4 Replies Latest reply on Mar 3, 2018 10:58 PM by jw0752

# Current question

Attatched I have a photo of a small circuit I set up (I know its simple but I am just starting out) and realized something weird. When I put my test leads over my resistor I got very low resistance on my resistor which I thought would take the majority of the drop since the led offers little resistance. Also odd was when I turned my MM onto amps the DC power supplies current reading went up. Why is that?

• ###### Re: Current question

You've got a 3.2V voltage source (your PSU) driving a resistor and LED in series. The LED behaves like a diode with quite a high forward voltage drop.  I found a nice cheap blue LED a bit like yours on Farnell ( 2377467) - if you download the data sheet you'll see that they specify the forward voltage as 3.3V typical at 20mA. Diodes and LEDs are very non linear for for their forward voltage/current relationship but for your diode to drop about 2.7V at 3mA (which I can read on you psu) seems quite reasonable.

Ohms law doesn't apply to diodes ! The current rises much more rapidly as the forward voltage increases than it does with a resistor.

Here's a tutorial on LEDs (looks OK but their are many others):  https://www.electronics-tutorials.ws/diode/diode_8.html

Hope this helps.

MK

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• ###### Re: Current question

Hi Mitchell,

If you turned your meter to current measurement with the leads hooked across the resistor you would have put a short across the resistor effectively taking it out of the circuit. Since the circuit would have less resistance the current would go up. The resistor is important in LED circuits as it acts like a limit so that the current of the LED can't go higher than the LED can tolerate.

John

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• ###### Re: Current question

John,

So as I understand it, if the meter wants to know current it creates a path of little resistance so that all the current is read through the meter. However how does this work in a parallel circuit? If my branch all of the sudden has 0 resistance all my current will drop across that branch and ill have none across the other branches. How do you effectively measure that?

• ###### Re: Current question

When using the Ammeter properly it is always necessary to break the circuit and put the ammeter in series with the rest of the circuit. You are correct the Ammeter has a very low precision resistor across its leads and the meter reads the voltage drop across this resistor and displays the output as Amps or milliamps.

John

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