Reading current on my analog MM

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
One of the things I've inherited from my late grandfather is an old Radio Shack analog multimeter. From the few things I've measured and understood, it seems to be surprisingly accurate. However, I'm a little confused as to some aspects of reading current. R and V seem pretty easy, but for some reason, current hasn't fully "clicked" yet.

The manual is sparse, and does not explain which scale on the VU to look at when using different ranges. I'm sorta' getting it piece by piece, but there are still a couple gaps in fully understanding this thing.

Here are the 4 DC scales (used for both I and V):

0 - 50 - 100 - 150 - 200 - 250
0 - 25 - 50 - 75 - 100 - 125
0 - 10 - 20 - 30 - 40 - 50
0 - 2 - 4 - 6 - 8 - 10

The ranges for DC current are:

10A (has a seperate + input for this range), 500mA, 50mA, 5mA, and 50uA.

There is also a range doubling switch.

Here's my understanding thus far, and a couple questions:

Say for example, to read up to 10A, naturally I'd use the 10A range, and look at the 0 - 10 scale on the meter.

Reading up to 50mA, I assume use the 50mA range, and look at the 0 - 50 scale.

BUT, how do I read everything else from 50 - 500mA? I know it's easy as pie, and the answers are staring me in the face, but it just hasn't "clicked" yet. Any help would be greatly appreciated.

JoeJester

Joined Apr 26, 2005
4,390
You would use the DC ... 0 to 50 scale for all your readings, except the 10 A reading. You'll adjust the decimal point [5.0 mA or 500 mA] as needed.

What model number is your multimeter? Does a pic of the face exist on the net?

awright

Joined Jul 5, 2006
91
JoeJester's advice on meter scales is good.

Bear in mind that an analog current meter is easily burned out if used incorrectly because it has a low resistance between leads and probably no or minimal protection against misuse. Some better meters do fuse the current circuit but a serious overload is still very bad for the meter. If you don't burn out the movement (generally irreparable) you can bend the pointer by slamming it against its stops.

The most common error leading to meter movement burnout is to perform a current measurement with great care, then put the meter down with the leads still connected in the current function. The next time you pick up the meter to perform a voltage measurement, you don't notice that the leads are still plugged into the current jack. You put the probes on a voltage source, draw a very large current through the ammeter, and burn out the meter or the test object. I've done this a half dozen times over my 60 years of playing with electrons and it always hurts bad. I now try to remember to NEVER put the meter down with the probes still in the current jack.

I don't want to scare you - just to alert you to a possible hazard.

You may be able to download a manual from Radio Shack, depending upon how old the VOM is.

Have fun.

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
You would use the DC ... 0 to 50 scale for all your readings, except the 10 A reading. You'll adjust the decimal point [5.0 mA or 500 mA] as needed.

What model number is your multimeter? Does a pic of the face exist on the net?
Joe - I'm sorry to say that doesn't fully make sense to me, as I still see gaps in certain current ranges.

Just to get this out of the way first, it's a Micronta (old Radio Shack brand) part# 22-214. Here are the scale and range pictures (this unit is not mine... grandpa kept his stuff in way better shape than this):

OK... now talk about being dense... what am I not getting? I'm googling everything possible about reading current, including images of other analog MM's, and it ain't happenin'. Anything log 10 that matches up with the last number of any scale is easy... no problem. Let me ask about current this way, as I'm usually good at reverse engineering an answer.

What range is used, and what scale is read to measure, say, 150mA? Or, 120 mA for that matter? Here's why I ask.

While recently rewiring a laser project mentioned in a couple other threads here, I was able to take some current measurements, which I didn't do while building it (it was a "kit" with predetermined safe values). With the MM set to the 500mA range, and looking at the 0 - 250 scale, the reading was approx. 153. The problem is, the laser diode has a threshhold current of 60mA typical - 75mA max., which means it doesn't even begin lasing until this range. It's typical - max current for rated laser output (80mW) is 150mA - 200mA.

Ignoring the IV curve of the diode and going strictly by Ohm's law, there is 3.2V from the batteries, going through a 2.7 Ohm resistor... so one might expect to see a minimum current of 118.5mA. With the IV curve factored in, one would expect to see a little more, but probably not much. Hence, asking how to read 120 - 150mA.

HELP!! This is driving me nuts. As mentioned, the answer must be right under my nose. As dad likes to say to this day, "if it was a snake, it would'a bit ya'". I also refuse to resort to an autoranging DMM before understanding how to kick it old school... lol.

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
awright - Hehe... well, I'll admit to almost messing up grandpa's meter the other night. Something (we'll call it beer, and non-thinking) possessed me to try the 5mA range on the laser circuit, and needless to say, it pegged the VU pretty quick.

Offhand, nothing appears burnt out, and will soon check the meter against various measurements taken before, to see if it threw off the calibration.

