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Electronics => Electronics => Topic started by: RVincelette on September 11, 2011, 09:10:49 PM

Title: Multimeter help.
Post by: RVincelette on September 11, 2011, 09:10:49 PM
I purchased this multimeter:
http://aidetek.com/mm5/merchant.mvc?Screen=PROD&Store_Code=A&Product_Code=VC97&Category_Code=DMM (http://aidetek.com/mm5/merchant.mvc?Screen=PROD&Store_Code=A&Product_Code=VC97&Category_Code=DMM)

Everything seems to work well but I am following the instructions in the book "Robotic Building for Beginners" to check a red LED using the diode setting. I did everything correctly. I put the black lead in the COM and the red in the V(OHM)Hz. When I connect it to the LED it lights up slightly but the meter keeps on reading .0L.  I tried changing leads, different LEDs and nothing seems to make it budge from .0L.  Every other feature on the multimeter works great, but this will not work as the book says.

Please help.
Thank you.
Title: Re: Multimeter help.
Post by: billhowl on September 12, 2011, 04:02:27 AM
I think the meter using 3V battery so may not have the same function as meter that used 9V battery.
Title: Re: Multimeter help.
Post by: RVincelette on September 12, 2011, 08:30:52 AM
It is a 3V multimeter but in the book they say that if it is open it should read roughly 3V. They also say the red LED should read only 1.6V so it should still work with this multimeter.
Title: Re: Multimeter help.
Post by: billhowl on September 12, 2011, 09:48:19 AM
Yes the red LED work
Quote
the LED it lights up slightly
.
but the resistance is too high so you get
Quote
the meter keeps on reading .0L.

Title: Re: Multimeter help.
Post by: RVincelette on September 12, 2011, 01:52:29 PM
The book says it should read 1.6V which is easily under the 3V which it maxes out on. The batteries in the multimeter are brand new. Electricity confuses the crap out of me.
Title: Re: Multimeter help.
Post by: Soeren on September 12, 2011, 03:09:24 PM
Hi,

The book says it should read 1.6V which is easily under the 3V which it maxes out on.
A common red LED (There are 3.something red types as well) will have a voltage drop of around 1.8V to 1.9V when 10..20mA is driven through it.

The tiny current used on the diode check range4 coupled with internal measuring method and wiring resistance makes it unable to register a drop below 3V - hence the "OL" reading.


The batteries in the multimeter are brand new. Electricity confuses the crap out of me.

It has got nothing to do with the batteries, but the diode check is for testing rectifier diodes, not Light Emitting Diodes.

Don't worry about the confusion, it's a lot of ground to cover and if you just learn a little bit each day, it will grow sorta exponentially, as the basis knowledge makes you understand the next steps better, making it possible to learn more etc. etc.
Title: Re: Multimeter help.
Post by: RVincelette on September 12, 2011, 03:23:14 PM
I just did a bit more research on the DMM I have and I found this:

Forward DC Current is approx. 0.5 mA. Reversed voltage is approx. 1.5 V

The multimeter take 2 AA batteries but I guess it only goes to 1.5V in diode mode.  Could this explain it?