Electronics > Beginners
LED voltage measurement question
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iMo:
While doing a measurement with your DMM with 10Mohm input resistance and with long DMM cables you have to be aware you may always see such effects.

A capacitive/inductive coupling to the AC mains in your voltage adapter, AC pickup from wires around you, high frequency signals from radio sources (ie wifi, gsm, ..), wrong grounding or decoupling, all this stuff may influence your measurements.

Those AC signals are rectified by your LED diode and with the high 10Mohm input resistance you see the DC voltages.

The signals come from "high impedance" sources, say 1V AC pickup signal comes from a source which has say 100Mohm internal impedance.

With your 10Mohm DMM input impedance/resistance it creates a voltage divider so you will see (after DC rectification) something like 90mV.
 
When you wire 10k in parallel to your 10Mohm DMM input you will get around 10k input resistance of your DMM in the result and the resulting voltage will be aprox. 1000x smaller.
iMo:
I would recommend you to download the LTspice simulator (it is free and easy to operate) and you may learn to draw schematics and play with your designs :)
Lucky-Luka:

--- Quote from: imo on March 31, 2019, 12:40:50 pm ---
When you wire 10k in parallel to your 10Mohm DMM input you will get around 10k input resistance of your DMM in the result and the resulting voltage will be aprox. 1000x smaller.

--- End quote ---

Why?
btw I've tried a 10k resistance in parallel to my led and now I read 0V.

Luca
iMo:

--- Quote from: Lucky-Luka on March 31, 2019, 01:45:10 pm ---
--- Quote from: imo on March 31, 2019, 12:40:50 pm ---
When you wire 10k in parallel to your 10Mohm DMM input you will get around 10k input resistance of your DMM in the result and the resulting voltage will be aprox. 1000x smaller.

--- End quote ---

Why?
btw I've tried a 10k resistance in parallel to my led and now I read 0V.

Luca

--- End quote ---

Why what?

I've taken the 10k as an example. You may take 100k if you wish. Or any other value which is much smaller than 10Meg.

Note: when you decrease the input impedance/resistance of your DMM (ie with a parallel resistor) you have to think about how it will influence the circuit in which you measure.

What is important for you is to get a feeling for what you are actually doing.
With high impedance/resistance circuits you have always to calculate with parasitic phenomena :)
hamster_nz:

--- Quote from: Lucky-Luka on March 31, 2019, 11:58:03 am ---
--- Quote from: hamster_nz on March 30, 2019, 08:08:10 am ---
One thing I am pretty sure I know is this: if you put a 10k resistor over the legs of the LED will make the problem go away. It will still light when you press the switch, and it will read (very close to) zero when the switch is open. The 10k resistor will swamp out any of these minuscule effects, and make things look more like the "ideal" LED.


--- End quote ---

Can you explain me why?
thanks

Luca

--- End quote ---

The 10k resistor is low enough to provides a path for whatever the stay voltage is to 'balance out'. Any voltage over the LED will cause current to flow in the resistor, which the heats up a tiny bit, getting rid to the energy.

But the resistor is also high enough that it doesn't really change the desired operation on the rest of the circuit in a meaningful way (which a low valued 10 or 100 ohm resistor might).
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