Simple guide to using a DMM for measurements

[HKJ]

Thanks for the great guide. I wasn't aware the meter turning off would cause no problems to measuring current. Am I right that no power is consumed from the internal battery either when the meter is off even if still part of a circuit?

Cheers

Turning the meter off might affect the current measurement, if the power is controlled with the range selector switch. But letting the meter turn itself off will usual not change range, because the range selector stays in the same position.

When the meter turns itself off, it will probably still use a small amount of power, this has nothing with being part of the circuit to do.

 

[AFAustin]

May a non-technical guy please pose a question? After many years of faithful service, my budget DMM is acting strangely. When I first turn it on, it measures a cell's voltage too low, and then gradually measures it more accurately as a few minutes go by.

I changed the battery, but that hasn't solved the problem. Just now, it seemed worse---it won't show a fully charged AAA NiMH cell at more than ~1.23v, when I know it should be closer to 1.35v.

Any advice would be appreciated. Thanks.

Andrew

 

[Mr Happy]

I have inexpensive meters that sometimes behave a bit like that after a period of non-use. In my case I put it down to oxidation on the contacts of the rotary switch leading to poor contact. Sometimes I can fix it a bit by rotating the switch vigorously between settings a few times before using the meter. I don't know if there is a really good cure -- you tend to get what you pay for, and such things may be more likely with budget meters.

 

[HKJ]

May a non-technical guy please pose a question? After many years of faithful service, my budget DMM is acting strangely. When I first turn it on, it measures a cell's voltage too low, and then gradually measures it more accurately as a few minutes go by.

I changed the battery, but that hasn't solved the problem. Just now, it seemed worse---it won't show a fully charged AAA NiMH cell at more than ~1.23v, when I know it should be closer to 1.35v.

Any advice would be appreciated. Thanks.

Andrew

My first thought was flat battery, but you sort of spoiled that (Are you sure the new battery is working).

Next point is oxidation on the rotary switch, if you can, try opening the meter and clean the switch, maybe use some deoxit.

The best solution is to buy a good meter (My preference is Fluke), but that is a bit expensive.

The next best solution is to use a few $ on a new meter and maybe a cheap calibration unit/voltage reference (The best/cheapest way to verify that a meter works).

 

[AFAustin]

Thanks, Mr Happy and HKJ.

I will follow your suggestions and report back.

Andrew

 

[AFAustin]

Oxidation on the contacts of the rotary switch must be it---I vigorously rotated the switch a number of times, and voila'---working like a champ again. :thumbsup:

I removed the cover, thinking I'd try cleaning and applying DeOxit to the contacts, but the circuit board is a bit difficult to remove. For now, I will leave well enough alone.

Thanks again, gents, for your help. I really appreciate it.

Andrew

 

[kosPap]

hmm if there is stoll room for DMM trouble shooting what is the reason for one not zeroing....at times it reads (standalone)0.11V or more..some things happens with A measurements.....

it is not a batetry thing not a calibrating issue (have accessed the rotary dial (a resitor) taht zeros the readings....

toasted maybe?

 

[Mr Happy]

hmm if there is stoll room for DMM trouble shooting what is the reason for one not zeroing....at times it reads (standalone)0.11V or more..some things happens with A measurements.....

it is not a batetry thing not a calibrating issue (have accessed the rotary dial (a resitor) taht zeros the readings....

toasted maybe?

What does it read on DC V if you short the probes together?

 

[kosPap]

still the same number

 

[uk_caver]

I had something like that happen after blowing a fuse, but then (fairly obviously) all the functions that ran through that fuse stopped working.

 

[Mr Happy]

Most DMMs give non-zero indications on the volts ranges while the probes are unconnected due to static charges on the inputs. They will only read zero if you short the probes together. I have never owned a meter that didn't read zero when I shorted the probes.

It's probably a bad idea to adjust any of the internal calibration pots unless you have suitable calibration references to use and you know exactly what you are doing.

If you have turned any of those potentiometers since the meter was originally calibrated you might have thrown the readings off.

 

[wquiles]

It's probably a bad idea to adjust any of the internal calibration pots unless you have suitable calibration references to use and you know exactly what you are doing.

+1

I got me one of these 1-2 years ago. It is awesome to routinely check my Fluke DMM's:
highly stable 5Volt reference

And my Flukes are still very accurate:

and the same guy now sells another one that also offers current and resistance checks (probably a better buy price-wise):
DMM DVM Tester Checker Calibrator Voltage Reference

I have no affiliation with the seller - I am just a happy customer.

 

[Bullzeyebill]

Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

Bill

 

[wquiles]

Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

Bill

Bill,

I don't have the current checker, only the voltage checker, but basically yes. Once you "correct" the calibration factor, which causes the true value to deviate as components age, drift, etc., the meter should then be again accurate to within what is designed to do. These are for example the specs on my Fluke 289:
http://us.fluke.com/fluke/usen/digital-multimeters/fluke-289.htm?PID=56061&trck=289specifications

Corporations/ labs/engineering groups routinely send out their measuring equipment to be calibrated (sometimes yearly) to ensure that their results are meaningful. It is no good to have great resolution if the result lacks accuracy.

 

[Meterman]

Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

Bill

The "DMM DVM Tester Checker Calibrator Voltage Reference" delivers 1.000mA, so you only test the range 1mA. A good closeness there does not necessarily imply precision in the 5A range.

When an instrument is to be calibrated, every range is examined and adjusted individually.

Wulf

 

[Bullzeyebill]

The "DMM DVM Tester Checker Calibrator Voltage Reference" delivers 1.000mA, so you only test the range 1mA. A good closeness there does not necessarily imply precision in the 5A range.

When an instrument is to be calibrated, every range is examined and adjusted individually.

Wulf

Thanks for correction. Yes, I did read the spec wrong.

Bill

 

[wquiles]

Thanks for correction. Yes, I did read the spec wrong.

Bill

Same here. Sorry about that :green:

 

[Bullzeyebill]

Going back to DC current measurements what would be an ideal resistor for measuring 3-8A's?

Bill

 

[HKJ]

Going back to DC current measurements what would be an ideal resistor for measuring 3-8A's?

Bill

That depends on what you are going to measure, i.e. how big voltage drop you can accept.
If it is a direct drive led from a single LiIon, without any series resistor, you need a very low voltage drop. A 0.01 ohm resistor would work, but you need to be very careful with the connection from the resistor to the flashlight and battery, they could add considerable resistance to the connection.
If you are working with 2xLiIon and a buck driver, you do not need to worry as much about voltage drop and a 0.1 ohm resistor (0.8 volt drop at 8 amp) would do fine.

 

[njet212]

It sound like you are measuring volts at the battery, not at the light. The meter and wires can easily drop a few tenths of volts.

I did things just like on your picture. I'm bit confused, if i'm doing thing just like your pic, I'm I measuring volt battery or current draw? :thinking: