Checking our meters - ultra budget voltage reference

[jewelie]

Hi folks

I've got a couple of cheap multimeters (a UNI-T UT201 that switches off at inconvenient moments and the other is a 4000-count Vicci VC97 that I got to replace it - sorry Dave, I didn't know better at the time I got them.)  My friend also has a couple of meters.

I've been wondering about a *very cheap* way to check *roughly* how accurate the meters are every now and then (I would imagine about +/-0.3% at either 2.5V or 5V as those are in the two ranges I most regularly use?)

I've searched the forums and read a number of the discussions regarding voltage references - they all seem to be discussing accuracy and costs beyond our needs and means.  I can't really justify more than about £10 / US$15 for something that's more for fun and learning than a necessity and will only be used rarely.  I've done the cliched thing of just bunging my two meters together and seeing how well they agree on voltage measurements (see pics below.) 

So.... 

• Is it worth getting, or even making, a very cheap reference voltage circuit of some kind (like, as Dave would term it, a one-hung-low device like eBay auction: #231781327574 which /says/ it's based on the AD584LH - although looking on RS Components website for a sanity check says even just the AD584K is £9 / US$13 ?)
  
• Or, frankly, would cheap meters like these tend to be more accurate than such a low cost reference device anyway?
  
• Or do you folks have any other ideas or suggestions?
  

I welcome your input and a "re-educating", although please be gentle? 

Regards
Julie
(Derby, UK)


Because pictures...





PS Because you've read this far, my background...

When I was twenty years younger I did a degree in Computer Systems Engineering (joint electronics and computing degree.)  It was at a bad point in my life, and I drowned in the maths on the electronics side and stepped aside from it all other than basic household repairs.  I'm always terribly nervous both of my ability and the likelihood of stuff breaking all the time, although I have a pretty good history of rarely breaking anything and utter amazement when things work (which has been pretty much always.)  I've only kept a basic knowledge going at all due to repairing things; we're not flush with cash, so if things break I'm bloody minded enough to attempt a repair on most things, and succeed more often than not (although I don't attempt to open or fix things that aren't broken!) 

I came to eevblog from reading skepticism blogs after reading an article about the battery "saver" scam devices (those little DC-to-DC converter battery jackets.)  I soon started watching Dave's videos and have started to rekindle an interest (thanks Dave!) in electronics again, the subject that the degree ruined somewhat for me; I was fascinated how much came back to me, how much I can remember, and now I've hankering to use the soldering irons for something other than just repairs! 

[ade]

I think most people just check one meter vs. another as above and if they're close enough then they're good enough. 

Whether or not it's worth making a budget voltage reference depends on what your interest is really.  E.g., given your background, you might enjoy playing with microcontrollers and doing embedded programming. If so I'd say spend your time on those kinds of projects and don't worry so much about multimeter accuracy.  Digital logic usually can withstand a huge level tolerances in 0s and 1s.

OTOH if you're into designing supplies, DACs/ADCs, analog control systems, or just love test & measurement stuff, then by all means making a voltage reference might be an interesting way for you to get back into electronics?

[biffster]

I had a quick look at the Vichy datasheet and it states a DC voltage accuracy of +/- 0.5% + 4d and the clamp meter is probably worse than that. As ade says it depends what you need it for - some projects require high accuracy and some don't so I wouldn't worry until you know what you need.

If you do want a voltage reference though I'd have a go at making one yourself; it'll teach you way more than buying something suspect off ebay. Also, RS are a rip-off and will also charge you high delivery fees if you aren't a company. Here's a simple and fairly cheap 0.1% voltage reference:

http://www.topqualitytools.co.uk/shunt-voltage-reference-lm4040a20idbzt/

I'm using one in a project at the moment and all you need is one external resistor. The problem then becomes how you know whether this has drifted when you come to use it in a year's time. That's when you realise you need a reference to calibrate your reference and enter the never-ending calibration spiral of doom.

[rich]

some projects require high accuracy and some don't so I wouldn't worry until you know what you need.

Yes, this.


