, 03:44:46 pm
content: Typical Linear technology schematic. They use a comparator to blink an LED that cost more than the entire BOM of other circuits. Stepping back from that, the circuit likely was never built. The VREF is clearly the output of the LT1236. LT1881 is not accurate enough for a Kelvin divider, overkill for a potentiometer. I think you can build better circuit if you really tried.

author: NNNI
author_rank: Regular Contributor
timestamp: January 20, 2025, 10:06:09 pm
content: QuoteAs shown in the schematic posted above, no, but in a different form:

author: guenthert
author_rank: Frequent Contributor
timestamp: January 21, 2025, 08:57:35 am
content: Quote from: tszaboo on January 20, 2025, 03:44:46 pmIndeed. The outputs of U4A and U4B are directly connected -- most certainly not what one wants.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 21, 2025, 09:03:52 am
content: Quote from: munorc on January 20, 2025, 03:33:20 pmUse it as you see fit. What spontaneously came to my mind were use cases of setting the reference voltage for a current source or for a reference voltage divider like a Fluke 750a (in lieu of the KVD used there).

author: 2N3055
author_rank: Super Contributor
timestamp: January 21, 2025, 09:51:43 am
content: Quote from: guenthert on January 21, 2025, 08:57:35 amIt is an comparator with open collector outputs. You can certainly do that to achieve OR function.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2025, 10:45:56 am
content: It's truly amazing how much time and Silicon they put into lighting 3 LEDs! It looks as though they gave a fresh grad a pencil, paper and a databook and no goals (other than to design in as many LT parts as possible! At first glimpse I assumed that all those transistors were a current boosting output buffer, but wtf? more LED related curcuitry.My advice to the OP would be to throw it in the bin and concentrate on the basics:1. 10V reference. Off the shelf IC, LT1236, AD587, REF102 etc. All will drive the Dekapot directly. Pay attention to temperature stability, noise reduction, fine trim - minimizing trim range on the pot - something that they forgot to do in that circuit in their LED frenzy.2. An accurate output buffer. Look through the various voltage reference threads for recommended opamps, DC accuracy, noise etc. Also check output current limit and short circuit protection. Expend 99% of your effort in this section.3. Power supply. Do you really need battery operation? Does your output buffer need a (small) negative rail to achieve true 0V output (probably).This Metrology section will give you loads of interesting ideas and information if you browse the voltage reference threads.

author: Evans2688
author_rank: Newbie
timestamp: January 22, 2025, 12:50:22 am
content: There seems to be a lot of negativity regarding this subject. For what its worth, and I hope I don't get flamed for this opinion, I built this circuit some time ago and have used it for several years now and it works great. I have used it extensively in my work. It sets a voltage to 5 place resolution to within 100 uV of the setting by the dekastat, as verifed by my two calibrated 3458A DMMs ( I also verify the DMMs with my two FLUKE 732A voltage standards that are periodically transfer calibrated against a known 732B standard). Regarding the cost, the parts were much cheaper when I built the Precision Voltage Source and I had the Dekastat and chassis on-hand so the cost was quite reasonable.I have attached the schematics, PCB layout, and photo of the unit. The schematic and PCB were created in ExpressPCB Classic and I can attach them if anyone is interested.

author: daisizhou
author_rank: Super Contributor
timestamp: January 22, 2025, 03:59:27 am
content: Hi Can you share a picture of the inside of this power supply?I would like to know the exact model of this potentiometer and the model of the terminal blockIt would be great if there is a real test video, thanks

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2025, 12:56:24 pm
content: Quote from: Evans2688 on January 22, 2025, 12:50:22 amNot negativity as such, they just seem to have paid much more attention to the LEDs than the reference and buffer. For instance noise reduction and a high current output without Kelvin sensing. If they had addressed these as well, then it could have been great.