About power resistors: their use and wattage and their heat development (calculation)

Please read the textbox. 2021=2022, sorry. A simple video where I show how Ohms law + the Wattage (power) law works in practice and how to calculate/get a good idea about the heat that you will find dissipating inside a series resistor when it is mounted in an electronic/electric circuit.

Correction: of course this video was made on 17 February 2022 (not 2021!...sorry…).

Ohms Law: U=I x R (Voltage = Ampère x Resistance)
And about the Wattage (heat) calculation: Watt = Volt x Ampère.

These laws work very properly and concise. Other relevant vide is here https://youtu.be/icTfvjOYgV8

Normal resistors (carbon film spiralized types, etc.) have in general an extremely low inductance, thus their inductance can be neglected here.

Of course that inductance only plays a role on AC sources. No problems can be expected with spiralized carbon film resistors on frequencies below 300 MHZ and on DC.

Often wire wound resistors are only used in low frequency applications, so also not a serious inductance problem here.

The video shows the very basics of Ohms Law in practice and the calculation of heat dissipation in a power resistor. Say rule of thumb calculations regarding the heat dissipation.

About cooling: it could be that the power resistor only needs a kind of “stand-off” from the circuit board to be cooled enough by the air flowing around it.

Could also be that such a power resistor must have a heatsink, or can be left “as it is” on the circuit board.

In general: with 2 to 3 Watt dissipating resistors or on higher wattages kind of or cooling measures must be taken.

It all depends on its heat development (wattage), calculated by the voltage drop parallel to the power resistor (measure it) and the current flowing in the circuit (measure it).

So this other example: a 6 Volt voltage drop over a resistor where a current of 1 Ampère flows in a circuit: that means a heat development of 6V x 1A = 6 Watt.

That could be (too) hot or could be acceptable with a properly selected power resistor combined with a heatsink.

So in this way (looking at the voltage drop/current parallel to a resistor) you can easily calculate how hot a certain (power) resistor will get under (maximum) load.

I talk somewhere in the video about the "load". I assume here in this video that it is a kind of fixed resistor of 10 Ohms. But important: when the internal resistance of an electric/electronic load varies during its working (say a class B audio amplifier, the current that it takes depends on the audio input level, it varies constantly) that will mean that both the voltage drop over the power-series resistor and the audio amplifier will vary.

Thus such a circuit (class B audio amplifier) is in fact driven/steered by the audio input level.

The use of a series power resistor in the voltage supply to the class B audio amplifier can (thus) take such a class B audio amplifier out of its working point (giving audio distortion) at certain audio input levels.

But: this is no problem in many cases, say audio amps. from 1-5 Watt. On higher audio output levels a stabilized voltage is advised/necessary to avoid these issues. It keeps the voltage and thus the current constant under all circumstances. Many circuits are on my YT Channel about how to make such a voltage stabilizer.

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in. My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.


I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc.

Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that this happens. Upload 18 Februari 2022.
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