Community Bench Meter

[smindinvern]

I'm sort of interested in the idea of this, so, realistically, what is required to make a meter?  I imagine that conceptually it is a pretty straightforward idea.  A voltage reference and an ADC.  Compare the incoming voltage with the reference.  Use a DSP to analyze the input and do whatever you want with it, whether that's measuring the average of the input, p-p voltage, Fourier analysis, etc.  Feed it to an mcu/mpu and display it on a nice little screen.  Of course that's just voltage.  I would think current could be measured in a similar manner.  Really, as far as quality goes, what would be required to make it as high-quality as possible?  I would think basically just an extremely accurate and precise voltage reference, high-quality ADC, and measures to minimize input noise.  Is there anything (probably a lot) that I'm missing?  Because it really sounds fairly simple to me.  Simple idea, hard to pull off, I would imagine.

[Bored@Work]

It might not be worth the effort to build a high accuracy meter or an extra cheap meter, but there are some niche applications where it makes sense.

So, you are volunteering to fix issues a) to f) above, or ...

Such as a meter that automatically calculates average and peak power. Or a datalogger that records voltages to a SD card at a sample rate of a few Hz or kHz.

... do you just want to add to the wish list?

BTW, one can get low-quality datalogging multimeters for 120 € onwards (new), e.g. http://www.unisource21.com/tDM620.html (many people allegedly have problems with it, but they will have with a DIY meter, too). Can you build one for that price?

And automatic calculation of averages and peak power can be done with any piece of rubbish that has a PC interface, plus some PC software. I never checked what the cheapest meter with a PC interface costs. I know a VA-18B can be had for 40 € (new).

In any case, we'll make the assumption that the accuracy needed is comparable to or less than that of the under $20 meters and that the builder already owns a meter (and calibration reference) much more accurate than the one being built.

That excludes all those who were after a high-precision meter for next to nothing.

One extremely high accuracy measurement that is relatively easy to make is time/frequency. Put a GPS in your homemade frequency counter and you'll very likely have more accuracy than you'll ever need.

1) What has that got to do with a bench multimeter?
2) The guys on timenuts would disagree with "GPS in your homemade frequency counter and you'll very likely have more accuracy than you'll ever need".

[Bored@Work]

I imagine that conceptually it is a pretty straightforward idea. 

Then do it. Don't talk, do it.

[smindinvern]

I imagine that conceptually it is a pretty straightforward idea. 

Then do it. Don't talk, do it.

Could you be a little bit more helpful?  I'm interested in learning as much as I can.  This forum is a treasure trove of knowledge.  That's why I asked.  My education certainly isn't your responsibility, but any help you would like to offer would be appreciated.

[Bored@Work]

Could you be a little bit more helpful?

Why? I owe you nothing.  Let me tell you a little story. It is the story of why I don't have a ham license.

When I was a boy ((C) Dave Jones) I was at some point in time very interested in amateur radio. So I started to study for a ham license. I memorized all the abbreviations, operating procedures and stuff. I even started to learn morse code, taking a course. This was the time when you only got a serious license if you knew morse code. I was about to become better, almost to the point that I could do morse.

And then, via the morse code course, I started to make contact with hams in the local amateur radio club. These OMs (see, I can still do the ham lingo) were all the same. They were always talking about the great things they plan to do, and what they would build and how great hams are, how special, because they build their own stuff. Yes, they do. Licensed by the government, no less.

But when visiting their shacks it was always the same. There was just commercial equipment on their benches, and somewhere in a corner was a half-finished project. Often literally covered in cobwebs. They all had their unfinished alibi project, but nothing else. They just talked, often about things they didn't really understand, because their technical ham training was in fact poor.

It was deeply disappointing. And I dropped the morse code course and stopped studying for a license, because I never wanted to be like them. Only talk, no do. Many years later, at some point in my EE career, I was operating RF equipment every ham would go apeshit about. Of course no ham license needed, and it gave me deep pleasure to remember these poor "we are so great, we are hams" guys with their cobweb projects.

And since the days of my aborted attempt to get a ham license I despise people who always talk, but don't do. Who wank about how great some project could be - if just others would do the work. They are time wasters.

Unfortunately, this type of people have become more ubiquitous in recent years. Scratching their private parts, grunting "uh, cool, eh, eh, eh", feeling entitled, and shouting "awesome" is all they do. I have seen many such "projects" like this one. Someone shouts Wouldn't it be great, if ...?, and the wanking croud responds Cool, you are awesome dude! You are the man! And it must have .... Of course non of them lifts a finger to actually do something.

I'm interested in learning as much as I can.  This forum is a treasure trove of knowledge.  That's why I asked.  My education certainly isn't your responsibility, but any help you would like to offer would be appreciated.

You just got an education. Don't talk, do, especially if you think

it is a pretty straightforward idea.

[smindinvern]

Why? I owe you nothing.

Am I detecting some animosity here?  I certainly hope not; I haven't really done anything to deserve that.

You just got an education. Don't talk, do

That doesn't really mean much coming from someone who just did a heck of a lot of talking, and a questionable amount of doing.

To be honest, I probably never will build a bench meter; I have enough to keep me busy for quite a while, and not enough money to go spending on projects whenever I want.  But it never hurts to procure another bit of knowledge, especially not about something which you'll probably use at some point in the future.

[NiHaoMike]

And automatic calculation of averages and peak power can be done with any piece of rubbish that has a PC interface, plus some PC software. I never checked what the cheapest meter with a PC interface costs. I know a VA-18B can be had for 40 € (new).

That's true of steady DC, but not for AC or DC with significant AC ripple. For AC, you'll need to sample both the voltage and the current, then multiply them together and take the average. A normal multimeter would also sample too slowly to catch a brief inrush. (And if your supply voltage changes over the course of the measurements, you'll need a second meter to be able to read both voltage and current. It still won't work for AC measurements since you need instantaneous voltages and currents, not RMS.)

[firewalker]

Maybe the project should divert to something more doable.

E.g. Community Bench PSU &&, || a Community Bench Function Generator.

[hans]

I'm sort of interested in the idea of this, so, realistically, what is required to make a meter?  I imagine that conceptually it is a pretty straightforward idea.  A voltage reference and an ADC.  Compare the incoming voltage with the reference.  Use a DSP to analyze the input and do whatever you want with it, whether that's measuring the average of the input, p-p voltage, Fourier analysis, etc.  Feed it to an mcu/mpu and display it on a nice little screen.  Of course that's just voltage.  I would think current could be measured in a similar manner.  Really, as far as quality goes, what would be required to make it as high-quality as possible?  I would think basically just an extremely accurate and precise voltage reference, high-quality ADC, and measures to minimize input noise.  Is there anything (probably a lot) that I'm missing?  Because it really sounds fairly simple to me.  Simple idea, hard to pull off, I would imagine.

I've seen people talking about 24 or even crazy 32-bit devices (with an ENOB of 21.3 max). If you're designing such devices in practice, it's not the case of slamming some parts together with the highest specs, it's nicer if you can get the maximum specification that's still reliable from every part in your circuit.

A 24-bit is really not necessary. If you want to design a 20,000 count multimeter it basically means the A/D converter needs a effective resolution of 20k steps. That's a resolution of about 14.288 bits. There are 16-bit A/D converters on the market that can get that resolution easily, even in bi-polar form to make things 'easier'. Remember these circuits are extremely low noise (anything from 14-bits is) which requires very well thought circuit design and prototyping.

I also hear people talking about power measurement. I think it's a community challenge enough, as it is proven, to even choose a single set of measurements to be taken. Adding power to that is not sane. Furthermore, it would push the budget far because you will need to select a multi-channel A/D converter. On top of that, good power measurements are done by simultaneously sampling both channels (current and voltage readings) instead of sequential, and those parts are more expensive as well.

