[Off Topic] 38khz 1,6w in a 0805 resistor?

[hmmpic]

Hi

I know off topic.

The Peak load on the resistor is about 3,6v and 0,44 amp = 1,6w.
Duty 50% Temp in the box max25c.


It is a 0805 resistor pulsed with 38khz in 150ms, with 1 second pause between next burst. There are NO heat at all, I have stressed it to run 1 day without problem. Normally it will only fire once or two for a day.

Will it be overkill to go in production with it?

[Ttelmah]

It depends totally on the environment.
Is the unit going to be boxed?. How hot is it likely to get inside the box?. Ambient temperature?.
It is not clear whether your .44A, is peak, or the average during the 38KHz pulse, and what the duty cycle of this is. If it is the peak, and the pulse is a square wave, the average during the pulse is only 0.8W. The average power then becomes (0.15/1.15) * 0.8, which is almost exactly 0.1W.
The rate of heat rise during the pulse, will depend on the board it is attached to, as well as the resistor itself. Most resistor manufacturers will also have a pulse rating, and you need to check you are inside this. I'd possibly suggest that for the very low extra cost involved, it'd be worth specifying a resistor with a high/known 'pulse rating'. For example, in the UK:
<www.welwyn-tt.com/pdf/application_notes/pwc_appnotes.pdf>

These are _rated_ to handle 0.9W pulses at 150mSec duration, so you can then have confidence.

Best Wishes

[asmallri]

[newguy]

More than 90% of the hardware-related weird behavior or outright failures I have to deal with on a daily basis are caused by design marginalities somewhat like your situation. Do yourself a favour and either opt for a resistor like the kind that Ttelmah recommended, or redesign the board to accept a higher power rated resistor.

As a side note, the people responsible for these designs at my workplace were fired long ago. For precisely this reason. There are good designs and there are "this should be good" designs. Don't become known for the latter.

[John P]

I assume this is part of an infrared light communication system as with TV remote controls.

Repeating Ttelmah's work on the topic:

You have 1.6W power dissipation while the current is flowing. But it only flows half the time, so the real power loss is 0.8W. Then it's operating at 150ms per second, so the duty cycle is 15%, which cuts the power loss down to 0.12W (assuming it heats up at an "average" rate).

Now an 0805 resistor is rated at .125W, so you're right at the top of its capability. But rather than dissipating power at that "average" rate, I would guess that a 1-second period imposes some significant heating and cooling each cycle, which probably adds to the stress level. This looks like a marginal selection. If I were your colleague on the design team, I'd be trying to get you to move up to a larger (physically) resistor. It really wouldn't add much size or cost to the product.

[asmboy]

ask yourself:
do you know how to do linear integration?

FILM and most types of resistors are very good at averaging (RMSing) the total power as "area under the curve" so long as you don't exceed :
1- peak voltage rating
2- " current rating
3 and as Mr. Tt points out, most critically "dissipation in-situ"
out of habit i never spec a resistor that will dissipate more then 66% of its free air rating. 50% in power stages, near other heat producing parts.

the rest is just algebra

[Ttelmah]

Absolutely.

However 0.125W, is normally the rating for industrial temperature ranges. The same resistor can handle well over this for a reduced temperature range.

Hence the question about the environment. Separately though, a short high power pulse brings other problems (fatigue as the part expands/contracts etc.), so you can't just integrate under the power curve, and expect everything to work.

Provided the environment is relatively benign, the power rating is probably OK.
However I'd not trust a resistor that hasn't got a pulse rating to take this, to be reliable 'long term'.
Also remember that 'unexpected' places can sometimes be very hostile. Inside a car for example. Park it in the Sun, and suddenly you have 70C temperatures.....

Best Wishes

[Mike Walne]

[hmmpic]

Thanks for all the reply, it help a lot.
I have decided to use something like "@Ttelmah" suggested resistor. I dont know they exist, know I know it to another time.

[hmmpic]

Hi

Will this get the job?

Panasonic ERJT06
http://www.mouser.com/ds/2/315/AOA0000CE4-64075.pdf
Page 5 table most left.

[gpsmikey]

A resistor pulsed beyond it's instantaneous limits (even if under the "average" specified) is often referred to as a "fuse" :-) I always stay clear of the rating when I can to avoid being the "common failure" a year down the road on 1000 boards in service.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[FvM]