GT Electronics forum FAQ
linear:
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linear:
A: Use one of the following calculators:
http://members.cox.net/brutal/Resistor/main.html (brutal)
http://led.linear1.org/1led.wiz (linear)
http://www.gideontech.com/content/articles/229/1 (x24--downloadable windows prog)
Otherwise, R >= (VS - VF) / ID,
where
VS = source voltage
VF = diode forward voltage, and
ID = diode forward current.
If you need a quick way to do this in your head, 20 mA is a common diode current. Find the necessary voltage drop in the resistor (VS - VF), multiply that by 100 and take half the answer. For example, for a 12V supply, and a 2V @ 20mA LED, the voltage drop in the resistor is 10V, so you want a (100 * 10) / 2 = 500 ohm resistor.
linear:
A: (originally posted by viridius)
Any LED you use in your computer should have a resistor to limit current. There are a few times when you don't need to put in a resistor because there's already one there:
When you change the LEDs that are connected to your motherboard (power, HDD, etc).
When you change the LEDs on your drives.
Any other time you put in a LED where there already was one.
Every other time requires the use of a current limiting resistor and a long explanation follows. You can skip to the end if you like. LEDs belong to the family of semiconductors known as diodes. LED stands for Light Emitting Diode. Diodes are devices that only permit the flow of current in one direction, specifically from anode to cathode. To accomplish this, they use a semiconductor substrate (usually silicon or germanium) doped, or seeded, with certain impurities such as arsenic or indium. When doped, the semiconductor substrate takes on electrical properties that change the way current moves through it. N-type semiconductors pass electrons through like a metal. P-type semiconductors use the movement of "positive holes" that move the other direction. The diode uses a N-type and a P-type semiconductor connected in what is called a PN junction. When the two materials are connected, some of the holes from the P-type region move into the N-type region and are filled by electrons and some electrons from the N-type region move into the P-type region and fill some of the holes. This only happens in a narrow band around where the two regions touch called the depletion region. After the movement of electrons and holes is complete, there is a thin layer of negatively charged ions on one side of the junction and a layer of positively charged ions on the other (since the ions themselves cannot move across the gap). At this point, neither electrons or holes can move across the gap and current flow stops. The opposing charge of the ions on both sides of the junction sets up an electrostatic field in the depletion region called the junction field. When a diode is connected to a current source so that the voltage is in opposition to the junction field (this is called forward bias), the junction barrier shrinks and current can again flow through the diode. When a diode is connected so that the voltage increases the junction barrier (reverse bias), little to no current flows. LEDs are doped with specialized impurities that enable them to emit light when forward biased. Aside from this, they are electrically very similar to normal diodes. This is why a current limiting resistor is necessary. If a LED is connected so that the supply voltage matches the operating voltage, a slight fluctuation in supply voltage will cause the junction barrier to shrink past acceptable limits and vast amounts of current to flow across the junction. Because of this, the LED will heat up and the junction will be destroyed in a matter of seconds, but not before tripping your PSU's overcurrent protection or possibly burning out some of your computer components.
A: (by linear)
The short answer: to limit the current in the LED to a safe value.
The long answer: LEDs are semiconductors, diodes in particular. The current flowing in an LED is an exponential function of voltage across the LED. The important part about that for you is that a small change in voltage can produce a huge change in current.
more info here: http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/
linear:
A: (originally posted by Skylined)
Reading a Resistor:
The color bands on a resistor give its value and tolerance (accuracy). The vast majority of resistors you'll encounter will be 5% tolerance, which makes reading them easier. 5% tolerance resistors have a gold band on one end, usually separated from the other three bands. With the resistor oriented so that the gold band is on the right, read off the color bands from left to right according to the following chart.
The first two bands will indicate the value of the resistor to two significant figures. Multiply this value by the correct factor given in the multiplier band to find the total resistance. The actual resistance will be within 5% of this value. For example:
Yellow-Violet-Orange-Gold
Yellow = 4
Violet = 7
Orange = 1,000
Gold = 5%
The value of the resistor is 47x1,000 or 47,000 ohms with a tolerance of 5%, so the actual value could be anywhere from 44650 to 49350 ohms.
