# What is the resistance of a small LED bulb?

Discussion in 'Electronic Basics' started by zalzon, Nov 16, 2003.

1. ### zalzonGuest

Hi,
newbie question here that's got me confused. I'm trying to figure
out the resistance of an LED in this circuit.

I have a series circuit with a 1000 ohm resistor, a 3mm LED red bulb
and a 6200 ohm resistor in series. The input voltage is 5.02 volts
and the input current is 0.46 mA. Using this info, I have calculated
the LED resistance to be 3713 ohms.

Is it possible that the LED has such a high resistance?

On my multimeter, I read 0.46 volts across the 1000 ohm resistor and
2.91 volts across the 6200 ohm resistor. So i guess it matches up
with the calculated values approximately.

I can't seem to get the resistance of the LED using my multimeter for
some reason.

2. ### John PopelishGuest

At that current, sure. But for devices that do not hold a fixed
ratio of voltage to current, called non-ohmic, this resistance has
little meaning.

If you change the current through the LED, the resistance (as the
ratio of voltage divided by current) will be less.

LEDs drop a voltage that is proportional to the log of the current,
plus a small drop proportional to the current (an ideal diode in
series with a resistance).
That is because the meter probably biases the LED with only a tenth of
a volt or so, and at that voltage, the current is unmeasurably small,
so the resistance reads open circuit.

3. ### zalzonGuest

Woah. i never knew that. So LEDs have variable resistance based on
current and voltage. So when designing a circuit, we have to take
into account the resistance of the LED.
When I set the multimeter to 200 Ohms resistance and touch it to the
leads of the LED, it lights the LED up. Is that not enough current to
make a resistance reading? All it shows is 1 which to me means to set
the resistance higher.

When I switch to 2000 Ohms (the next resistance level on the
multimeter), the LED light is no longer lit. Probably as you said the
current is too small. The reading still shows 1.

I keep on going up the scale all the way to 2000K and all I see is 1
and no light on the LED.

4. ### John PopelishGuest

A better way to say it is that you have to take the V versus I curve
into account. For currents within a decade of rated current, the
voltage varies less than a tenth of a volt. So if the data sheet says
that the typical forward drop is 2.1 volts with 20 ma, that voltage is
a good guess for the forward drop for all currents down to about 2 ma.
If it lights up, it should give a resistance reading, but since you
don't know the current, that resistance is not much use to you. I

Many meters have a junction test range (labeled with a diode symbol)
that applies about 3 volts open circuit with a few ma current limit
(there is an internal resistance). That range displays the forward
drop at that small (but unspecified) current. But since the voltage
of diodes does not vary much over a range of forward bias, this
reading is more helpful when figuring out what supply voltage will be
left over for the dropping resistor to consume as it sets the LED
current.
Higher resistance ranges bias the device under test with smaller
currents.

5. ### Watson A.Name - Watt Sun, Dark RemoverGuest

That's because the current thru the LED is non-linear with respect to
voltage. The LED won't conduct at voltages much below 2V, and when
the voltage gets to 2V, it starts conducting heavily, so that it would
burn out if the current was not limited. This is the way any diode
acts.

If you measure the voltage and current at many points, you will find
that the LED does not have a straight line V-I plot, like a normal
resistor. So you can't say it has a certain resistance at any point
other than at that point where the measurement was taken.

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6. ### Watson A.Name - Watt Sun, Dark RemoverGuest

Well, if the digital voltmeter has a maximum range of 1.999V, and it's
putting enough current thru the LED to cause it to drop 2.1V, then the
meter is going to indicate that it's out of range, or a flashing 1.
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