Increasing a resistors power rating ;)

[pittuck]

Ok,

I have an IR LED which is rated at 100mA for a <20ms 'burst'. I plan to check the sensor about 20Hz (perhaps 40) and have the LED on for a matter of 50us or so for the reading to take place.

Problem is i did not think to check the power of the resistor, i am using 0805 SMT package. They are rated at 0.125W and in order to get 100mA with a 3.7V voltage drop (5 - 1.3) i need a 39Ohm (closest value) resistor. Which makes for 0.350W

Too much, but for such a small time period (max of 10ms of a second lets say) would it be acceptable to use just one resistor, or am i going to have to stack a load of values to get a parralel one?

Thanks,

- Martyn

 

[Someone Electro]

you can parallel 2 resitors duble the value to get duble the power of one

 

[pittuck]

i know but i will need 3 or 4 to get the desiered value, its whether or not i actually need them!

 

[stevez]

Just some thoughts.

The rating of the resistor is likely based on some standard conditions. Air circulation, heat sinking via the PCB, among other things probably impact the power rating. You might try to understand how your specific application compares to the conditions used for rating the resistor. While it seems unlikely that the power handling ability would double or triple - it might also be only a portion of the 1/8 w.

Regarding the pulsing of power - it would seem that the rating would describe a steady state average. You describe what I think is a 1% "on" time at 0.35 w - on average that's way below 0.125 w. Regardless of how the average works out there certainly must be a peak current limit - I wonder if such a rating exists for the resistor? If so, some quick math might reassure you.

My guess is that most people would just give it a try - or set up a simple experiment- possibly increasing the duty cycle to allow for a factor of safety. It does seem worthy of some thought.

 

[pittuck]

yeah

a watt is actually Joules per Second so i am acutally only putting through 3.5mJ a second compared with the 125mJ a second maximum. So should work, if not it will get hot and go bang

 

[CoTang]

yeah

a watt is actually Joules per Second so i am acutally only putting through 3.5mJ a second compared with the 125mJ a second maximum. So should work, if not it will get hot and go bang

Never seen a resistor go bang. More like a flare and then silence follow by "$#@!!"

 

[Nigel Goodwin]

As already suggested, if you're pulsing the power the resistor only averages low power just as the LED does - work out the dissipation from the full current and voltage, then divide it by the percentage of time it's actually ON.

From what's been said so far, it sounds like the resistor has a huge safety margin.

 

[Dean Huster]

As Nigel said, the LED is only on for a fraction of the the time. The period of 40Hz is 25ms; your pulse width is 50µs; 50µs/25ms is 0.002

Multiply your calculated on-all-the-time power dissipated by the resistor by 0.002 and you'll have a better number to work with for actual power being dissipated by the resistor.

A 125mW rated resistor would be able to top out at 62.5W with that pulse width. No worry there!

Dean

 

[instruite]

Hi
Normally resistors pulse power rating is different than the normal power rating of the resistor
The resistors which I use in our designs for example the normal power rating is 0.0625W but for say 100 us it can handle power upto 1W (we get this specifications from supplier directly)
In your case it may not be possible to get those specifications

Dean: its not exactly the pulse power calculation to get the maximum power because in the data we get from suppliers below a certain pulse width say for example 100us the max power rating will be constant

pittuck: in your case the pulse width is small and the power disspation required is also not much so it will work. (nfact it might work with much smaller wattage resistor)

 

[awright]

Chip Resistor Power

It is clear that the AVERAGE power dissipation in your application is well within the rating of your chip resistor, so the question is only whether the resistor will be reliable with the power-time product of the individual pulses in your pulse train.

My intuition, like the other posters, is that you are very safe, but the proper evaluation would consider the thermal time constant of the mounted resistor. That is, for a step change in current, the time it takes for the resistor to reach 63% of its final temperature is the thermal time constant of the resistor as it is mounted on the board. If that time constant is much longer than the duration of the individual current pulses in your application, then the average power dissipation is the determining parameter. If the thermal time constant is comparable to or shorter than the duration of pulses in your application, you are in trouble because the resistor will respond thermally to the peak power of each pulse.

You can use a very fine thermocouple epoxied to the resistor chip to measure the time constant. You don't even need a precision readout or cold junction reference, since you don't really need the true temperature, just the change of temperature. In fact, you don't even have to determine the true time constant. Just look at the amplified TC output on a 'scope as you apply the worst-case pulse train in your application. You need enough amplification to see a small change in temperature when you start your pulse train, showing that you are actually seeing the thermal response of the chip. If you do not see any significant response to the individual pulses, the thermal time constant is long compared to your pulse durations and you only have to be concerned about the average power dissipation. If you see the temperature ramping up and down in synchronism with your pulse train, it is responding to each pulse and either more detailed thermal analysis or more conservative design is required.

I doubt that it is worth your time to perform this measurement, but it is really easy to do and would give you confidence in your design. See OMEGA for thermocouples. The bead and the wires must be very small compared to the chip so they do not add significant heat sinking to the chip or conduct significant heat away from the chip. Use a pair of TCs in series bucking configuration with one attached to the chassis or a handy thermal mass and the other attached to the chip. This will cancel out the ambient temperature and show the differential temperature between the two beads. You don't care about the calibration for this test.

Have fun.

awright

 

[pittuck]

ok, well i am not going to get the sensors working till tomorrow, i still need to make some routines and test the transistor footprints for their driving circuit.

Today i should be doing my nan's lawn, but its pissing it down so thats out. Then i gotta do my paper round (pays the electronics bill ) and then i gotta sort some make shift wheels out so i can see if my motors can take the weight of my robot!

Finally this evening i will try and find something to make the vertical posts (otherwise i gotta 10km cycle to the model shop )

 

[DigiTan]

Aye, aye, aye this is getting complicated. Can't you just integrate the waveform and divide by time?

 

[Styx]

resistors normally have two ratings.

1) continouse power dissipation
2) pulse power, with some time info, ie 1us pulse at 1W per 1second

From the duty you are talking abt it will be fine.
If you really want to be sure work out the RMS power in the resistor, if this is below the rating, then you are fine.

RMS for a squarewave is the same as the average.


The other option is to fit a mini-MELF style resistor to the pads, they have the same pitch as what you have, but handle alot more power

I

 

[Dean Huster]

instruite:

Dean: its not exactly the pulse power calculation to get the maximum power because in the data we get from suppliers below a certain pulse width say for example 100us the max power rating will be constant

I'll certainly agree with that. Most parameter calculations assume "normal" situations. Wild pulse widths, high frequencies and weird waveforms usually affect these "normal" calculations. That's why (especially in the old days of vacuum tubes) a lot of folks wrecked their meters by hanging them off the assumed-safe 350-volt measuring point only to find that there was a multi-kilovolt pulse there they didn't know about. Maybe I should have been so broad with the calculation assumption, but knew that even though off by a bunch, he would be well-within his resistor's rating. Just an example, though not perfect.


Styx:

resistors normally have two ratings.

Since we're playing with power, let's also not forget that resistors have an oft-abused voltage specification. Folks playing with higher-voltage attenuators and voltage dividers often forget that little fact.

Dean