Was wondering how one can measure the resistance of an LED (load ?)
I searched and found that one can't do it with a multi-meter - i thought i was doing something wrong when i tried it - was going to post a thread here when i thought i'd try putting it in a voltage divider circuit and then calculate given the reading at the Arduino analog pin - like so;
But this also doesn't work - the AnalogRead(val) = 523
And when i changed the 10K to a 1K, i got 356 !
I took out the LED and then got 762 which is correct.
So how does one measure the resistance of an LED ?
Am trying to set up a multi-button layout on a single analog input and have LEDs in series with the buttons just as a confirmation that the button has been pressed - i'm setting up a spreadsheet to calculate various values like this;
but if i'm putting in LEDs that changes the calculations, right ?
fungus:
You can easily measure volts and amps, after that you need a thing called "Ohm's Law".
PS: It's not a constant, it varies with voltage.
Ohm's law does not apply to non-linear devices as the law states that there is a constant relationship between the variables., which for non-linear devices is clearly not the case.
However the basic equation as determined by the law may be used to determine the "resistance" under the conditions of measured voltage and current ie R=V/I
For what does or does not follow ohms law, look at:
Specifically half way down the page under the heading Linear approximations
Edited from wikipedia:
There are, however, components of electrical circuits which do not obey Ohm's law; that is, their relationship between current and voltage (their I–V curve) is nonlinear (or non-ohmic).
An example is the p-n junction diode.
The current does not increase linearly with applied voltage for a diode.
One can determine a value of current (I) for a given value of applied voltage (V) from the curve, but not from Ohm's law, since the value of "resistance" is not constant as a function of applied voltage.
Further, the current only increases significantly if the applied voltage is positive, not negative.
The ratio V/I for some point along the nonlinear curve is sometimes called the static, or chordal, or DC, resistance,
the value of total V over total I varies depending on the particular point along the nonlinear curve which is chosen.
This means the "DC resistance" V/I at some point on the curve is not the same as what would be determined by applying an AC signal having peak amplitude ?V volts or ?I amps centered at that same point along the curve and measuring ?V/?I.
However, in some diode applications, the AC signal applied to the device is small and it is possible to analyze the circuit in terms of the dynamic, small-signal, or incremental resistance, defined as the one over the slope of the V–I curve at the average value (DC operating point) of the voltage (that is, one over the derivative of current with respect to voltage).
For sufficiently small signals, the dynamic resistance allows the Ohm's law small signal resistance to be calculated as approximately one over the slope of a line drawn tangentially to the V-I curve at the DC operating point.
fungus:
At any instant in time though, Ohm's law applies.
+1
UnoDueTre:
Or the fungus extrapolation of ohms law.
The only extrapolation i knew is the difference between "Static" resistor and "dynamic" resistor (around a polarization point).
The one apply to DC currents and the second to AC currents.
In the case of linear component like resistor they are the same but with active components (diode [LED = Light Emissing Diode] , transistors, and so on..) they are different.
In either case for a DIODE you must consider only the current, voltage is imposed by the current,
jackrae:
Ohm's law does not apply to non-linear devices as the law states that there is a constant relationship between the variables., which for non-linear devices is clearly not the case.
That's a circular argument really, since ohms law says "devices are linear". So its
a law that applies to everything except for those things it doesn't!
The best way to explain ohms "law" is to state the kinds of material/physics is does apply to:
"Uniform conducting solids and ionic solutions/liquids mostly exhibit a linear relationship
between applied steady-state electric field and current-density at constant temperature and
sufficiently low current-densities".
Gases/plasmas it doesn't apply to, semiconductor junction devices is doesn't apply to
(not uniform). Insulators it doesn't apply to. It does apply to uniform slabs
of semiconductor (such as a hall-slice).
In reality it breaks down in some obscure exotic cases (quantum hall effect for
instance, superconductors for another).
For many conductors of electricity, the electric current which will flow through them is directly proportional to the voltage applied to them. When a microscopic view of Ohm's law is taken, it is found to depend upon the fact that the drift velocity of charges through the material is proportional to the electric field in the conductor. The ratio of voltage to current is called the resistance, and if the ratio is constant over a wide range of voltages, the material is said to be an "ohmic" material. If the material can be characterized by such a resistance, then the current can be predicted from the relationship: I = V/R
The way I was taught, Ohm's Law -only- applies to materials for which current is linearly proportional to voltage divided by resistance.
And of course, it only applies over the range of voltages and currents in which it is still a linear relationship. So Ohm's Law doesn't apply any more if you put 50kV across a 1/4W resistor that is only rated for a max of 250V. It really is a guideline, not a law.
So we can play semantics 'til the cows come home, or realize that calculating a resistance number for an LED based solely on the voltage drop divided by the current is a meaningless number, because as soon as you change anything, you'll come up with a different answer.
When Georg Ohm established his "law" (1827) the world (and physics) was a much simpler place (and linear)
+1
Two centuries later the Ohms law applies as long as the component can be represented by a mathematical equation and it true for a diode.
Of course if we need a computer with a special program (a hammer to kill a fly) to find a numerical solution we don't attempt to apply.
But it is not because we do not know how solving the equation U (x, y, z) = R (x, y, z). I (x, y, z) that the law does not apply.
Methinks enough dialogue has been wasted on this argument.
There are those who argue the earth is flat and those that argue it is otherwise and it's unlikely that both parties will ever agree.
An important question for the original poster, is in what context is he asking for how to determine the "resistance" of an LED.
After all, there is something called small signal resistance of an emitter junction on a BJT. It is, as described in another message, the slope of the curve at a given point, and is only valid over a narrow range of voltage and current. It isn't really a linear resistance, it is an approximation used to simplify calculating the parameters of an amplifier.
jackrae:
Methinks enough dialogue has been wasted on this argument.
There are those who argue the earth is flat and those that argue it is otherwise and it's unlikely that both parties will ever agree.
Well I know first hand that the earth is not flat. As a child I had to walk to school 2 miles, uphill both ways. That wouldn't have been the case if the earth was indeed flat.
