9v Temperature Compensated Zener newbie questions - Page 1
Forum section: electronics / general

author: Victorman222
author_rank: Regular Contributor
timestamp: September 09, 2022, 03:26:38 am
content: So i've been having great dreams of making a multimeter and i figured a vref is a decent start. Thus i started frantically running around my house looking for something suitable. The best i found were soviet D818 good tempco zeners.Stabilization voltage: 9vOperating current: 3-33mARated current: 10mARated tempco: 10/20/50 ppm (different types)Time instability of voltage: ± 0.12% (nothing said about what timeframe or anything)This is basically all info i could find on it (maybe didn't look hard enough),reallydon't understand the time instability rating.When i tried to hook one up it seemed to work alright, by varying current i was able to go into positive and negative tempco regions using the soldering iron to the side of the diode heating method(no other choice when 3.5 digits is all i got). And by looking at the data i chose a point right in the middle of the range where the voltage didn't show any change and soldered appropriate resistor to diode.Please tell me what i got wrong. It would be real nice if someone could elaborate on the "Time instability of voltage" rating and what exactly it means. If someone has some great literature to share on this topic, please do. What are some other concerns i should worry about?

author: cellularmitosis
author_rank: Unknown
timestamp: September 09, 2022, 03:50:29 am
content: Nice, I saw these diodes recently and have been curious about them.You have the right approach with modifying the current to fine tune the temperature coefficient.Working with the limitations of a 3.5 digit meter, if you assemble a second reference, you can measure the difference between them and get a much finer resolution on the voltage change when measuring the tempco. A very simple setup might be to assemble two refs, but wrap one of them up in a towel with a small bottle of hot water, then watch the voltage difference change over the next hour or so (while also measuring the temperature). Hopefully you will get a curved graph which has a hill-top, which is the zero tempco point for that amount of current.What is meant by the “time instability of voltage” is likely long-term drift. Zener references tend to drift in value in an asymptotic fashion over a period of months to years, becoming more and more stable over time.

author: cellularmitosis
author_rank: Unknown
timestamp: September 09, 2022, 04:05:59 am
content: Since this is in pursuit of creating a meter anyway, you might be interested in creating a temporary meter just to measure the difference between the two refs, perhaps based on the LTC2400 series of ADC’s. Take a look at the Scullcom millivolt meter project:https://youtu.be/wxCW3pbvRi8

author: Mickle T.
author_rank: Frequent Contributor
timestamp: September 09, 2022, 05:08:21 am
content: Quote

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 10, 2022, 06:03:13 pm
content: Build two references using the circuit from reply #62 here-https://www.eevblog.com/forum/metrology/adr1399-reference/50/Then compare them with the most sensitive scale on whatever meter you have, or build an opamp amplifier. Put one in a box and change the temperature with a small heater or light bulb. Keep the other one stable. That's sort of step #1 in getting a handle on this stuff. You can look at the Mini-Metrology articles on my web site, but remember they're many years old so you might do things a bit differently today-http://www.conradhoffman.com/mini_metro_lab.html

author: cellularmitosis
author_rank: Unknown
timestamp: September 10, 2022, 09:39:53 pm
content: Conrad’s articles are highly recommended!As you get deeper into your DMM design, also check out the “theory of operation” section of old DMM manuals. I have a list of many old models here:https://github.com/USACalClub/gear/blob/master/dmms/README.md(Rotate to landscape if on a phone)

author: Victorman222
author_rank: Regular Contributor
timestamp: September 17, 2022, 11:08:12 pm
content: Thanks for the great advice and suggestions.I have assembled 2 boards of the bootstrapped zener circuit, one with set voltage and current and one variable. Measurements were done between two references using the 200mv range of my multimeter but with a 10x opamp amplifier (so 20mv fs). The process was heating the diode slowly using 12v 30w lightbulb on 5v, measuring the temperature using some cheap digital thermometer (The temperature sweep was 10*C from 28 to 38), I looked at voltage change with temperature and tweaked diode current accordingly (if the voltage increases then decrease current, if voltage decreases increase current). I think its important to do this at your application temperature because the optimal current is different for different temperature ranges (I suspect it's as if there is a point after which the voltage starts going in the other direction and we are trying to put that point in the middle of our temperature range (just an observation)). The results i was able to achieve were really nice. I was unable to tweak all diodes to 0tc point, maybe I need to add a fine adjust potentiometer or use non wirewound one.Once again thank you very much, I am very satisfied with the project and I hope my research can be of help to somebody.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 17, 2022, 11:19:51 pm
content: Interesting, is it possible to use something likehttps://www.analog.com/en/products/lm334s.html#product-overviewto fine tune the current?

author: Victorman222
author_rank: Regular Contributor
timestamp: September 18, 2022, 05:01:09 am
content: Quote from: opa627bm on September 17, 2022, 11:19:51 pmHmm.. The voltage does change with current but I guess it could be possible, but only in computer controlled system where the diode would be ridiculously characterized. But great idea and an interesting part, if I get my hands on one I will try it.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: September 18, 2022, 05:45:26 am
content: Multiple 9V Zener diodes can be used in the Datron-like "statistical" voltage reference.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2022, 06:47:29 am
content: With those temperature compensated zeners the TC depends on the current. There usually is a more positive TC with a higher current. With the right current one can often (depends on the diode type / sample) find a current to get near zero linear TC (e.g. get about the same voltage at 28 and 38 C in the example). For this there is not need for an extra temperature sense. However there is usually also a 2nd order term. So one may have a positve TC at lower temperatures and a positive one at high temperature.Compensating for this (e.g. use more current with a higher temperature) can be tricky, as the compensation circuit can also introduce dirft with aging. Often one also adds a linear term and may need a quite diifferent current / linear TC to start with.If the power consumption is not a big issue, the alternative here is to use a kind of oven to stabilize the temperature. With the linear part is trimmed the oven can be quite effective. Even if just stable to +-1 C this helps a lot with the 2nd order term. The effort for a small oven is not that large, especially if the environmental temperature range is small (e.g. lab temperature and not possible extremes during transport).Trimmers are often not very stable. So the adjustment should be more done with a combination of selected resistors.