complete beginner.. please explain?

Using Arduino General Electronics
1

I'm going right back to the basics with electronics as there's a lot I've never quite understood.

I was just messing around with the dimmer tutorial and thought I'd take the voltage and current measurements with 1, 2 and 3 LED's in series.

Here are all the values I got;

Can someone please explain why in the scenario with 3 LED's in series, the voltmeter would show ≈0.4V when checking them individually but then give a higher total voltage?

Also, why is the total voltage lower when only 1 LED is connected?

And finally, why is there no/little voltage across the resistor when there are 2 and 3 LED's?

Thanks!!

2

Your image didn't come through.

Have a look at this: (Out of Date) Guide: How to Insert Uploaded Images in a Post - Website and Forum - Arduino Forum

3

...I didn't see your image either, but I was able to "find it".

One of your "problems" is, 3V is not enough. You are operating the LEDs at "undefined" conditions.

LEDs are a little tricky because (like all diodes) they are non-linear. That means the resistance changes when voltage changes. The resistance is high below the forward operating voltage.* Above the operating voltage, resistance is low.

Red LEDs operate at about 2V, so you need at least 6V to "properly" run 3 of them in series.

Voltages across series components sum-up. Or we can say the voltage "divides" among the series components, proportional to their resistance. If you apply 3V, then the individual voltages will sum to 3V. (If you measure something different, you're getting measurement errors with your meter, and no meter is perfect.)

So normally, here's how an LED & resistor work - Because of the non-linearity of the LED, it operates at "constant voltage". If you increase the voltage, current increases and the LED gets brighter, but since the resistance of the LED drops, you only get a very-small voltage increase across the LED and most of that increased voltage ends-up across the resistor. In other words, the correct voltage across the resistor "falls into place". (Except in your case you don't have enough voltage for 3 LEDs.)

When we choose a resistor for an LED, we look at the LED specs and available voltage. We subtract the LED voltage from the power supply voltage to determine the voltage across the resistor. Then, knowing the voltage across the resistor, we calculate a resistor value to give us the desired current.

and current measurements

If you are using the current-range on your meter - It's better to measure the voltage across the (known) resistor and use Ohm's Law to calculate the current. (The current is the same through all series components.)

Measuring current with a meter is tricky because you have to break the circuit and insert the meter, and it's "dangerous" because in the current-mode your meter is a short circuit... If you accidently connect it wrong you can blow the fuse in the meter or fry your circuit under test. I measure voltage & resistance every day at work, but I almost never measure current (except that my bench power supply has an ammeter and I do keep an eye on that).

  • On a "regular" diode, this is the forward "breakdown" voltage, and it's about 0.7V for a regular silicon diode.
4

Technically, you could talk about led forward voltages as 'diode drops' but this isn't done normally to avoid
confusing it with rectifier and signal diode drops (0.7 & 0.3 respectively ). Led diode drops, called 'forward voltage' vary with the color so blue could be very different from red or yellow.

Led forward voltage by color

5

willduino:
Can someone please explain why in the scenario with 3 LED's in series, the voltmeter would show ≈0.4V when checking them individually but then give a higher total voltage?

Take a photo of your measurement for your single LED - including the LED and the multimeter reading. This measurement was taken with a 9V DC supply and a series resistor for limiting the current right?

It is a good idea to know the specifications of this LED. Or you could assume a particular forward voltage, and then you could use a variable DC voltage supply (eg, a bench power supply) to wind up the DC voltage (and use a suitable half-watt 220 Ohm resistor in series) until you get some illumination. But don't wind up the voltage too much, or else it might destroy the LED and/or resistor. Then you could maybe measure the current and get an idea of what level of current this LED can operate at. That's if you don't have specs for this LED.

6

willduino:
I'm going right back to the basics with electronics as there's a lot I've never quite understood.

I was just messing around with the dimmer tutorial and thought I'd take the voltage and current measurements with 1, 2 and 3 LED's in series.

Here are all the values I got;

Can someone please explain why in the scenario with 3 LED's in series, the voltmeter would show ≈0.4V when checking them individually but then give a higher total voltage?

Also, why is the total voltage lower when only 1 LED is connected?

And finally, why is there no/little voltage across the resistor when there are 2 and 3 LED's?

Thanks!!

Because voltage is relative to something, always. There is no "5v" there is only "5v relative to ground". The voltage on one side of a single LED in the chain compared to the other side of same LED will be the same for each LED.