Current measurement 240315

Thread Starter

allenpitts

Joined Feb 26, 2011
166
Hello AAC forum,

Ok, please don't jump on me because this is such a basic question.

Voltage measurements are made frequently but current tests not so much.
What is remembered from the basic electronics class is that voltage is measured in parallel
and current is measured in serial, right?
Current_test_240315.jpg
So it was conjectured to measure the current going thru the LED in 'LED Tester' a break
in the circuit is made, anywhere, and the multimeter is connected in serial
as shown in 'Current Measurement'. The probes must be connected to Common and
the mA/uA meter input and the dial has to be turned to mA.
But the LED does not light and the meter continues to read 00.00.
What am I missing?
Thanks.
Allen Pitts
 

Reloadron

Joined Jan 15, 2015
7,543
You got it, we measure voltage across the load and current in series with the load. Placing a meter setup to measure current across the load bad things happen. :) I agree with spook as something is open circuit and likely a blown fuse in your meter.

Ron
 

dl324

Joined Mar 30, 2015
16,988
Voltage measurements are made frequently but current tests not so much.
I'm more inclined to measure the voltage drop on a resistor and calculate current.

If you don't think about what you're measuring, the meter can perturb the circuit and give erroneous readings.

However, there are times when the convenience of a direct measurement makes it worth the bother. I wanted to measure the gate leakage current of some MOSFETs I purchased on AliExpress that were packaged improperly (and likely mishandled too). The maximum Igss was specified to be 100nA, so I was looking at having to make an amplifier with the offset voltage nulled so I could measure leakage current. In the end, I decided I'd use my HP bench meter and just measure directly on the 200uA scale. That was much more convenient, but also an exception for me.
 

Sensacell

Joined Jun 19, 2012
3,471
When the meter is set to "current mode" - it's placing a low value resistor across the input leads, and measuring the voltage developed across it.
I seldom use the current mode on my meter, it requires a lot of extra caution not to short or damage something. (post #3)

You need to stay focused so as to never apply a voltage source when a meter is configured to measure current.
 

MisterBill2

Joined Jan 23, 2018
18,934
Use the meter to measure the VOLTAGE across the series resistor, then calculate the current. It will also be needed to know the actual resistance of that series resistor if an accurate determination of current is required.
 

Ya’akov

Joined Jan 27, 2019
9,230
I concur with @nsaspook—I think you will find a blown fuse in your meter. This is very common and could have happened long ago without you realizing it.
 

Tonyr1084

Joined Sep 24, 2015
7,943
I concur with @nsaspook—I think you will find a blown fuse in your meter.
And I concur with Ya’akov. The fuse blew.

[edit] correcting numerical mistakes [end edit]

I gave a gift of a meter to a friend. He immediately blew the fuse in it by testing wrong. The fuse - if I remember correctly was 13mA (0.013A). In your diagram you show a 9V battery with a 220Ω fuse. We don't know the forward voltage of the LED, which differs from manufacturer to manufacturer and varies with color. I'll assume you have a forward voltage (Vf) of 2.2 volts (2.2Vf)

(9V - 2.2Vf) ÷ 220Ω = 31mA (0.0309Amps). That's high enough to blow a 13mA fuse quickly.
Change current scale to 10A (or the equivalent higher current scale) and connect the lead to that instead of mA/uA.
Not all meters have this range option. If yours does have this option then follow this advice. The cheap meter I gave my friend didn't have that option and he blew the fuse when he set the meter to read current then tested the voltage across a 12 volt car battery. His father (like mine) always referred to voltage as "Current". I grew up in a home with a current of 120. That was how my dad understood it. I had to unlearn that and learn more about both AC and DC. Aside from the obvious difference, there is still other things to understand about AC, and that took me much longer to understand. And now I only partly understand AC.
 

nsaspook

Joined Aug 27, 2009
13,418
I concur with everyone who mentions a blown fuse. That includes I concur with me also. Concurrence is a wonderful thing. :)

Ron
It's easy to concur when it goes boom measuring voltage in the current configuration.
1710696977148.png
This is when you're glad you have a Fluke meter in your hands instead of a Harbor Freight cheapie.
 

Reloadron

Joined Jan 15, 2015
7,543
It's easy to concur when it goes boom measuring voltage in the current configuration.
View attachment 317831
This is when you're glad you have a Fluke meter in your hands instead of a Harbor Freight cheapie.
Nice current transformers. 0 to 10, 0 to 20 and 0 to 50 amp ranges settable with jumper. I have seen the same CTs with different manufacturer names on them. Here is a pretty old school application. Just be real careful what we touch as a mistake can be quite shocking.. :) Three Phase 480 volt delta.

Current_Transformer.jpg

Ron :)
 
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