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mA meter 1 day build

A 1 day build of a digital millimeter for my bench.

Sidecar Bob

Dec 19, 2021
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Sidecar Bob submitted a new Project Log:

mA meter 1 day build

I've been doing a lot of projects lately (mostly involving LEDs) that draw less current than any of my multimeters can measure with any accuracy. I didn't want to spend an arm & a leg on more meter than I really need for anything else so I did some shopping and bought this little meter on eBay
https://www.ebay.ca/itm/402993119895

The specs say
Measuring Current Range: 0-50.000mA
Minimum resolution 1UA
Power consumption: <= 15MA,
Operating voltage:5v(If don't need High precision also can use 3.8-30v power )
Refresh rate: about 3 times / sec
Test accuracy: error: ± (0.05% + 2)

It arrived this week and the other project I was working on is waiting for parts so I gathered the parts and got to work. The first step was to tape up the top of a project box, mark out where I wanted everything and drill some holes


A razor saw is good for cutting holes in plastic but I really need to get a smaller one for projects like this


The box isn't deep enough for the big connector on the meter so I removed the connectors and soldered the wires to the board. Other than that the wiring was about as simple as it could be so it didn't take long to connect them to the switch and the lugs for the binding posts. Note that the battery is held between the meter and the switch (I also added a piece of foam tape inside the lid so it doesn't rattle).


The final step was trying it out. I connected it in series with a 47KΩ resistor, an LED and a 9V battery. The voltage across the resistor was 7.6V so the calculated current was...

Read more about this project log here...
 

Martaine2005

May 12, 2015
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Hi Bob (with a sidecar),
Maths is by far my worst enemy, which is why I failed my City & Guilds exam years ago.
Trying to work out your calculations doesn’t help me at all.
Is the resistor you used 47kΩ or 4.7kΩ? (Lower case k).
You wrote 47KΩ (capital K) and then 4700Ω. Either way, I still get a few more zeros after the decimal point than you have.

Martin
 

Sidecar Bob

Dec 19, 2021
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You're right. It was a typo (corrected now). I did say "the calculated current was 7.6V / 4700Ω = 0.1617mA" in the next sentence

When did this business of KΩ and kΩ meaning different things start? And what exactly is the difference? I tried looking up KΩ vs kΩ but Google but it doesn't recognize upper and lower case so I couldn't find anything...
When I was in school we always used capital letters for metric multipliers that were more than one (KΩ, MΩ &c) and lower case for multipliers less than 1 (mA &c).
We were also taught that MF and μF were interchangeable, especially in typed reports because typewriters didn't have μ and encountering a capacitor measured in a megafarads was pretty unlikely.

BTW: The first thing our teacher said in our first college Maths class was "Gentlemen, I'm going to teach you how to become lazy." He went on to explain that all of the great mathematicians in history were lazy because instead of calculating the answer the hard way they looked for the easiest way to get the correct answer.
I can multiply and divide on paper but I only do that when there isn't a calculator handy. In the same way I can calculate Ohm's law formulas (E / I x R, P = E x I and their derivatives) but these days I use a Windows app called Ohm's law calculator to do it.
 

Sidecar Bob

Dec 19, 2021
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When did it change from everything bigger than the unit had upper case and everything below was lower case?
Using lower case for everything up to kilo seems like a step backward to me. It used to be deci was d and deka was D but Now deka has to be da instead.
But these days they say things like 1000mL (instead of 1L) are commonly used too so who knows what will become acceptable in the future.

Personally, until someone tells me that KΩ means something different from kΩ I think I'll continue to use the upper case.
 

Sidecar Bob

Dec 19, 2021
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It sounds like the bureaucrats have been allowed to make decisions again. I mean, no educated person would think that a kilogram was a basic unit so why would anyone feel the need to explain that it wasn't?
 

Harald Kapp

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When did it change from everything bigger than the unit had upper case and everything below was lower case?
I don't know if that ever changed. I've known the lower case form all my life.

no educated person would think that a kilogram was a basic unit
Huh?
From Wikipedia:
The kilogram (also kilogramme[1]) is the base unit of mass in the International System of Units (SI),
Of course, that may be not so easy for those still living in an imperial environment ;)
 

Sidecar Bob

Dec 19, 2021
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The kilogram is by definition 1000 grams. The gram is the base unit. Anyone with basic understanding of how the metric system works should see that anything with a metric prefix cannot be a basic unit.
As I said, that sounds like bureaucrats with no real understanding of the system were allowed to make decisions.

I live in Canada, which hasn't been an "imperial" environment for a long time. We used the Imperial System until 1970 and then changed to Metric since for most purposes, although we still buy building materials in feet and inches (not that a "2x4" has been 2" x 4" for a very long time) and in grocery stores the prices of meat and produce still indicate both with the price per pound larger (although all of the prepackaged goods have either metric only or metric first).
We learned both systems in school, Imperial first but from about grade 6 or 7 we learned about the metric system in science class (for me that would have been about 1965) and electronics was metric long before I started studying it in 1969.
(I know it was officially Systeme International from 1960 but didn't hear it called that until well into the 1970s and our government referred to the change as "Metrication")
My children (all born after Metrication) often struggle when they come across inches, Fahrenheit, gallons &c
(BTW: we used real, full sized Imperial gallons here, not the miniature US ones).

When I was a teenager working on bicycles I found that I liked using metric wrenches because I didn't have to juggle the denominators of fractions to figure out which was the next size up or down (50+ years later I know the inch sizes but I still prefer metric wrenches, fasteners &c).

I was the engineering department at 2 different speaker manufacturers for a total of 12 years in the '70s & '80s, which involved designing the speakers and preparing all of the drawings for manufacturing them. When I started the cabinet parts were all measured in inches & fractions but over time the material suppliers changed the thicknesses from fractions of inches to mm and CNC woodworking machines (programmed in metric units) replaced the older equipment so at some point during my time at the 2nd company the fellows in the wood shop were converting every dimension to mm and it made a lot more sense for the drawings to be metric in the first place. From that point on all the drawings I produced for new models were metric (which meant the maths I had to do to figure out how the parts fit together was much easier) but also for over a year I spent at least part of every day converting the drawings for older models to metric as they came up in production.
 

Harald Kapp

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The kilogram is by definition 1000 grams. The gram is the base unit. Anyone with basic understanding of how the metric system works should see that anything with a metric prefix cannot be a basic unit.
I'm afraid you're wrong here. The kg is the basic unit of the SI system. Despite the kilo prefix. The Wikipedia explains the details and the history well.
The SI system used to be called MKSA system, too, an abbreviation for Meter-Kilogram-Second-Ampere the four basic units from which all other units could be derived.

We face her the, admittedly weird, situation that the gram is actually 1/1000 of a kilogram. Trust me.
 

Sidecar Bob

Dec 19, 2021
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Well, I've been using an uppercase K for kilo so long that I will continue doing so unless someone tells me that KΩ means something different from kΩ.

None of the information presented here so far does.
 

bertus

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Nov 8, 2019
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Hello,

The capital K is used for the kelvin temperature scale:
K kelvin temperature (T) BASE UNIT
See also the PDF in post # 11

The lower case k is used as prefix as 1000.

Bertus
 
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