The specifications advertised for resistors tend to be measured under ideal conditions. Real life environmental factors like high ambient temperatures or inefficient neighboring components can make components less efficient or less able to dissipate power. We call this disconnect a derating, because we literally have to design for a lower rating than optimal as a result of our environmental conditions.
Resistor Power Rating Considerations
Resistors used in electronics are made of materials with very low positive thermal coefficients, meaning even cheap resistors will only vary in resistance a few hundred parts per million per degree Celsius. Considering most resistors have an ohmic tolerance of 1 to 10%, this variance in resistance from heat is negligible compared to variance allowed within the tolerance spec.
Where you run into trouble with resistors is the power rating. Say you have a 100 ohm resistor rated to 1 watt. This one watt rating is likely measured at 25 degrees Celsius, which is considered room temperature. A typical resistor power curve starts to drop off dramatically after about 70 degrees Celsius, often hitting the axis before 200 degrees. Your “1 watt” resistor may only be able to handle half a watt at 100 degrees C and cause failure in your circuit.
Now, 100 degrees C is hot. That’s boiling water hot. How often do you design a circuit for boiling water temperatures? Well, the temperature you have to worry about isn’t the ambient temperature – it’s the hottest point of the resistor body. If you have a flat surface mount resistor dissipating even an eighth of a watt from a tiny package, it can get warm. Put that on a PCB opposite a big power switcher or high brightness LED that can easily hit 150 degrees C, and you see how you could easily develop a hot spot you need to worry about.
Resistor Derating in Real Life
Some things are better shown than explained, so I am going to show you exactly what can happen in a real life example. Surface mount resistors developing a hot spot against the board is the most common mode of failure but isn’t very visual, so we are going to heat the ambient air around a through-hole resistor using a heat gun.
Our resistor is rated to 100 ohms with 1% tolerance, a temperature coefficient of 100 parts per million per degree C, and a power rating of a quarter watt. It’s not the absolute cheapest resistor you can get by any means, but it’s about 5 cents in quantity of one and has pretty good specs.
I am going to hook it into our DC power supply and set the limit to 50mA. Power is current squared times impedance, so 50mA times 50mA times 100 ohms is a quarter watt – exactly the spec of our resistor. This claims a maximum operating temperature of 155 degrees but if you go look at the derating curve, that is literally where it hits the x axis. It will start being able to handle less and less power starting at about 70 degrees.
To complete the picture I’ve set up thermocouples under and next to the resistor body, as well as one in ambient air for reference. I also have a multimeter on the resistor to show any change in resistance we might see as it heats up. We will thermally stress the resistor with a hot air rework gun.
I’ll start by measuring the resistance to make sure we are within that 1% advertised spec. So far, so good. We are probably seeing the resistance in the cables and connections, but you will definitely notice if this goes to 50 ohms or anything. Now, I will turn on the power supply to start pushing a full quarter watt through the resistor and let the temperature stabilize for a few minutes.
Results and Aftermath
It looks like its stabilized 50 degrees C, which is already warm to the touch. You can imagine how easy it would be to accidentally enter the danger zone with power-hungry components on the same board. Now, I am going to set the air to about 300 degrees and hold it about an inch or two from the resistor body to heat up all the surrounding air with some degree of consistence. Watch what happens to the voltage and body temperature as the environment heats up.
We can also stress the resistor by just exceeding the power rating, but that’s an obvious design mistake. In this demo, the resistor is performing so well at high heat that we will increase the power to have a more exciting failure. It’s easy to forget about derating a resistor especially when you don’t think of your ambient temperature as being that high, but managing power on your PCB extends to every single component in a system.
For a deeper understanding of resistor technology, check out our concise explanation of the law behind resistance calculation and crack the resistor color code with us.
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