Resistor power dissipation

[ringo888]

Hi Guys, quick qusetion for you,

If i had a resistor of say 10R on 15V supply, how can i caluculate the power dissipation at 50% duty cycle at 20KHz?

Thanks in advance,
Ringo

 

[karesz]

Hi,
at 20 KHz & 10R is the value of frequency can be irrelevant...
You can calculate with 50% of DC dissipation (or double of the 10 Ohm).
K.

 

[AdvaRes]

For a voltage V and a resistor R the power consumption is V²/R.
If the ducty cycle is 50% you divide this Power by two and that's all.

 

[ringo888]

Yes i know the calc for power dissipation is V2/R, surely its not as simple as halfing the value for 50% duty cycle?? and the pwm of 20KHz hasn't been taken into consideration.

Ringo

 

[karesz]

Hi Ringo,
Your pwm is=50%->> the half value of dissipation must be correct at (only) 20KHz...
If your PWM is no more=50%; you can calculate the actual dissipation as time proportional value (100%=1 & 10%=0.1 of the DC value) :_)...
K.

 

[WimRFP]

Hello Ringo888,

Yes it is that simple, draw yourself the voltage waveform in the time domain.

Now draw at the same time scale the V^2/R curve. Now you have the instantaneous power versus time curve. Do an average on that and you have the average power.

As the thermal time constant of the resistor will probably be far above 0.5/frequency, the resistor experiences the average power only.

 

[ringo888]

So in this example since we're at 20KHz at 15V using a 4.7R resistor the power dissipation will be 23.9W??

So the frequency doesn't really matter, or does it, so its just the duty cycle that afftects it? so wattage is the same with 1hz and 100khz provided the duty cycle is the same??

 

[betwixt]

Ignoring effects that become more prevalent at very high frequencies (> 100MHz) it really is that simple.

Consider that at 0% duty cycle, the voltage isn't there at all so the dissipation is zero, at 100% duty cycle it's there all the time so in your example W = V ²/R = 47.87W. If you turn the voltage off and on repeatedly, the average power lies somewhere between zero and 47.87W. The 'somewhere' is proportional to the duty cycle.

Brian.

 

[WimRFP]

Hello,

For the avarage power over multiples of periods, yes you are right (your calculation also). There is a big however: The resistor ""feels" the avarage power/temperature only when the thermal time constant is large with respect to the pulse duration.

You can compare this with the time constant of an RC circuit, when the frequency is well above the -3 dB bandwidth, the output is the avarge voltage. When the frequency is too low, the voltage will swing above and below the average value.

With the resistor it is the same, when the time constant is not large enough, the temperature of the resistor will swing above and below the avarage temperature.

The pulse power handling of resistors is limited because of maximum operating temperature of the materials, but also because of too large temperature gradients in the material that may result in cracks.

But don't worry, when you are talking about 20W resistors and 20 kHz, the resistor will not notice any difference between 20W DC power, or 20W average because of a 40W with 50% duty cycle (at 20 kHz)