But I have to take exception to this:

Also, RS are a rip-off and will also charge you high delivery fees if you aren't a company. Here's a simple and fairly cheap 0.1% voltage reference:
http://www.topqualitytools.co.uk/shunt-voltage-reference-lm4040a20idbzt/

RS and Farnell both offer free shipping for all online orders. For the shunt reference, a two pack from RS works out cheaper than buying from the posted 'topqualitytools' link (they seem to charge 1.99 postage ).   The moral: it's always worth shopping around, even from the big distributers.

[biffster]

Sorry, you're absolutely right there rich. I didn't realise RS had removed their £5 shipping charge on orders under £20. Good news!

[Gyro]

'topqualitytools' really winds me up - specifically their ebay shop. I got caught out a couple of times before I realized. A stiff 100%+ mark-up in addition slow delivery. The only thing it's useful for is that ebay search is a slightly faster way of checking that either Farnell or RS stock it (usually Farnell) than their own search functions. 

'PCArenaBristol' come a close second cost-wise, but at least they are free postage.

The only useful one (sometimes) is 'littlediode_components' who often have parts in obsolete packages, eg. Cerdip or TO99 which RS and Farnell don't stock.... at a price though.

It's also worth checking CPC (distributor, not ebay) who are owned by Farnell. Smaller components range but sometimes have the same component at a startlingly lower price.  A recent example: Fairchild J112 JFET £0.06 from CPC vs £0.24 from Farnell but.....£0.54 from 'topqualitytools'

EDIT: Sorry folks, that was a bit OT.

[Arjan Emm]

I wouldn't bother with a questionable voltage source. I'ts all about confidence in the measurement of your meters. To put all trust in a single cheepy reference would not give me much confidence.
Get together with your friend. Bring all the meters you both have and start measuring voltages and resitors. Full batteries, a power supply, some random resistors whatever.
The meters that agree are most likely spot on.The ones that disagree are probably off. If you have ten meters that say a 9v battery measures 9.2V and one says 8.8V the last one is probably out of calibration. Ditch that one and get together next year to do the same test.

[G7PSK]

I have one of these, a year or so ago many people  on the forum were getting them and some were even tested on high end meters,and proved them to be remarkably accurate.

[uncle_bob]

Hi

Ok, the lowest cost way to do this is to buy a nice fresh battery. Measure it's voltage before use. Better yet, buy a pack of 10 of them. Measure all of them and pick one. Check all your meters against that battery and what it should read. The result is going to be pretty close to the accuracy of the gear you are using.

A year from now buy another pack of AA cells (or whatever you use a lot of) and repeat the process. Check against the same "what it should read" number. You can now maintain calibration against the same "standard".

Net cost ... zero if you use up the batteries in all of your battery powered gizmos.

Next lower cost:

One resistor, one cheap NPN transistor, one cheap 6.2V zener diode. The transistor gets wired with collector and base shorted. Wire them in series, bring out two leads. One is  from the "ground"  leg of the zener. The other is from the junction between the resistor and the collector / base combo on the transistor. The emitter of the transistor just goes to the cathode of the zener and does not come out to the outside.  Any fairly good metal film 1% resistor will do. You want a miliamp or three through the contraption with a source picked in the 12 to 24V range powering everything.

You now have a sub 50 cent temperature compensated reference.

Grab your bench supply and set it to some voltage (let's say 12V, yes the resistor would have to be the right value). B+ goes to the top of the resistor. Ground goes to a separate lead to the anode of the zener. Set it with a good DVM borrowed from a friend. Read the voltage on the zener after everything has stabilized. Check the room temperature and record it. Also record the supply voltage and zener voltage. Go have a cup of coffee. Repeat the process in a half hour. Bump the supply up and down 2% and record that effect (just so you know).

You now have a cheap reference that is much better than your DVM's. To use it, you will have a little back and forth between the power supply set and the zener reading (it's a ratio and meters are pretty good at that). The trick is to read both the zener and the supply on the same range of the meter. That drops out most calibration issues on the meter. In use, you should be within a couple of mv (call it 10 mv max) of the recorded value. 0.1% of 6.8V is 6.8mV.

For added precision, run the beast at home for 30 days before you take it over to your friends house. For even more fun, pot the parts inside a piece of copper tube. Ground the tube to the supply ground lead. Scrap tube from a plumbing job will do just fine. A scrap is a sub 25 cent item if you have to buy it at the hardware store.

