Hobbyist Voltage Reference - Page 1
Forum section: electronics / projects

author: rpetitho
author_rank: Contributor
timestamp: October 13, 2019, 02:06:28 am
content: Currently working on a voltage reference to calibrate multimeters and stuff. I am going to use the MAX6350 powered by a 12 volt battery. I choose this reference because it has good accuracy, and I have not got the equipment to calibrate it. I put a spot on a board to trim it if I want to later.The circuit has 2 sides. First is the reference side, taken from the data sheet. Second is the LED light to let you know everything is awesome. It will be green when on, but will turn yellow as the voltage approaches the minimum for the 6350. Plan is to fit it into a small aluminum enclosure with banana jack outputs. I post updates when I get them! If you see anything crazy please let me know.-Bob

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 13, 2019, 02:15:25 am
content: Your output capacitor should be hung off the MAX6350'sNR pin--that is what its there for. As suggested in the datasheet:

author: rpetitho
author_rank: Contributor
timestamp: October 13, 2019, 03:05:43 am
content: Sounds good. I stick one on when I build it up, maybe I'll do a test to see how much of a difference it makes.

author: teksturi
author_rank: Regular Contributor
timestamp: October 13, 2019, 07:49:57 am
content: Datasheet says that 1uF capacitor for NC pin. In figure 3 there is also output capacitor so you can just add NC capacitor and not move it from output pin.Layout looks good to me. Couple small thinks.Do you need mounting holes now they are missign?You should use thermal relief pads so it will be easier to solder.Maybe move your wirepads that way that if you want to you can add connectors.You do not have very much room sides of your board. Maybe add couple mm to your board size an less you have case for this already.Edit: And you can add decoupling capacitor for LM358. Datasheet says 10nF. And also add some hysteresis to yellow led driver.

author: MosherIV
author_rank: Super Contributor
timestamp: October 13, 2019, 12:44:12 pm
content: HiHave you thought about using the AD584?It gives you 3 reference voltages: 2.5V 5V and 10V

author: 001
author_rank: Super Contributor
timestamp: October 13, 2019, 02:41:22 pm
content: Strange thing about DIY voltage reference is that most of them are DCDIY AC reference not so popular

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: October 13, 2019, 03:09:08 pm
content: Quote from: 001 on October 13, 2019, 02:41:22 pmMaybe it's just not as common. Especially among hobbyists.

author: MosherIV
author_rank: Super Contributor
timestamp: October 13, 2019, 03:56:29 pm
content: DIY AC references are very hard to make.There are no off the shelf parts that just do it like there are for DC references.I am sure that as soon as someone designs a simply AC reference, it will be copied and made common place.The AC that you get on the cheapish hobby reference standard is actually just a square wave that has been measured with a high accuracy rms meter.

author: 001
author_rank: Super Contributor
timestamp: October 14, 2019, 12:37:30 pm
content: Quote from: MosherIV on October 13, 2019, 03:56:29 pmIs it a bad idea?

author: MosherIV
author_rank: Super Contributor
timestamp: October 14, 2019, 07:12:01 pm
content: QuoteWhat do you think the rms value of a square wave is?Using a square wave for an AC reference is a pretty bad idea to begin with!Digitally creating pure, clean sine wave us very hard.

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2019, 07:26:26 pm
content: Quote from: 001 on October 14, 2019, 12:37:30 pmIt is if you're trying to calibrate a non True RMS voltmeter. Also a square wave has sharp edges, which means that the value will vary depending on the high frequency response of the meter.

author: rpetitho
author_rank: Contributor
timestamp: October 27, 2019, 03:05:00 am
content: Got the PCBs yesterday. I ended up adding a bodge cap on the bottom on the noise reduction pin. Not sure if it will do much since it is battery powered. Since I have no standard to compare to, this is now the official 5v standard of my household. I have the parts to build up one more just to make sure I didn't get a bum chip. Battery should last about 12 hrs, but I made sure it was easy to change. I'm happy with the product! Pics below.

author: magic
author_rank: Super Contributor
timestamp: October 27, 2019, 06:52:26 am
content: Nice, I have built a similar MAX6250 reference on a piece of perfboard but never bothered with enclosure.One thing you could add is reverse supply protection, otherwise you need to be careful not to touch the battery connector the wrong way even for a moment.Noise reduction capacitor is pointless for DMM calibration. The datasheet shows its effect on noise and input ripple rejection - it only starts to make a difference above 10Hz. That means the output resistance of the zener cell has to be some 16kΩ and it would take 100µF to push noise reduction down to 0.1Hz. Foil capacitors in this size are rather big and electrolytics have leakage approaching 1µA which would drag the output several mV down. Not sure about MLCCs but IIRC they aren't that great either.

author: iMo
author_rank: Super Contributor
timestamp: October 27, 2019, 08:19:52 am
content: Quote from: magic on October 27, 2019, 06:52:26 amSimply wire an 1N4001 between Bat+ and Bat-. I smoked 2 my references recently that way..Also I made bad experience with trimpots. Do not populate the trimpot, stay with the voltage as-is.