Why does the potentiometer only change the voltage when there is an LED? - Page 1
Forum section: electronics / beginners

author: Kahenraz
author_rank: Contributor
timestamp: November 03, 2018, 04:04:39 am
content: Why does the DC voltage only decrease as I adjust the potentiometer if there is an LED in the circuit?I think this has to do with a load being on the circuit but isn't the potentiometer considered a load by its resistance?If I use a 0R resistor instead then it reads 0V.Why is this?Please be kind. I am new to electronics.

author: mariush
author_rank: Super Contributor
timestamp: November 03, 2018, 04:31:45 am
content: The led behaves like a diode with a forward voltage of around 1.7v...1.9v (for a red led like in the picture) but this diode only conducts once there's some amount of current flowing through it.There is also a very narrow region in which the led behaves like a resistor, it's not fully closed and it's not fully on.So when you have the potentiometer in the circuit, you practically have a resistor in the circuit which limits the current flowing through the led. If the resistance is small enough, enough current flow that the LED is turned on fully and there's a voltage drop across the led.You are measuring the voltage drop between the led terminals with your multimeter.If you adjust the potentiometer, you're either reducing the current flowing through circuit too much, so the led starts to turn off, or you're letting more current flow and you could either damage the led or the batteries could suffer and lower their output voltage, if they can't supply the demanded current.if you use a 0 ohm resistor, you're measuring the voltage drop between the multimeter probes. Since the resistor is 0 ohm, the resistance of the wires is very small and the current is very small (because it's limited by the potentiometer) then the voltage drop is practically 0.Remember Ohm's law ... Voltage equals current x resistance .. you can use it pretty much everywhere.

author: ocset
author_rank: Super Contributor
timestamp: November 03, 2018, 10:39:11 am
content: yes, from your question it sounds like you are expecting the voltmeter to be able to read the current through the zero ohm load.As you know, the voltmeter just measures the voltage between the two points in the circuit.......and if there is zero ohms, as Mariush says, then you have zero volts. It must be so.No matter what current flows through zero ohms......there will always be zero volts across it.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 03, 2018, 04:44:06 pm
content: The problem with the way your circuit is shown is that when you use a zero ohm resistor and you adjust the pot to minimum resistance, the current can be high enough to burn out the pot. Having zero ohms across the battery is a dead short and would be the same as shorting the battery with a screwdriver or some other metal object.An LED acts a lot like a zener diode in that it has a characteristic voltage drop when it is producing light. A red LED may have about 2 VDC drop and that voltage will vary a little as the current through the LED changes. Most small LEDs will have a maximum current rating of around 20 Ma. If you exceed the current rating of the LED by much, it will burn out and this could happen with the circuit you show because when the pot is adjusted to the high end there is 3 VDC connected directly to the LED and nothing to limit the current.This circuit will work and limit the current through the LED to 15 Ma.

author: bson
author_rank: Unknown
timestamp: November 03, 2018, 09:28:37 pm
content: You're measuring the voltage across the 0R resistor. U=R*I, so if R=0 you also get U=0 no matter what the current.

author: mariush
author_rank: Super Contributor
timestamp: November 04, 2018, 12:41:45 am
content: Normally, he would get a voltage reading that's slightly different than 0, because the leads themselves have some resistance which will not be 0 ohms. The measurement is done inside the multimeter, so the probes/leads become part of the circuit. But my guess is the current was already limited significantly the by potentiometer's high resistance and his meter may just not be sensitive enough (or it's set on a range that's too big, showing only one or two decimals after zero).But yeah, as a problem , assuming ideal wires with 0 resistance... I already explained.