Yes you can go a bit crazy building a number of these things and temperature compensating them. You can also have four of them and check one against the other to be sure you are using one that has not broken. You can enhance the thing by using a 78L12 to power the resistor. That gets rid of most of the power supply dependance and could easily double the cost to (yikes!!) $1.

Like any project, this can quickly spiral off into enormous un-needed complexity (and lots more fun).

Bob

[German_EE]

How accurate is your meter? If the meter is 1% accurate on the DC ranges then you need a reference accurate to 0.1%, ten times better. The same theory applies to current and resistance measurement, ten times more accurate than the instrument you're testing.

[biffster]

If you have ten meters that say a 9v battery measures 9.2V and one says 8.8V the last one is probably out of calibration. Ditch that one and get together next year to do the same test.

Again it depends on what accuracy you need to calibrate to but comparing un calibrated devices isn't the way that calibration is done in industry. That's because, even if you can find 11 meters it's unlikely that only one would have drifted. More likely they would all have drifted either in the positive or negative direction and maybe by the same amount so you'd have a distribution of readings.  Just because devices agree it doesn't mean they're correct.

[mojoe]

MAX6350, 5 VDC reference. $11.60 at Mouser.
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyMAX6350CPA+

One IC and three bypass caps. I built one on a breadboard over a year ago and still use it. See my post: https://www.eevblog.com/forum/metrology/siglent-sdm3055-comparison/

[Arjan Emm]

If you have ten meters that say a 9v battery measures 9.2V and one says 8.8V the last one is probably out of calibration. Ditch that one and get together next year to do the same test.

Again it depends on what accuracy you need to calibrate to but comparing un calibrated devices isn't the way that calibration is done in industry. That's because, even if you can find 11 meters it's unlikely that only one would have drifted. More likely they would all have drifted either in the positive or negative direction and maybe by the same amount so you'd have a distribution of readings.  Just because devices agree it doesn't mean they're correct.

This is the beginners section, not the voltnuts one. 3 or 4 digit meters don't drift over time, they are spot on or they fail. They are not accurate enough to show long term drift usually.

[mojoe]

<sarcasm>Hey, all you beginners. Stop asking questions or you just might learn something. And then you might develop a need for even better test equipment. Besides, as a beginner, you don't even need an accurate 3.5 digit DMM. A 1.5 digit DMM is good enough for you.</sarcasm>

Let me see... a beginner probably doesn't have a large budget and may not even know what test equipment to invest in. So, he/she either buys a Harbor Freight special DMM, or buys an older piece of test equipment at a bargain, so they have something to learn with (what a concept). Now, we all know how accurate those give-away meters from Harbor Freight are. And that 20-30 year old piece of gear is just as accurate as the day it came from the factory (not). Been there, done that for many years.

What is so wrong with spending a few dollars (about $12 total for the solution I posted) and actually seeing whether your DMM meets its specifications. You may find out that it does. That doesn't mean it was a wasted $12. You had the opportunity to learn some basic metrology, you now have confidence in your meter, and you have satisfied your curiosity (which is very important).

[mojoe]

I just looked at that ebay link provided by the OP. That actually looks like a decent solution for less money. I would be concerned about the use of a 15V battery, but an external battery pack could certainly be made on the cheap.

[Rycroft]

I built this,



I didn't bother with the battery monitor portion.

[jewelie]

Hiya

Yes, we've recently come across the Scullcom videos both of the reviews of the Chinese boards and that example board.

I think we're both going to build a variation of Scull's circuit but at 2.5V (as we've both got 4000 count meters) with a MAX6225 and because it's easy to get hold of the chips at affordable prices from genuine sources. 

We nearly ended up getting some chips via aliexpress but got very suspicious of the prices and noticed there does seem to be chip counterfeiting - and inaccurate counterfeits would defeat the purpose of doing this.

It's actually been really interesting working together going through datasheets and finding gotchas, making appropriate compromises and so on - this is far easier when you've got someone else to be bouncing off of (and who'll find holes in your suggestions.)

Now to get the bits whilst getting some stocks of parts.