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Author Topic: GT Electronics forum FAQ  (Read 18623 times)

linear

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GT Electronics forum FAQ
« on: September 16, 2005, 01:53:59 PM »

This is the consolidated FAQ thread.

You can reply if you have a FAQ submission, but posts will periodically be trimmed as submissions are consolidated into the FAQ by the moderator team.
« Last Edit: September 16, 2005, 02:28:43 PM by linear »
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Q: What resistor should I use with my LED?
« Reply #1 on: September 16, 2005, 01:58:22 PM »

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.
« Last Edit: September 16, 2005, 02:03:58 PM by linear »
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Q: Why do I need a resistor with an LED?
« Reply #2 on: September 16, 2005, 02:03:41 PM »

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/
« Last Edit: September 16, 2005, 02:27:33 PM by linear »
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linear

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Q: How does the resistor color code work?
« Reply #3 on: September 16, 2005, 02:08:27 PM »

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
« Last Edit: February 12, 2006, 04:35:24 PM by viridius »
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Q: What is Ohm's law, and how do I use it?
« Reply #4 on: September 16, 2005, 02:13:13 PM »

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.
« Last Edit: February 12, 2006, 03:20:08 PM by viridius »
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Q: How much resistance does wire have?
« Reply #5 on: September 16, 2005, 02:16:45 PM »

A: To determine the resistance, you need to know the length and diameter of the wire, or it's AWG (American wire gauge) from which you can use a table to find the resistance per unit length.

A: (table originally posted by Skylined)
Code: [Select]
AWG = American Wire Gauge size from 0000 to 40
Dia-mils = Diameter in mils (1 mil = .001 inch)
TPI = Turns Per Inch [Note that this is for BARE WIRE. Insulation thickness varies]
Dia-mm = Diameter in millimeters. This was included to help when dealing with metric Coilers.
Circ-mils = Cross sectional Area in Circular Mils. ( circular mils = diameter in mils squared )
Ohms/Kft = Ohms Per 1,000 ft.
Ft/Ohm = Number of feet required for 1 Ohm of resistance
*AMPS = Conservative Amp Rating based on 750 circulare mils per Amp
MaxAmps = Maximum allowable current based on 500 circular mils per Amp. Do NOT exceed this rating.


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

0000       459.99    2.1740    11.684    211592    0.0490     20402    282.12    423.18
000        409.63    2.4412    10.405    167800    0.0618     16180    223.73    335.60
00         364.79    2.7413    9.2657    133072    0.0779     12831    177.43    266.14


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

0          324.85    3.0783    8.2513    105531    0.0983     10175    140.71    211.06
1          289.29    3.4567    7.3480     83690    0.1239    8069.5    111.59    167.38
2          257.62    3.8817    6.5436     66369    0.1563    6399.4    88.492    132.74
3          229.42    4.3588    5.8272     52633    0.1970    5075.0    70.177    105.27
4          204.30    4.8947    5.1893     41740    0.2485    4024.7    55.653    83.480
5          181.94    5.4964    4.6212     33101    0.3133    3191.7    44.135    66.203
6          162.02    6.1721    4.1153     26251    0.3951    2531.1    35.001    52.501
7          144.28    6.9308    3.6648     20818    0.4982    2007.3    27.757    41.635
8          128.49    7.7828    3.2636     16509    0.6282    1591.8    22.012    33.018
9          114.42    8.7396    2.9063     13092    0.7921    1262.4    17.456    26.185


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

10         101.90    9.8140    2.5881     10383    0.9989    1001.1    13.844    20.765
11         90.741    11.020    2.3048    8233.9    1.2596    793.93    10.978    16.468
12         80.807    12.375    2.0525    6529.8    1.5883    629.61    8.7064    13.060
13         71.961    13.896    1.8278    5178.3    2.0028    499.31    6.9045    10.357
14         64.083    15.605    1.6277    4106.6    2.5255    395.97    5.4755    8.2132
15         57.067    17.523    1.4495    3256.7    3.1845    314.02    4.3423    6.5134
16         50.820    19.677    1.2908    2582.7    4.0156    249.03    3.4436    5.1654
17         45.257    22.096    1.1495    2048.2    5.0636    197.49    2.7309    4.0963
18         40.302    24.813    1.0237    1624.3    6.3851    156.62    2.1657    3.2485
19         35.890    27.863    0.9116    1288.1    8.0514    124.20    1.7175    2.5762


