Hi! first post here. (voltage drop problem).

click_here

Joined Sep 22, 2020
195
Expanding on what 1an0 said, the voltage drop across the LEDs will be very close to one another.

The voltage drop seems to be about 1.89V.

So the voltage on top of the second LED is approx 2*1.89V, and 3*1.89V which is approx 5.66V (5.67V)
 
Last edited:

DickCappels

Joined Aug 21, 2008
7,442
Welcome to allaboutcircuits.com!

Another way of looking at it: You lost ~3.345V across the top half of the 'potentiometer' leaving ~5.66V across the diodes
View attachment 240606

Technically the potentiometer is a rehostat - a variable resistor. No current flows through the rheostat until the diodes are connected and the voltage across the LEDs is nearly constant with a tiny dependency on the resistance of the rheostat. The remaining voltage across the rheostat divided by the resistance of the rheostat determines how much current goes through the LED.
 

MisterBill2

Joined Jan 23, 2018
8,781
All of the responses are correct. What you have is the series voltage drop of three LEDs biased into conduction.This voltage will increase only a very small amount as you reduce thr resistance so that the current increases towards 20 mA, and the LEDs will illuminate more brightly. So there i no problem, there is just the forward conduction voltage drop of three diodes. At 1.89 volts my guess is that they are green LEDs.
 

Dave Lowther

Joined Sep 8, 2016
57
Technically the potentiometer is a rehostat - a variable resistor.
Yes, that's why I put single quotes around potentiometer in my post. I was using the term the OP used and I wanted a get out clause if anyone pointed out it was a rheostat :) I didn't want to confuse the OP because when you buy one of those 3 terminal twiddly things, it's called a potentiometer. I didn't feel it was necessary to clarify potentiometer vs rheostat in my response to the OP.
 

Delta Prime

Joined Nov 15, 2019
1,022
Hello there!
:)
The crucial difference between potentiometer and rheostat.Is that a potentiometer,is used to determine the unknown( EMF) ElectroMotive Force (EMF, denoted and measured in Volts) controls the circuit voltage. As against a rheostat, is an instrument that controls the flow of current through a circuit.
;)
 

MisterBill2

Joined Jan 23, 2018
8,781
The three terminal device IS a potentiometer, even if it is wired as a rheostat. The current election of rheostats is very small, and so it does not make sense to hammer some body for such a detail.
 

Ian0

Joined Aug 7, 2020
2,252
It's probably not our job to cause the maximum confusion for the new member, to have bought a potentiometer and to be told it's actually a rheostat. Almost as bad as being sent to stores for some glass nails.
It's a potentiometer if the two ends of the track are connected to two different potentials (one of them often 0V), and moving the wiper outputs a potential between the two.
The term "rheostat" was coined by Wheatstone, and means a current regulator.
 

MisterBill2

Joined Jan 23, 2018
8,781
It is a potentiometer if it has terminals at each end and a third terminal connected to an adjustable tap point.
AND this is a really dumb argument about something that does not matter.
 

MaxHeadRoom

Joined Jul 18, 2013
23,123
It's probably not our job to cause the maximum confusion for the new member, to have bought a potentiometer and to be told it's actually a rheostat. Almost as bad as being sent to stores for some glass nails.
"A resistor by any other name would control as sweet"

(With apologies to the the Bard ! )
 
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