Looking at my recent posts on Ohm’s Law and Resistors part I, part II, and part III, I’ve realized that I didn’t discuss a few things that are very essential in resistors and that’s tolerance ratings, various compositions, their identification, and standard power ratings. I’ll talk about the tolerance ratings first.
Tolerance
Since resistors aren’t perfect (ideal), resistors will sometimes be off a little bit of their value. How much a resistor will be off by is usually given through a percentage and is called the tolerance. Tolerances come in different ranges depending on the resistor composition which is discussed below. But the typical tolerance ratings that most hobbyists work with are 0.5%, 1%, 2%, 5%, 10%, or 20%. So what these ratings really mean goes something like this: say you have 2KΩ resistor that has a tolerance rating of 5%. That means that the actual value can range anywhere from
2000-(2000 x 0.05) = 1900Ω
up to
2000+(2000 x 0.05) = 2100Ω.
The given value is just the base resistance for the resistor and is adjusted by the construction of the resistor with stuff such as how precise the cutting is for how much of the material is there.
Compositions
There are many compositions of resistors but to make this post fairly short, I’ll only list two of the common ones found to get you started:
- Metal Film
- Carbon Film
Before I get into describing the different compositions and their applications, there are a couple of terms I should go over:
- TC: Temperature Coefficient- Also called TCR (for Temperature Coefficient of Resistance), this is a variable that’s put in the datasheets of resistors that tells you how much change there is in the resistance of a resistor as the resistor gets warmer than usually normal room temperature (about 25°C) per 1°C and is given in parts-per-million (ppm). A positive TC will mean that the resistance increases and a negative TC means that the resistance decreases.
- Rise Time- This variable tells you how fast a resistor is able to respond to a pulse and is measured in µs (microseconds) or ns (nanoseconds).
- Temperature Rating- Pretty self explanatory but this is measured in °C and is usually given two numbers, one for them maximum allowable temperature when there’s a load attached and the second one that’s for when a load isn’t attached. So if you see a range such as +85°C-+145°C, that means that with a load attached, the resistor can still operate with the ambient temperature around being +85°C and when there isn’t a load attached, it can withstand up to +145°C without melting or burning.
Now to move onto the different compositions:
A metal film resistor. Photo credit: solvingelectricalcircuits.wordpress.com
Metal film resistors work well in circuits that deal with high frequencies or if the circuit has fast rise times in the µs range. Metal film resistors are also very cheap and come in tight tolerance ratings as low as 0.01% and have low TC levels. They also generate less noise, are cheaper, come in surface mount packages as well which are highly used in the surface mount world of electronics, and are more accurate than carbon film. Their real downside is that they don’t come in high resistance values compared to carbon film.
A carbon film resistor Photo credit: ck3.co.uk
Carbon film resistors are probably the most popular resistors that you’ll find regardless of how well metal film resistors are. Carbon films work in most general applications which is why they’re also called general purpose resistors and they respond fairly well to various frequencies but not as well as metal film in high frequencies. You can get these resistors with a tolerance as low as 1% but the issue is that due to their construction at 1%, the TC gets really high so if you’re looking for a 1% resistor, a metal film resistor will probably do just fine.
Power Ratings
I’ve talked about power before as well as power ratings for resistors but I didn’t list the standard ratings so here all the standard power ratings measured in watts: 1/16, 1/10, 1/8, 1/4, 1/2, 1, 2 5, 10, 15, 25, 50, 100, 200, 250, and 300W. As a refresher to find the proper power rating, use P=IV or P=I2R or P=V2/R.
Color Bands and Markings
The way that manufacturers show what size resistor they made, there are two different ways:
- Color code bands
- Printed directly on the resistor
In color code markings for resistors, there are 4 band, 5 band, and 6 band types but the 4 band system is most common so I’ll focus on that system here.
A carbon film resistor with 4 bands. Photo credit: http://www.minikits.com
The way that the 4 band system works is that each color has its own number:
| Color Name | Color | Value | Multiplier | Tolerance | |||
|---|---|---|---|---|---|---|---|
| Black | 0 | 1 | – | ||||
| Brown | 1 | 10 | 1% | ||||
| Red | 2 | 100 | 2% | ||||
| Orange | 3 | 1,000 | – | ||||
| Yellow | 4 | 10,000 | – | ||||
| Green | 5 | 100,000 | 0.5% | ||||
| Blue | 6 | 1,000,000 | 0.25% | ||||
| Purple | 7 | 10,000,000 | 0.1% | ||||
| Gray | 8 | 100,000,000 | – | ||||
| White | 9 | 1,000,000,000 | – | ||||
| Gold | – | 0.1 | 5% | ||||
| Silver | – | 0.01 | 10% | ||||
| None | N/A | – | – | 20% | |||
The first color is the first digit, the second color is the second digit, the third color is multiplier, and last color is the tolerance. For an example, in the last picture with the resistor, the colors are red, purple, brown, gold. So looking at the chart, we get:
1st digit: 2
2nd digit: 7
Multiplier: x10
Tolerance: 5%
Put it together and you get 27 x 10 = 270. So the resistor is a 270Ω 5% resistor.
These are cement resistors used for high power applications. Notice the printing on the resistor. Photo credit: m4.sourcingmap.com
In the above picture, the values are printed right on the resistor. The tolerance is given through a letter. The complete list is below:
0.1%: B
0.25%: C
0.5%: D
1%: F
2%: G
5%: J
10%: K
20%: M
In this case, the resistor is a 1KΩ 5% resistor.
Variable Resistors
What I have been showing up until this point are fixed resistors meaning they only have 1 value. But, there are also variable resistors (also called potentiometers or pots for short) which you’ll find in lots of places and you’ve probably used them a lot before, such as turning up the volume on the radio in your car. The knob is a variable resistor of a sort. There are basically three types, they all do basically the same thing but each look different:
- Rheostat
- Potentiometer
- Trimmer (also called a preset)
Rheostats
A rheostat is what you’ll find in volume controls for things like radios, mixers, etc.
A rheostat. You’ll find these in lots of electronics and are a lot of times used as volume controls. Photo credit: Wikipedia
These are also called volume controls because they’re extensively used in a voltage divider circuit to make a variable voltage divider for audio because of a physical shaft that you can use to change the voltage.
Potentiometers
Note that all variable resistors are potentiometers but have different names to differentiate them from this type of potentiometer that gets the name “potentiometer”.
A potentiometer type variable resistor. Photo credits: rfparts.com
These type of variable resistors are adjusted using a screwdriver in the slot on the top. Potentiometers are a lot of times used to limit current like a fixed resistor but can be used to dial in a specific resistance.
Trimmers
Lastly, there are trimmers which look like this:
A trimmer resistor, also called preset resistors Photo credit: m4.sourcingmap.com
Trimmers are a lot of times used for calibration. You use a flat head screwdriver to turn the knob (the metal piece in the above picture) to adjust the resistance to a precise measurement, a lot of times more precise than a potentiometer. When being used for calibration, the trimmer will be adjusted ever so slightly until the proper resistance needed is reached.
That’ll do it for this blog post on more information of resistors. Thank you so much for reading this and I hope you’ll be able to use the information that’s given. Next time, I’ll be posting about a brand new component and a little bit more electrical theory. But before I do that, what’re some things you’d like to see on the blog? Leave a comment below or post to the blog’s Facebook page, Twitter, Google +, or even send me an email. Until then, I hope you’ve enjoyed this series and have learned something new from it and I’ll see you in the next one.
Volts 'n' Bolts 