[SOLVED] How to determine wattage for resistor with different current profile?

hello all, how to determine wattage for the resistor for the following input condition.

the total time of signal length is 1.5second. the signal starts with a peak current of 70 amps for a period of 1ms and falls to 4 amps. after 1 second the current rise to 40 amps for period of 0.1s and falls to zero. the voltage of the input is 12v.

for the above condition how to calculate power consumption and select suitable wattage.

thanks in advance.

Re: how to determine wattage for resistor with different current profile?

If the voltage is 12v, what is causing the variation in current?

Need to know the resistor resistance value.

Re: how to determine wattage for resistor with different current profile?

Assume that the voltage is fully dropped across the resistor (the other end of the resistor is grounded).

calculate the total number of joules: 12V x 70A x 0.001 + 12V x 4A x 1 + 12V x 40A x 0.1=96.84J over 1.5s. The power dissipated is 100/1.5=67 say 70W. Use a 100W resistor. If the voltage is not fully dropped across the resistor, just scale the result.

Re: how to determine wattage for resistor with different current profile?

Assume that the voltage is fully dropped across the resistor (the other end of the resistor is grounded).

Click to expand...

That would mean the resistor has a variable value, which is not likely in that time-frame.

Re: how to determine wattage for resistor with different current profile?

crutschow said:

That would mean the resistor has a variable value, which is not likely in that time-frame.

Click to expand...

Yes, true.

The problem is over specified: you have given a specific voltage but several values of current at different times.

If I ignore the voltage, assume that R is constant, that the problem is intractable because the R is not known.

A diagram will be useful. But I cannot guess the objective of the task because grounding the other side of the resistor is also not meaningful.

I give up.

Re: how to determine wattage for resistor with different current profile?

can you include the current profile diagram showing the current-time relationship?

Also include a detailed description .

Re: how to determine wattage for resistor with different current profile?

hey all, sorry for the hidden description. the actual load is an inductor(DC motor). by varying load the given current profile is achieved. there will be series resistor for current measurement. in this case what should be the resistor watt.

@c_mitra: is your reply valid now?

thanks

Re: how to determine wattage for resistor with different current profile?

For an exact calculation, you need to know the resistor thermal time constants or an explicit pulse rating.

In practice you want reasonable low voltage drop and power dissipation of the shunt resistor. I would choose it for e.g. 30 or 40 A continuous current, you may end up with something like 1 mohm/2W.

Re: how to determine wattage for resistor with different current profile?

still the diagram does not give time information.

Find the RMS values of each segment and take the final RMS values from the calculated segmented ones.

Re: how to determine wattage for resistor with different current profile?

Hi,

in post #1 you say 12V...
But after I read post#7 I assume the 12V is never seen by the resistor.
Then this 12V value is irrelevant for resistor wattage calcutaion.

The power dissipation in the resistor is: P = I x I x R
This is true for RMS current (it is calculated of a dedicated period of time) as well as
for instantaneous current.

The peak current is 70A.
The RMS current for the given waveform is somewhere below 4.5A for the given waveform.

Now if you decide to dissipate not more than 1W during this time. then R = P / (I^2) ... about 5mOhms

This 5mOhms is for constant current, but in times with high current the resistor will temporarily overheat.
Therefore I recommend to use a 2W rated resistor. Then you have 1W of headroom for the current peaks.

Calculate peak pulse power dissipation: 70A x 70A x 0.005 Ohms = 24.5W
RMS pulse current for the first 1ms (70A linearily down to 4A in 1ms) = about 45 A --> P = 10W
All other values are more relaxed..

Now take the datasheet of a 2W, 0.005 Ohms resistor and look at the SOA graph.
Not all datasheet show this chart, but at least pulse load rated resistors should have.
Here an example of a 1W 0207 pulse rated MELF resistor (Vishay, CMB0207)

On the right side it shows the 10W limit (red circle)

For the very peak (1us) it shows more than 1000W
--> you are relaxed with your 24.5W

and for the 1ms limit it shows about 300W.
--> here also you are very relaxed with your 10W.

****
Please don´t port this result to standard resistors (non pulse load resistors).
Use a resistor that shows an SOA chart in it´s datasheet, then you can rely on that.

Klaus