What I think first is needed for a project always is a budget. I never saw people talking about prices, except it might become too expensive. Let's say we make it reasonable of 200$. I can tell you that is going to be really tight for an accurate meter, if you look up the specs below. MCU, A/D, 'basic parts', mechanical (PCB, casing, buttons) will cost >$125 easily for a small project. Then again, I guess a case wouldn't be required if it's not going to be markted product. People might be able to built it into their own casings.

VDC: 20000 count (V), 2000 count (mV) in high ranges 0.25%
VAC: 2000 count, 2%
AAC: 2000 count, 2%
ADC: 2000 count, 0.5%
Ohms: 2000 count, 0.5%
(Cap: 100 count, 2%)
Diode test: 1mA test , up to 2V, 2000 count
Frequency: 100kHz with a fast proccesor should be possible...

Adjustable current sounds fun and useful, but I have some arguments in not incorpating that:
1) When do you actually need to measure accurately the voltage of a LED?
2) If you do, however, you can built a 'Diode tester' more easily. There are dozen schematics on the internet that do just that, with adjustable current (tenths of mA to several mA's) and respectable resolution.
3) It sounds kinda cheap, but high currents drains battery.

In my mind, the easiest solution on making a multimeter is to have a MCU with an external ADC linked together (SPI/Parallel). On the A/D is , ofcourse, some amplifying, protection, detection and range selection circuits. For a MCU it will has to be a 16 or 32-bit device for sure. MSP430 sounds reasonable because it's famous for mobile device operation, but microchip has some interesting controllers as well in their dsPIC, PIC24 and PIC32 ranges. I think Microchip, being a hobbyist community project, is more accessible. But the exact selection is not necessary now, as I will explain.

Communication to outside world could be done with USB (FTDI chip to make a virtual COM port). Easy, always works. Add a digital isolator and it's a bit safer, but I still wouldn't want to measure the mains whilst having it connected to PC. Optical is best.

Just to note something, it's funny that people are discussing ARMs, connection methods, but I haven't seen much attention to the analog part at all.

How do you design overload protections? Surges, be able to handle high voltages on every input setting, etc.

Determine required accuracy of the built electronics instead of gambling for high-spec-unaffordable-parts?

Create ultra low noise pre amplifying circuits?

Set up a reliable form of ranging that doesn't blow up components (i.e. 230V~ on mVdc)?

Calibration, reliabality, accuracy, bandwith, noise levels, power supplies, bipolar measurements, diode/cap/resistance measurements..?
etc.

To me, the digital part of a multimeter is just a way of common sense on what microcontroller and A/D converter fits the bill the most. But you do this after you investigated what you want to capture from the analog frontend and at what degree. These kind of circuits has the A/D and MCU coupled tight together. If a project can get away with a 10-bit resolution, it might be fine to use onboard A/Ds. If a high speed measurements are in order, having a high speed A/D connected to a 16-bit wide PMP on a PIC32 might be necessary.  The A/D converter is selected on the required speed, SNR or SINAD calculated from the desired ranges and resolutions, margins and stuff like that.

I think we should discuss on getting those figures and schematic(!) idea's on table first, before adding anything else to the discussion on the 'digital section'. In my opinion a high-count multimeter is more about the analog circuitry, because that needs to be done well. A digital circuit is more like connecting the pins correctly when the right parts are chosen for the job.

[001]

I think we should discuss on getting those figures and schematic(!) idea's on table first, before adding anything else to the discussion on the 'digital section'. In my opinion a high-count multimeter is more about the analog circuitry, because that needs to be done well. A digital circuit is more like connecting the pins correctly when the right parts are chosen for the job.

Yea! I see input circuits and signal path in 1980th gear are kind of art
The endless tell about diy bench meter is silly since everybody who REALLY need it know how it cost