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
BTW... doesn't the 0 - 125 scale seem superfluous? What's it there for? Most AMM's I've seen pics of don't have that.

recca02

Joined Apr 2, 2007
1,212
can we not set a fsd for 500 ma ,the resolution will suffer but atleast we can have readable values between 0-150 + above. i have little experience of using amm
and that too which had not more than two or three scales so sorry if i m wrong here.

or may be a shunt resistance for mm to convert fsd to 150 ma from say 50ma ?

m i getting the question all wrong?

mozikluv

Joined Jan 22, 2004
1,435
let's just consider the 125ma scale as just a matter of design, anyway it would cost them only 1 more resistor to install.

moz

n9352527

Joined Oct 14, 2005
1,198
What range is used, and what scale is read to measure, say, 150mA? Or, 120 mA for that matter?
You use the lowest current range that covers 150mA, which in your case is 500mA. Read the value on the appropriate scale for the range, for example, if the range is 500mA maximum, then you can use 1-10 multiplied by 50, or 0-50 multiplied by 10 or 0-250 multiplied by 2.

Notice that for DC and AC the scales are linear, so you can use any scale with an appropriate multiplier so that the maximum range value equal to the maximum scale value.

This doesn't work with non-linear scales, such as resistance and dB.

Gadget

Joined Jan 10, 2006
614
Analogue meters still have a valued place on my bench. When you know how, they are great for checking most semiconductors... and when voltages are varying, the meter swings nicely in a logical manor, rather than random digits appearing on a digital scale.
Because of their lower Ohm per Volt, they can tend to load a circuit more, but this means you normally dont get those weird readings around high voltages that tend to be induced into high impedance digital meters.
Don't get me wrong, I use a digital most of the time.... but I don't think I could do without the analogue either.

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
You use the lowest current range that covers 150mA, which in your case is 500mA. Read the value on the appropriate scale for the range, for example, if the range is 500mA maximum, then you can use 1-10 multiplied by 50, or 0-50 multiplied by 10 or 0-250 multiplied by 2.
That's exactly what would make sense, but I think I know what's been messing me up here. The amount of current when testing the laser was over double than what was expected, thereby throwing me into confusion the last couple days.

The "kit" with the predetermined values (which consisted of the diode, resistor, and cap) is supposed to be running a little under the typical output rating of 80mW, which therefore should be consuming less than 150mA of power.

So, if the meter was on the 500mA range, and looking at the 0 - 250 scale gives a reading of 153, multiply by 2, and that means 306mA??? My laser should be dead by now! Anyone putting 300+ mA through these diodes knows they won't live very long, hence my confusion. There CAN'T be that much current going through this circuit... it's only 3.2V going through a 2.7 Ohm resistor, and the diode.

For what it's worth, current was tested between batteries and switch, not between resistor and diode. Does that matter?

Edit: As far as I can tell, it shouldn't matter where current is tested, as it should be the same anywhere in the circuit. It's likely that my confusion somehow ties in with voltage drop across the diode, and/or the IV curve someone mentioned to me recently. Unfortunately, I don't have two meters yet to make a plot of that curve. Also don't have a variable bench supply, so the plot would have to be created by testing with a bunch of Alkaline, and Lithium primaries in various levels of depletion.

n9352527

Joined Oct 14, 2005
1,198
If you have the circuit of the kit, post it. Is it a linear driver or a PWM? 3.2V over 2.7 ohm and a diode could well be over 300mA, depending on the Vf.

Ron H

Joined Apr 14, 2005
7,014
The scale says "range doubler". That would explain the 125 scale. How are those two little knobs labeled?

Thread Starter

Avalanche Breakdown

Joined Mar 22, 2007
20
If you have the circuit of the kit, post it. Is it a linear driver or a PWM? 3.2V over 2.7 ohm and a diode could well be over 300mA, depending on the Vf.
I can scan the schematic if needed, but offhand, this pic should explain the entire circuit. My layout of components is different, but follows exactly this design... which as you can see, has no driver.

Here are the electrical specs of the diode. I'm assuming what they call "operating voltage" is what we call Vf.

I just re-read a bunch of posts regarding the group buy for this laser diode over on CandlePowerForums (which I participated in), and saw that the resistor value was chosen to give approx. 150mA to the diode when run on 2 alkaline batteries. I'm using E2 Lithiums, rated at 1.7V each.

Very interesting that current appears to be over 300mA. Perhaps I got a freak diode with a really low Vf? It happened to at least one person in the group buy, where he used the same resistor, no cap though, and two alky D batteries... and got a reading of 318mA! The person running the GB plotted a rough power chart for these things, using a calibrated laser meter, and 318mA translates to 182mW of output!!

Reading that thread again, and related ones, also reminded me of the toughness of these diodes. Yes, some people have had them fail at 300mA, but others have lived... although nobody knows for how much longer.

Edit: I've been saying 3.2V this whole time without realizing it. The E2 lithiums are giving 3.4V total.