Electronics      Reviews, Teardowns, & Guides

Introduction


Multimeters-- even digital ones-- can be surprisingly inaccurate.  If you're going to calibrate one, how do you obtain a voltage of some known value?  Alkaline batteries vary too much, even when new.  They also drop in voltage when tested.  Bench power supplies, on the other hand, are not accurate enough unless you use a very expensive one.

So, you need a "voltage reference standard". 

Voltage references can cost hundreds of dollars, but there's one that costs only about $3 to $10.  Let's take a look.





Reader-Supported Site

Articles like this one are possible only with the support of readers like you, when you use the links on here to purchase your gear. 

The small commissions from sales are what allow me to keep this site going.   Thank you in advance for your help.




In This Article

Description & Basic Use

Power Input

Factory Calibration

Drift

Cheap DMM's

AMM's

Good DMM's

The $300 Electronics Test Bench

Conclusion





Description & Basic Use


It's basically a circuit board without any type of enclosure or box.  (Some do have enclosures, but they're higher-priced and they don't all use the same board.)  There are a couple of surface-mount diodes, a few capacitors, and of course the specialized AD584 integrated circuit.  It looks like a metal-can transistor, because it uses a TO-99 metal package.  IC's with this basic radial-pin design have been around since the first commercially-available IC.

A single AD584 chip can cost $20 or more from the big component suppliers, so getting the whole circuit for $3 to $10 is pretty great.

If you plug in the required amount of DC voltage, the unit will output a very specific, calibrated voltage.  Jumpers on the board let you choose between 2.5 volts, 5 volts, 7.5 volts, and 10 volts. 

So, you can use this to calibrate a multimeter.  (Don't forget to turn on the "ON" switch on the circuit board.)



Table of Contents



Power Input


There's a place for an odd battery that no one seems to have.  Aside from that, there are two basic ways to power this unit. 

First way:  use a center-positive barrel connector that's run from your favorite DC power supply.  I think this may be a size "N" barrel connector, but I'm not 100% sure.




Other way:  use alligator-clip leads to connect the outputs of your favorite DC power supply to the battery clips on this unit.

The supply voltage should be at least 12.5 volts.  A brand-new car battery should be 12.7 volts or so.  That could power this voltage standard if you had the right connectors.

14 to 18 volts is better, to avoid a very slight drop in output voltage (microvolts). 

The barrel-connector has to be center positive, which is what most electronics use.  However, it seems to have diodes for reverse-polarity protection. 




Table of Contents



Factory Calibration


The item description on the "bay" often mentions an Agilent 34401, which I guess means they calibrated it with that.  Mine didn't include a calibration certificate, though:  not even a fake one. 

For most of us, the only way to test this unit is to check it against a fairly accurate multimeter.  I used an old Hewlett-Packard which hasn't been calibrated in a quarter of a century.  The 10-volt reference gave a reading of 10.00 volts on the HP.  It's seems likely that they're both accurate, as opposed to one unit having just the right amount of error to offset the other one's error. (That's always possible, though;  just not as likely.)



Table of Contents



Drift


If there was any, it required more resolution than my HP multimeter could display.  Then again, I let the voltage standard "warm up" for about two minutes before using it. 

Temperature stability?  Not sure at this point.  That said, it was at 10.00 volts the entire time, according to the HP. 

I realize that the multimeter's two-decimal place resolution kind of limits the test.  Still, better that it should read 10.00 volts than 10.47 volts or something.

When I get a better multimeter, or take this to a friend's who has better equipment, I'll update this section.




Table of Contents



Cheap DMM's


This voltage reference reminds me of why cheap digital multimeters are cheap.  You can calibrate the cheap multimeter by turning the potentiometer inside it.  But then it drifts.  A week later, 10.00 volts could measure as 9.42 volts or something.  (In fact that's the measurement it was showing... 5.8% error!!)  Even a basic analog multimeter usually stays more accurate than that. 

The potentiometer in my $10 digital meter is awful.  As soon as you stop adjusting it, it un-flexes enough to drift.  And then when you put the cheap DMM back together, it drifts again.  So, a cheap multimeter cannot be made into a truly good multimeter, unless you basically want to rebuild the whole thing from scratch.

Today, the threshold for an acceptably decent one seems to be around $40 or $50 US;  I would at the very least get an Amprobe.  The DM-7C is not always available anymore through this link, but the AM-510 should be




Table of Contents



Analog Multimeters


Most of these have DC accuracy 3% of full-scale.  Better ones advertise 2%.  Sometimes you get an individual meter that's much better than that, especially on a particular range.

My new $20 one measured 9.85 to 9.9 volts on this 10-volt reference.  So, that's only at most 1.5% error, well within spec for analog multimeters. 

This voltage reference is more than accurate enough to calibrate AMM's.


Get a low-priced AMM here

Table of Contents



Good DMM's


Any halfway-decent digital multimeter should have basic DC accuracy of 0.5% or better.  Try to get one with 0.2% or better.  Also look for high numeric count for best results.  That will give you the highest actual DC measurement accuracy. 

The better multimeters also don't drift as readily.  

Since I've started really noticing the limits of cheap multimeters, I'm increasingly wanting one of these, one of these, or possibly one of these.  I used to be a big fan of Hewlett-Packard equipment, not familiar with them after they became "Agilent".  I guess Keysight is probably just as good as any of the earlier two brands were.  There are other good brands, I'm sure, but those are the ones that come to mind offhand.



Table of Contents



The $300 Electronics Test Bench:  Shopping List


Just some notes on what I'm finding useful.  I'm not an electronics engineer, so your needs may vary, but here's some stuff that's really helpful for basic electronics. 

- The voltage standard is a core item, especially if your multimeter hasn't been calibrated in some years.  For three to ten bucks, you need this.

- The LCR-T4 meter.  Essential. 

- The DSO-138 oscilloscope.  It doesn't have nearly the bandwidth of a benchtop scope, but it's good enough for audio stages, 555 pulse generator circuits, and that sort of thing. 

- Speaking of which, check through those links for an inexpensive 555 pulse generator.  So much the better if it can output square, triangle, and sine waves.

- An inexpensive linear DC power supply

- A good-quality digital multimeter.  Here's one I've been wanting for a long time;  also try this link





Conclusion


Well, there you have it.  A $5 or $10 voltage reference that goes along nicely with several other "instrument on a board" pieces of test equipment.  You can start your electronics test bench for about $100, or let's say $300 if you get a good multimeter as your core piece of equipment.


If you found this page useful, informative, or entertaining, please help me out by purchasing your stuff through these links.   Your help is greatly appreciated;  it allows me to keep this website on-line and adding more articles to it.

Thanks for visiting this page!








Contact me:

3 p o.t o . 1 2 0 s t u d i o.. c o m


This won't directly copy and paste.  Please manually type it into your mail program.
No spaces between letters.





Home Page


Audio Guide




Disclaimer

Copyright 2018. All rights reserved.






 



Back to Top of Page