E01 Resistor Value
Bradley Zweig
The following are tools to calculate the ohm value and tolerance based on resistor color codes, the total resistance of a group of resistors in parallel or in series, and the resistance of a conductor based on size and conductivity.
An electronic color code is a code that is used to specify the ratings of certain electrical components, such as the resistance in Ohms of a resistor. Electronic color codes are also used to rate capacitors, inductors, diodes, and other electronic components, but are most typically used for resistors. Only resistors are addressed by this calculator.
The color coding for resistors is an international standard that is defined in IEC 60062. The resistor color code shown in the table below involves various colors that represent significant figures, multiplier, tolerance, reliability, and temperature coefficient. Which of these the color refers to is dependent on the position of the color band on the resistor. In a typical four-band resistor, there is a spacing between the third and the fourth band to indicate how the resistor should be read (from left to right, with the lone band after the spacing being the right-most band). In the explanation below, a four-band resistor (the one specifically shown below) will be used. Other possible resistor variations will be described after.
In a typical four-band resistor, the first and second bands represent significant figures. For this example, refer to the figure above with a green, red, blue, and gold band. Using the table provided below, the green band represents the number 5, and the red band is 2.
The third, blue band, is the multiplier. Using the table, the multiplier is thus 1,000,000. This multiplier is multiplied by the significant figures determined from the previous bands, in this case 52, resulting in a value of 52,000,000 Ω, or 52 MΩ.
The fourth band is not always present, but when it is, represents tolerance. This is a percentage by which the resistor value can vary. The gold band in this example indicates a tolerance of 5%, which can be represented by the letter J. This means that the value 52 MΩ can vary by up to 5% in either direction, so the value of the resistor is 49.4 MΩ - 54.6 MΩ.
Coded components have at least three bands: two significant figure bands and a multiplier, but there are other possible variations. For example, components that are made to military specifications are typically four-band resistors that may have a fifth band that indicates the reliability of the resistor in terms of failure rate percentage per 1000 hours of service. It is also possible to have a 5th band that is the temperature coefficient, which indicates the change in resistance of the component as a function of ambient temperature in terms of ppm/K.
More commonly, there are five-band resistors that are more precise due to a third significant figure band. This shifts the position of the multiplier and tolerance band into the 4th and 5th position as compared to a typical four-band resistor.
On the most precise of resistors, a 6th band may be present. The first three bands would be the significant figure bands, the 4th the multiplier, the 5th the tolerance, and the 6th could be either reliability or temperature coefficient. There are also other possible variations, but these are some of the more common configurations.
Resistors are circuit elements that impart electrical resistance. While circuits can be highly complicated, and there are many different ways in which resistors can be arranged in a circuit, resistors in complex circuits can typically be broken down and classified as being connected in series or in parallel.
This tool is used to decode information for color banded axial lead resistors. Select the number of bands, then their colors to determine the value and tolerance of the resistors or view all resistors DigiKey has to offer.
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Strange one. I created my own resistor library, importing a symbol from a working library. I went ahead and created my package, then device, etc. I have a device with a symbol, package, connections and prefix as I understand from the video series. But when I place the symbol on the schematic, it i the value (R1206), not a blank value. I go to the properties, and there is no value field to modify. Just device and package - both of which are named R1206. Name shows correctly - R1, position in the schematic, Gate G$1, what library it is from. It is smashed. I bring it a part from another library - resistor-power supplied by Eagle, and I get the same thing except there is a value field and some attributes. What is going wrong here? I really need to enter resistor values.
I have been having the same issue. I have a schematic with lots of custom parts. On some of them the value is not showing up. The radio button is definitely set to on. As far as I can tell, all of the other parameters are the same between the parts with values showing up, and the parts with values NOT showing up. any suggestions?
Since my intention is to order the board from JLCPCB and avoid any manual soldering, I've opted for the Crystal X322516MLB4SI. To make this choice, I referred to the STMicroelectronics Datasheet, specifically "AN2586," which provides guidance on designing with an HSE crystal.