E24 Standard Values:
Because a resistor's value is not exact (it can vary within the tolerance range), only certain discrete values of resistors are manufactured. For example, 102 ohm 5% tolerance resistors are not made because it is possible for a 100 ohm 5% tolerance resistor (an E24 standard value) to have the same resistance. Therefore, 5% resistors can be found only in the following values:
100
110
120
130
150
160
180
200
220
240
270
300
330
360
390
430
470
510
560
620
680
750
820
910
And any multiple of 10 thereof (such as 62 or 240000 ohms). If a more accurate resistance value is necessary, a variable resistor, resistors connected in series (resistances add in series), or a fixed resistor with a tighter tolerance (and therefore come in more standard values) may be used. Tighter tolerance resistors necessarily have more value bands. These are read normally. For example:
Red-Red-Blue-Yellow-Red
Red = 2
Red = 2
Blue = 6
Yellow = 10,000
Red = 2%
The value of the resistor is 226x10,000 or 2,260,000 ohms with a tolerance of 2%. Notice that the tolerance band is red, making proper orientation of the resistor for reading difficult. However, the tolerance band is usually set apart from the value and multiplier bands.
*Note for modders: when selecting a current limiting resistor for an LED, if the calculated value for the resistor is not a standard value, the next greater standard value should be used.
Different ways of writing resistances:
When reading schematics, you may encounter different systems for indicating resistances. In some places, the United States, for example, 470 ohms is usually written as 470, 2,200 ohms is written as 2.2K, 1,200,000 ohms is 1.2M. In other places, such as the UK, 470 ohms is written as 470R, 2,200 ohms is 2K2, and 1,200,000 ohms is 1M2, where the metric prefix replaces the decimal point.
Power considerations:
The most commonly encountered resistors come in 1/8 (0.125) watt, 1/4 (.25) watt, and 1/2 (0.5) watt flavors. Which one is right for you? The power dissapated by a resistor can be calculated by the following equations:
P = (I^2)xR
or
P = (V^2)/R
Where P is the power dissipated by the resistor, measured in watts; I is the current through the resistor, measured in amps; V is the voltage across the resistor, measured in volts; and R is the resistance of the resistor, measured in ohms. Example:
A LED draws 20 milliamps at 3.7 volts. You want to run it off of the 12 volt rail with a 415 ohm resistor (the calculated current limiting resistor, value is not standard, but we will use it for the purposes of this example). What power resistor should you purchase?
Using the current, the power dissipated is 0.02 amps (current through the resistor) squared multiplied by 415 ohms, which is 0.166 watts.
Using the voltage, the power dissipated is 8.3 volts (voltage across the resistor, 12 volts - 3.7 volts) squared divided by 415 ohms, which is 0.166 watts.
Since 0.166 watts is more than 1/8 watt, a 1/4 watt resistor should be used.
x24 has created a downloadable program which will translate resistor values and calculate current limiting resistors for LEDs. It is available at:
http://mywebpages.comcast.net/x24/prog_LEDcalc.htm
linear:
A: (originally posted by viridius)
Ohm's Law is a very useful relationship between voltage, current, and resistance. It is usually stated:
E=IR
Where E is voltage in volts, I is current in amps, and R is resistance in ohms. So if you have a lightbulb with a resistance of 30 ohms and the current in the circuit is measured to be 0.5 amps, the supply voltage is:
30*0.5=15
15 volts.
Solving for current:
I=E/R
If you have the lightbulb from the previous example (30 ohms) hooked up to your 12 volt rail, the current draw on that rail would be:
12/30=0.4
0.4 amps.
Solving for resistance:
R=E/I
If you have a device of unknown resistance hooked up to your 12 volt rail and you have measured the current draw on the rail to be 0.3 amps, the resistance of the device is:
12/0.3=40
40 ohms. This equation is also useful for determining the value of the current limiting resistor for a LED. In this case, you just use the necessary voltage drop for E and the LED current for I. So if you have a 5 volt, 20 milliamp LED that you want to hook up to your 12 volt rail, the voltage drop would be:
12-5=7
7 volts and the equation would then be:
7/0.02=350
So the current limiting resistor would be 350 ohms but since 350 ohm resistors don't exist, the next highest standard value (360 ohms) would be used instead.
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