Simple 10V reference project - LT1021 - Page 1
Forum section: electronics / general

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 15, 2025, 01:52:52 pm
content: Hi group,I am sharing the design and build of a 10V battery powered reference based on the LT1021-10. This is a DMM-check level of reference. I wanted something that I could take to a meter, instead of taking the meter to the calibrator.InspirationThis application circuit from the datasheet shows the LT1021 being used in shunt mode. The circuit can be modified to the positive version.This application circuit shows how to implement a restricted range trim circuit.SchematicThis is the schematic. The current source is implemented using a green LED and and BJT. The forward voltage of the green LED is around 2.2V. Minus the Vbe of Q1, 0.7V, results in 1.5V across the 510resistor. This results in a 3mA current source.PCBThe PCB was designed using KICAD version 5.BuildA 3D-printed base was attached to the PCB.TestingThe trimmer was adjusted for 10.000VThis is not the world's best reference, but it can be used for checking DMM's on the 10V. It is relatively easy to build.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 15, 2025, 02:50:37 pm
content: Hi group,The LTspice model shows that the diode illuminates when the output is valid:Regards,Jay_Diddy_B

author: zlymex
author_rank: Frequent Contributor
timestamp: August 16, 2025, 12:36:25 am
content: A more simple way is the plug-in type that eliminates manual switch and leads.Red box is a micro switch.

author: CalibrationGuy
author_rank: Unknown
timestamp: August 18, 2025, 04:33:20 pm
content: Doesn't the DMM check version have diode(s) in their compensation network to minimize the tempco? Your circuit is simple, but the quality of the LT1021 makes the project suitable for many DMM checks. You have to start with a calibrated meter to check it initially, though.TomG.

author: edavid
author_rank: Super Contributor
timestamp: August 18, 2025, 04:55:46 pm
content: What is the reason for choosing shunt mode?

author: zlymex
author_rank: Frequent Contributor
timestamp: August 18, 2025, 05:37:42 pm
content: Quote from: edavid on August 18, 2025, 04:55:46 pmIt requires a more economical current source compare to a voltage regulator in series mode.More over(not applies here though), it requires less voltage supply, say 10.5V in shunt mode, while it requires 11.5V(recommended, 11V at least) in series mode.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 18, 2025, 07:28:02 pm
content: Quote from: CalibrationGuy on August 18, 2025, 04:33:20 pmI implemented the trimming as described on the datasheet. The datasheet suggests a diode when using the 5V version of the LT1021. The DMMCheck uses the 5V version.Athttps://voltagestandard.com/you can buy a voltage reference based on the LT1021. Considerable value is added by burning in the references for 2000 hours and calibrating them with a 3458A DMM.Yes, you need to get it calibrated one way or the other. I have a Fluke 732A, and a Datron 1281. I can get it close enough for most applications.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 18, 2025, 07:30:43 pm
content: Quote from: zlymex on August 18, 2025, 05:37:42 pmzlymex is correct. In addition, the LED provides two functions, it is part of the current source but also indicates if the battery voltage is too low.Jay_Diddy_B

author: iMo
author_rank: Super Contributor
timestamp: August 19, 2025, 06:35:17 am
content: .. also the green led is the lowest noise among the LEDs, afaik, much less than when a zener there in the current source, people say..On the other hand I would be skeptical to use an LT1021 in plastic package while targeting 5+ digits.It is a reference created many decades back for 12bit ADC/DAC mainstream usage (as any others except perhaps LM399)..

author: mawyatt
author_rank: Super Contributor
timestamp: August 19, 2025, 12:06:38 pm
content: Quote from: zlymex on August 16, 2025, 12:36:25 amHere's something we did along these DMM checking ideas. Also served as a AC rms waveform as well as the DC reference.https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/Best

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 19, 2025, 12:43:16 pm
content: Quote from: iMo on August 19, 2025, 06:35:17 amThe green LED has a -2mV / C temperature coefficient, this provides a first-order correction to the current source.I had some plastic LT1021 in my collection of parts, so I used one. There are better options.Jay_Diddy_B

author: zlymex
author_rank: Frequent Contributor
timestamp: August 23, 2025, 10:17:43 am
content: Here is mine.

author: iMo
author_rank: Super Contributor
timestamp: August 23, 2025, 12:07:47 pm
content: With my first 7106 meter I wanted to save battery money, so I made DIY "Finger ON Switch with Automatic OFF Timer".It was wired somehow like see below, no mechanical switch needed and it had automatic OFF timer.Worked fine, the ON time depends on which finger you will use..C1 is foil..

author: zlymex
author_rank: Frequent Contributor
timestamp: August 23, 2025, 01:06:53 pm
content: Quote from: iMo on August 23, 2025, 12:07:47 pmGreat!I once made a similar LED switch for kids as well which is finger on/finger off/auto off.

author: iMo
author_rank: Super Contributor
timestamp: August 23, 2025, 01:44:11 pm
content: For people starting with electronics I should have added to the "Your Finger" label above following:"Tap here shortly with Your Finger"

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2025, 09:00:52 pm
content: Quote from: zlymex on August 16, 2025, 12:36:25 amzlymex,I see that you have implemented this. It works great! The battery should last a long time.This would not work for some of my meters. My Datron DMMs all have binding post connections.Regards,Jay_Diddy_B

author: zlymex
author_rank: Frequent Contributor
timestamp: August 24, 2025, 12:02:27 am
content: Quote from: Jay_Diddy_B on August 23, 2025, 09:00:52 pmYou are absolutely right. I plan to use two paralleled switches in the next version that can be seen from my last photo, one is momentary the other is alternate. Or to use finger switch.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 03, 2025, 01:35:19 pm
content: if you looked at schematic of 1021 , it already has current source inside , so design as shunt basically doesn't make sense for me personally , rather exercise ...probably better redesign for a compensation .i wouldn't be so pessimistic about plastic case .. Korean designers using ADR421 in soic for its 200000 dmm , still reasonable