Teh Voltage Drops

fullNelson

Joined Nov 14, 2011
46
Two questions:

Is a voltage drop a change in the voltage across an element (diode, resistor)?

For my next question I have a circuit with R1 and R2 in series and R3 and R4 in Parallel:

So if I have two elements in series where there is a voltage drop, and I am supplying my circuit with 9v, then the voltage that exists beyond R1 and R2 is decreased. So the voltage that R3 and R4 receive is different that for R1+R2. Does this sound right?

Well apparently the code block tag doesn't format my nice ASCII (or thereabouts) graphic of my circuit. So fill in what you can with your imagination

Rich (BB code):
                             ------R3-----
-----R1------R2-----|                |
|                           |-----R4-----|
9V                         |                 |
|                           |                 |
----------------------------------

Austin Clark

Joined Dec 28, 2011
412
Yes, voltage drop is equal to the voltage dropped across any component or circuit. All the source voltage must be dropped across the circuit, ground must equal 0 Volts.

If you have a 10 volt source, and you have one 500 ohm resistor and one 1000 ohm resistor in series, the total current would equal (10/(1000+500)) amps, to calculate the voltage drop across one of the resistors, you simply find out how many volts would be necessary to draw that much current through that particular resistor (or load in general). Another way to think about and calculate voltage drop would be to find the ratio of the resistance of the component you want to know the voltage drop across to the circuits total resistance, and multiply by the source voltage, so in the example above, the 500 ohm resistor would drop 10*(500/(500+1000)) or a third of the source voltage, because that particular resistor makes up a third of the total resistance.

Hopefully that makes sense.

fullNelson

Joined Nov 14, 2011
46
If what you say is true then that makes perfect sense.

So is a voltage drop only valid for active elements - elements with a resistance of some sort?

I guess what I am driving at is LEDs are often labeled with a voltage drop, is this another way of saying the LED has a resistance?

crutschow

Joined Mar 14, 2008
34,427
A LED can be viewed as having a resistance but it's a very non-linear resistance (the voltage drop is not directly proportional to current). An LED is rated for a particular operating voltage, depending upon it's type and color, and that voltage varies only slightly with the current through it at typical operating currents. That's why they normally have a resistor in series with them to limit the current to the proper value.

Diodes, in general, behave that way. They have a forward voltage drop that varies only slightly with current at there typical operating current. There is actually a logarithmic relationship between voltage and current. Thus if you plot voltage versus current through a diode on semi-log graph paper, it will be essentially a straight line (see the bottom right graph of this for example). There is a small resistive component to the diode impedance which will cause some deviation on the plot from a straight line. This resistance value is higher for small signal diodes as compared to large rectifier diodes.

fullNelson

Joined Nov 14, 2011
46
A LED can be viewed as having a resistance but it's a very non-linear resistance (the voltage drop is not directly proportional to current). An LED is rated for a particular operating voltage, depending upon it's type and color, and that voltage varies only slightly with the current through it at typical operating currents. That's why they normally have a resistor in series with them to limit the current to the proper value.

Diodes, in general, behave that way. They have a forward voltage drop that varies only slightly with current at there typical operating current. There is actually a logarithmic relationship between voltage and current. Thus if you plot voltage versus current through a diode on semi-log graph paper, it will be essentially a straight line (see the bottom right graph of this for example). There is a small resistive component to the diode impedance which will cause some deviation on the plot from a straight line. This resistance value is higher for small signal diodes as compared to large rectifier diodes.
Yeah you can see that it is "straight" but not quite. Interesting.

Do you say its a foreward voltage drop because the current only flows in one direction for a diode?