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

20         31.961    31.288    0.8118    1021.5    10.153    98.496    1.3620    2.0430
21         28.462    35.134    0.7229    810.10    12.802    78.111    1.0801    1.6202
22         25.346    39.453    0.6438    642.44    16.143    61.945    0.8566    1.2849
23         22.572    44.304    0.5733    509.48    20.356    49.125    0.6793    1.0190
24         20.101    49.750    0.5106    404.03    25.669    38.958    0.5387    0.8081
25         17.900    55.866    0.4547    320.41    32.368    30.895    0.4272    0.6408
26         15.940    62.733    0.4049    254.10    40.815    24.501    0.3388    0.5082
27         14.195    70.445    0.3606    201.51    51.467    19.430    0.2687    0.4030
28         12.641    79.105    0.3211    159.80    64.898    15.409    0.2131    0.3196
29         11.257    88.830    0.2859    126.73    81.835    12.220    0.1690    0.2535


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

30         10.025    99.750    0.2546    100.50    103.19    9.6906    0.1340    0.2010
31         8.9276    112.01    0.2268    79.702    130.12    7.6850    0.1063    0.1594
32         7.9503    125.78    0.2019    63.207    164.08    6.0945    0.0843    0.1264
33         7.0799    141.24    0.1798    50.125    206.90    4.8332    0.0668    0.1003
34         6.3048    158.61    0.1601    39.751    260.90    3.8329    0.0530    0.0795
35         5.6146    178.11    0.1426    31.524    328.99    3.0396    0.0420    0.0630
36         5.0000    200.00    0.1270    25.000    414.85    2.4105    0.0333    0.0500
37         4.4526    224.59    0.1131    19.826    523.11    1.9116    0.0264    0.0397
38         3.9652    252.20    0.1007    15.723    659.63    1.5160    0.0210    0.0314
39         3.5311    283.20    0.0897    12.469    831.78    1.2022    0.0166    0.0249


AWG       Dia-mils    TPI      Dia-mm   Circ-mils Ohms/Kft   Ft/Ohm    *Amps    MaxAmps

40         3.1445    318.01    0.0799    9.8880    1048.9    0.9534    0.0132    0.0198
« Last Edit: February 17, 2006, 01:48:45 PM by viridius »
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Q: What are the minimum and maximum voltage specifications for my PSU?
« Reply #6 on: September 16, 2005, 02:21:34 PM »

A: (originally posted by Skylined)



A: The current ATX12V spec (as of this posting) is essentially unchanged from the above.
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Q: What are some good electronics references online?
« Reply #7 on: September 16, 2005, 02:26:37 PM »

A: (originally posted by viridius)
brutal.tk - (Some of you may be too young to remember Brutal.  Since he's apparently no longer around to pimp his excellent site, I guess it's my turn to take up the hat.)  Contains:
LED Resistor Calculator
ATX Power Supply/IDE/Floppy/RJ-45 Pinout Chart
Case Fan Reference

Network Tech - Thank Cars1000 for this one.  Contains:
Pinouts for pretty much everything not on Brutal's site

Gideon Tech - Shocking, isn't it?  Contains:
Downloadable LED Resistor Calculator
Guides up to here.

Electronics Calculators - From Bigal.  Contains:
Calculators for Resistors/Capacitors/Ohm's Law/LEDs

555 Timer Pinouts  Contains:
Handy tools for helping you configure 555 timers.

KPS E-Club  Contains:
Lots of good stuff for beginners.

Electronics in Meccano  Contains:
More good stuff for beginners.

Lessons in Electronic Circuits  Contains:
Free textbook series.  If you want to learn a lot, check this one out.

A: (originally posted by x24)
555 calculators for both mono and astable.

A: (originally posted by Tank)
The Electronics Club at Kelsey Park School. Lots of good information about the basics, for those new to electronics.

Electronics in Meccano. Also good for electronics n00bs. Has a nice soldering tutorial.

A: (originally posted by Meander)
free online electronics textbooks

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viridius

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Useful Programs
« Reply #8 on: February 07, 2006, 04:29:26 PM »

Eagle 3D - Eagle schems to raytrace.
« Last Edit: February 07, 2006, 04:56:51 PM by viridius »
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viridius

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Q: How Do I Read Capacitors
« Reply #9 on: February 12, 2006, 04:42:54 PM »

For smaller capacitors which do not have the capacitance printed directly on the side, there is a simple procedure for reading the capacitance.  The first two digits give the capacitance in picofarads to two significant figures, and the third, if it is present, gives the number of zeroes to add to the end to get the correct value.  For example:
104 = 100000 picofarads (10 with four extra zeroes)
472 = 4700 picofarads (47 with two extra zeroes)
33 = 33 picofarads (33 with no extra zeroes)
Remember: 1 picofarad = 10^-12 farads
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