How simple a resistor is but it is very important in any circuits. A resistor role is to limit the amount of current flowing to the circuit. Without it, other electronic parts, circuits, modules or sub-circuits will not work. There are some factors to consider on how to select resistors. All these factors will be discussed below. These will give you the right guide in selecting resistor in whatever applications. These are all the parameters I considered when doing resistor selection for my designs.
1. Selecting Resistor Type
Let us start this article on how to select resistors by identifying your application perhaps and then you can select what resistor type you are looking at. If the circuit you want to build needs a variable voltage, then you need a variable resistor. It could be a trimmer or a potentiometer. If your application is just having a fixed voltage, then concentrate on a fixed value resistor. Is your application involved high power or just small signal circuits? Well, this can be answered if you already have the power dissipation data either by calculation or simulations. You may also think of a wire wound, carbon or a film composition…But these are not that important. I mean no need for you to brainstorm regarding this. Because if the power rating you need is very high, most cases that resistor will be of a wire wound type. On the other hand, if the power rating you need is small, it is mostly of carbon or film compositions.
2. Resistor Selection – Resistance
The electrical property of a resistor is the resistance. It is the resistance that will oppose or limit current flow. It is identified on a unit ohm (Ω). Resistance very important item in resistor selection. How to determine the resistance size? This will depend to the amount of current you are going to allow. It will depend also to the required voltage you want. Let us site examples to understand clearly.
Sample 1: Supposing the circuit current is limited only to 1A, what will be the resistance needed to operate the circuit from a 10V source? See below circuit.
Using ohms law principle,
I = V/R, R = V/I
So, R = 10V / 1A = 10 ohms
Select a standard resistor value (10 ohms is already a standard value).
Sample 2: In below circuit, you need to determine the value of R1.
By ohms law, the current on R2 is I = 7V / 10 ohms = 0.7A.
R1 and R2 are in series, thus they will be having the same value of current. From ohms law again,
I = V/R, R = V / I, R2 = 3V / 0.7A = 4.2857 ohms.
Let’s recheck the computation:
I = 10V / (R1+R2) = 10V / (4.2857 + 10) = 0.7 A. Our computation is correct.
There is no 4.2857 ohms standard value. So, choose a standard value near to this. Take note that the circuit current will change a bit once you are using the standard value resistor.
Sometimes, there is no need to compute for the resistance value. Instead, assigning a predefine value will work. Like you want a 100-ohm resistor then just compute the actual current, voltage, power dissipation and evaluate if this value serves your purpose.
3. Selecting Resistor Power Rating
One of the most important ratings to take note in selecting resistor is the power rating. Resistor will burn out if too high, power stress is applied. Therefore, know the actual power dissipation of the resistor.
The actual power dissipation of the resistor is can be computed as
Pdiss = I X I X R or Pdiss = V X V / R
Pdiss – power dissipation of the resistor
I = current flowing to the resistor
V = voltage across the resistor
R = resistance value
Let us take as example the simple circuit below on how to select resistors in terms of power rating.
Since the resistor R is directly connected to the voltage source, then the power dissipation is can be computed straight forward.
Pdiss = V X V / R = 10V X 10V / 10 ohm = 10 Watts
You can also compute the circuit current as I = V / R = 10V / 10 ohms = 1A. Then, the power dissipation is
Pdiss = I X I X R = 1A X 1A X 10 ohms = 10 Watts.
In my designs, I always preferred not to exceed 80% on power stress. This means I need to select a resistor with a power rating of not less than 12.5 Watts (10 Watts / 0.8). The 80% limit is the maximum allowable. You can always set the maximum limit to a lower percentage than 80%. There are just few considerations that you may need to go this high (80%). For example, on an applications where the resistor choices are limited and there is a big added cost in going to a higher power rated part. If you are doing the design, you gonna evaluate all these and decide based on the available options and facts.
Power rating of resistor will decrease with temperature for power resistors. Need also to consider this. Below is a power de-rating curve I get from TE Connectivity type HS series. As you can see, the power capability decreases somehow when attained certain temperature level.
