Find Duty Cycle base on Time to max temp and time back to ambient

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#1 21671924 12 Jul 2014 10:06

Ben Newman Ben Newman

Anonymous

Post #1
21671924 12 Jul 2014 10:06

Is there a formula to calculate the Duty Cycle percentage based on time it takes to reach max temperature and then to fall back to ambient.

for example if I want to run 200A through a circuit but not exceed 120C, and it take 4 minutes to reach 120C from ambient and 20 minutes to cool back down to ambient.

how can I calculate the duty cycle percentage?
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#2 21671925 13 Jul 2014 20:47

John Schmitz John Schmitz

Anonymous

Post #2
21671925 13 Jul 2014 20:47

Duty cycle = 4/(20+4) = 16.7%
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#3 21671926 14 Jul 2014 21:57

Ben Newman Ben Newman

Anonymous

Post #3
21671926 14 Jul 2014 21:57

Well yes that would be a duty cycle, but is it the max duty cycle the application could handle?

For example in the time period at the 6 minute mark couldn't I turn the power back on for 16 seconds, then do the same at the 9 minute, and 10? and it still won't exceed my max temp? and wouldn't that increase my duty cycle? Or maybe it would only alter my period?
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#4 21671927 15 Jul 2014 08:09

John Schmitz John Schmitz

Anonymous

Post #4
21671927 15 Jul 2014 08:09

Sorry Ben, did not fully understand your question the first time.
What you need to solve this problem is to know the thermal impedance of your device.
Do you know it's thermal impedance - sometimes called Theta JC?
If not then there is a bit of math and measurements you need to go through to get all the information you need.
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#5 21671928 15 Jul 2014 08:45

Ben Newman Ben Newman

Anonymous

Post #5
21671928 15 Jul 2014 08:45

I do not know the thermal impedance. I Googled how to calculate it just now, but I couldn't find anything that explained it very well.
#6 21671929 15 Jul 2014 09:59

John Schmitz John Schmitz

Anonymous

Post #6
21671929 15 Jul 2014 09:59

Ok, your problem can be roughly modeled as a capacitor in parallel with a resistor. The resistor is the thermal impedance of your device, the capacitance is it's thermal mass. Watts in get modeled as a current source feeding the network and delta temp from ambient is voltage across the network.
Does this give you enough information to measure and model your system to find your answer?
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Find Duty Cycle base on Time to max temp and time back to ambient

Find Duty Cycle base on Time to max temp and time back to ambient

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