I've selected C_L1 and C_L2 values of 20pF. Subsequently, I calculated an R_ext value of 497 Ohms. To match this value with a standard resistor from the E-Series, I've decided to use a 510 Ohms resistor.
A series resistor to the crystal, R2 in your case, would lower the gain margin significantly: with 510ohms to 0.819 (much smaller than 5), which would make the HSE unstable. With 15ohms you would reach 5.086, which would be just acceptable. In general, however, there is no need for a series resistor to the crystal.
Much more important is the correct layout, for which very important hints are given in AN2867, section 7. Specifically, for maximum stability under all possible environmental conditions, it is necessary to:
This results in a GND structure around the crystal that looks like a hand in which the crystal lies, so to speak, with the "arm" representing the GND connection that must be connected separately to the GND pin.
The STM32 families not only have oscillators with differing parameters, but also the parameters of the crystals used and their frequency play a major role in stable operation under all conditions. Unfortunately, the AN2867 is too rarely given the necessary attention, although a stable oscillator is a fundamental and absolutely necessary prerequisite for stable work of every microcontroller.
The internal pull up isn't a true resistor. It can be thought of as an 'active circuit' or a current source, with the rating being held in the electrical characteristics table show below. this information was take from the datasheet.
In most of the getting started tutorials for Arduino (like this Blink example) it is recommended that we have place a 220 Ohm resistor. I know that it is calculated using Ohms Law, but I am interested to know how this value is calculated?
Initially I thought it was to protect the LED. I followed this guide from Evil Mad Scientist. The articled explained that a typical Red LED has a voltage drop of 1.8 V and a current of abound 25mA and Arduino Pin has an output of 5V. Using these values and the formula from the article, the calculation is
These are really good questions Sudar. I'm sure many people will consider them to be basic but when you are just starting out (like me) it is confusing when people say, "Don't worry... 220? will be fine." Ok, but how do you know that?
Your first set of calculations are correct. However many simply round numbers up to a common value which means a little less current flows through the LED. For most cases this is irrelevant, unless you actually want to push the brilliance to the maximum rated value. Many of us would be satisfied using something like 470ohm but then the LED current would only be around 7mA, but probably bright enough as an indication that the output was active.
The actual point is that there is simply no reason to seek to deliver the "whole" 20 mA. The difference between 20 mA and 15 mA with a 1.7 V drop in the LED will not be visible and you are just being conservative with your ratings.
An interesting point by the way, is that if you look at the ATmega328 datasheet, Figure 35-22 and 35-24 you will note that the output will lose half a volt either way when drawing 20 mA, so it effectively adds 25 Ohms to the circuit.
Those resistors/LEDs are not exactly in parallel. Each LED-resistor pair is connected to a different output pin. With about 2V across the LED and 3V across the resistor you'd have about 14mA (3V/220 Ohms) coming out of each pin and through each LED-resistor pair and that's OK.
But until then I'll set it all up again and double check I was using the correct resistors. I've looked again at the schematic in the book and see what you mean about the resistors/LEDs not being exactly in parallel. So to be clear, each resistor/LED pair can be calculated separately (i.e. I didn't need to try and calculate equivalent resistance as I did in my previous post)?
sudar:
In most of the getting started tutorials for Arduino (like this Blink example) it is recommended that we have place a 220 Ohm resistor. I know that it is calculated using Ohms Law, but I am interested to know how this value is calculated?
I think its just chosen as its large enough to be driven to ground (ie without an LED), safely, and its an E3
resistor value. Its not critical and most modern LEDs are blindingly bright at 25mA, a 1k resistor is actually
more sensible.
I concur with MarkT's statements. Also the perceived brightness of LEDs is not linearly related to the current used to power them. At 50% of the midpoint (ie 10 out of 20mA) they look about 75-80% as bright as they do at the midpoint current. Doing this will also extend the lifetime of your LEDs.
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