4. How to Select Resistor Voltage Rating
Another important rating to consider on how to select resistor is the voltage rating. Datasheets provide limits for the maximum working voltage. This is the actual voltage applied across the resistor. Still from TE Connectivity type HS series, its specified a maximum working voltage below. If I am the one to do the design, in my understanding of this is that I will not allow the resistor to have actual voltage more than 1,900V for the HSC100 series. This is the absolute limit of this series.
Take note that this is tricky somehow. The rating is for the series, not for a single resistance value. Supposing you are using the 10 ohms version from the HSC100, is the maximum working voltage still 1,900V? Let’s find out.
Based on the above table, the power dissipation allowable for the HSC100 series is at 100W and 50W for with heatsink and without heatsink. Let’s compute the actual power dissipated when the voltage is allowed at 1,900V.
Pdiss = V X V / R = 1,900V X 1,900V / 10 ohms = 361,000 Watts. This is ridiculous amount of power dissipation and will burn the resistor in just microseconds.
Considering the higher resistance value from this series that is 100K ohms, let us compute again the power dissipation.
Pdiss = V X V / R = 1,900V X 1,900V / 100K ohms = 36.1 Watts. This is within the resistor power rating of 50W and 100W regardless of with or without heatsink.
If you increase the actual voltage to 2,000V, the corresponding power dissipation is
Pdiss = V X V / R = 2,000V X 2,000V / 100K ohms = 40 Watts. This is still less than the power rating of the resistor. Can I do this? The answer is NO. you need to stick to the datasheet.
In short, the maximum working voltage rating must be verified using the power rating and both must be satisfied.
5. Selecting Resistor Tolerance and Temperature Coefficient
There is no perfect resistor, so tolerance must also be taken cared of when selecting resistor. Resistors are come from several tolerance like 10%, 5%, 1%, 0.1% and so on. The higher the percentage, the higher the resistance can vary. For example, a 10K ohms resistor with 10% tolerance. The resistance range will be 9K – 11K. This is a huge variation. If your application is very critical, then select a lower tolerance part. In my designs, I make a standard to use 1% tolerance for the chip resistors for general use. For critical circuits like feedback and protections, I choose 0.1%.
Temperature coefficient is also provided in the datasheet. This is an indicator on how the resistance vary in terms of the operating temperatures. The smaller this value, the better as it means the resistance will not so dependent to the temperature. This is critical concern in using resistors in high ambient applications. In my designs, I select 100 PPM/C or below. It does not always true that a lower tolerance part will have a lower temperature coefficient. I get some data from Mouser Electronics page below.
6. How to Select Resistor Operating Temperature
In selecting resistor, do not forget the operating temperature range. If you know that the product you are working on will be exposed to 85’C maximum ambient temperature, then select a resistor with an operating temperature of more than 85’C. In my designs, I set the maximum temperature stress to 80%. Meaning I need a resistor that has a maximum operating temperature of 106.25’C for an application temperature of 85’C.
Likewise, if the minimum application temperature is -20C, then select a resistor that can operates down to -20’C.
The resistor operating temperature should be measured on the body. For small power resistors, the temperature rise due to power dissipation is negligible therefore the body temperature is can be equated to the ambient temperature. However, for high power resistors the temperature rise is significant. Thus, need to measure the actual body temperature. In power resistors also, the power capability is de-rated when going to the maximum temperature. Below is an example from TE connectivity HSC series.
7. Mounting Style and Physical Size
Mounting style is also a factor in resistor selection. You may need a chip or surface mount device or a through hole part. You may need a chassis mount or a heatsink mount resistor, etc. The decision for this is govern sometimes by the application, power levels or availability of the part. Physical size is also an important consideration especially in space critical products. Chip resistors like 0402, 0603, 1206, 1210 and so on are smaller in sizes but limited in power rating as well as voltage. Through hole resistors, heatsink mount or chassis mount are bulky but offer higher power dissipation and voltage rating.
Sample Resistor Ratings
Below is a sample resistor rating sheet that I get from Mouser Electronics overview page. The parameters discussed above in resistor selection are shown below.