Question on Voltage & Current

Thread Starter

drjackool

Joined May 23, 2021
34
Hi
[Voltages are somewhat]
I wonder on some thing!
When connecting a simple LED to 220V to reduce the voltage I used a resistor and the circuit works fine and when measuring the led voltage by multimeter is is 3v but when disconnecting the led and measuring the voltage on open circuit after resistor it is 220v! why?
Thanks
 

BobTPH

Joined Jun 5, 2013
9,110
Because the rest of the voltage is across the resistor. When the circuit is open, there is no current through the resistor, so it has no voltage across it.
 

Thread Starter

drjackool

Joined May 23, 2021
34
Ohm's law. :rolleyes:
sorry My english is bad so my question was not clear.
I found the answer on web: Resistor just limits the current and keeps input voltage at output.
LED just need 0.01 amp across it to turn on and the voltage is not matter!
before today i thought the resistor reduces the voltage.
 

dl324

Joined Mar 30, 2015
16,988
sorry My english is bad so my question was not clear.
I found the answer on web: Resistor just limits the current and keeps input voltage at output.
LED just need 0.01 amp across it to turn on and the voltage is not matter!
before today i thought the resistor reduces the voltage.
When the LED is removed from the circuit, no current is flowing in the circuit, so there will be no voltage drop across the resistor.

Since the meter isn't ideal, some current flows, but the resistance of the meter is much much greater than the current limiting resistor, so the result is the same for all practical purposes.

If you don't understand the basics, you shouldn't be working with lethal voltages.
 

BobTPH

Joined Jun 5, 2013
9,110
Resistor just limits the current and keeps input voltage at output
That is absolutely wrong. The resistor reduces the voltage by

V = I x R

that is Ohm’s law, the most basic equation in electronics.

If you put 220V across the LED it would die an instant death.
 

Thread Starter

drjackool

Joined May 23, 2021
34
That is absolutely wrong. The resistor reduces the voltage by

V = I x R

that is Ohm’s law, the most basic equation in electronics.

If you put 220V across the LED it would die an instant death.
I know the answer now!
exactly my question was here, If resistor reduces voltage why without led it is not reduces?! the voltage drop is side effect of resistor job and it is depend to current flows the resistor.

I come to electronics in middle of story :Dand damn to my bad teachers

Also Don't blame me, until a few years ago, scientists thought that positive elements move and not electrons, which are also used!
 
Last edited:

MisterBill2

Joined Jan 23, 2018
18,926
When the supply is an AC voltage there is another issue, which is that the voltage across the LED in the direction it is NOT CONDUCTING will be very high and so the diode will fail because of excess reverse voltage. Two different situations .
 

crutschow

Joined Mar 14, 2008
34,671
until a few years ago, scientists thought that positive elements move and not electrons,
What "positive elements" are you referring to?
Certainly there have never been moving positive elements proposed as being in an electronic circuit since the electron was discovered in 1897.
 

WBahn

Joined Mar 31, 2012
30,230
I know the answer now!
exactly my question was here, If resistor reduces voltage why without led it is not reduces?! the voltage drop is side effect of resistor job and it is depend to current flows the resistor.

I come to electronics in middle of story :Dand damn to my bad teachers

Also Don't blame me, until a few years ago, scientists thought that positive elements move and not electrons, which are also used!
On what do you base the notion that scientists thought that "positive elements" move and not electrons until a few years ago?

Many of the things you have said in this thread indicate that you are getting really bad information. This is not a good situation when you are messing around with 220 VAC mains supplies -- this is a really good and quick way to get seriously injured or dead.
 

MisterBill2

Joined Jan 23, 2018
18,926
Certainly experimenting with mains voltages requires a greater amount of care because of the much higher power available. But it is entirely possible to do a lot of safe and successful experimentation even with mains voltage circuits. The secret is to always disconnect the power before reaching in to make changes. And a handy safety tip is to never work in an area with a conductive floor. That means even a dry concrete floor on the ground, even with a layer of asbestos floor tile on top of it, such a floor will have a resistance to true ground of only a few hundred ohms, at the most, and that will allow plenty of current to deliver a nasty jolt. Been there and done that when I was NOT experimenting at all.
 
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