I have a resistance circuit that dissipates ~1.2 watts. If I use
11 one watt resistors in series (physically parallel) I can spread the heat out across the width of the board. I measured ~66C under each resistor at worst case ambient (~32C) in the test setup.What will happen to to the board over time (>10 years) of 24x7 operation? (Resistors in contact with the board.) Does FR4 behave like wood that is exposed to temperature over time, slowly charring?
TIA, Ed
Mormal practise is to space the resistors off the board with ceramic beads...... then you don't have the problem. If you can put a copper pour all under the resistors that will help dissipate the heat evenly as well, as opposed to leaving localised 'hot spot' sources.
I'm inclined to put the resistors in parallel and with lots of copper under them. If they're those small body type 1W, the heat goes out through the leads, need copper there to spread the heat.
Sorry, my crystal ball is bust.
Grant.
-- http://bugs.id.au/
C'mon, you must have some Consumer Electronics on hand! I see it all the time, discolored boards under diodes/resistors. Phenolic and epoxy both turn brown over time. I don't know how much this affects mechanical stability; it's not like it's crumbling under the hotspot.
(Although I did once overload an ATX power supply, which had the result of turning the output filter choke into a heating coil, merrily charring the PCB to a combination of carbon and glass fibers.)
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
ehsjr schrieb:
Hello,
why do you want to use 11 one watt resistors to spread about 1.2 watts? If it is only 1.2 W, two or three resistors will do, if it is 12 W, 11 of those resistors will not be enough. There are resistors for 4, 5 and
11 W to be soldered in a FR4 board too. There are also resistors for chasis mount available for 25 or 50 W. If you think about 10 years of 24 x7 operation, you should chose a robust solution.Bye
Is that out on the bench outside fo the intended enclosure? If you it will be much hotter when you enclose it.
"ehsjr"
** So that is 110 mW dissipation per 1W resistor or 11% of rated. ** Probably nothing.I see old equipment all the time with resistors laying flat on the PCB that dissipate more than 25% of rated and there is not even a mark left on the PCB.
You could space the body up a few mm from the surface for even greater immunity.
.... Phil
Thanks, Phil. I was hoping someone would answer based on experience with the long term effects, so your answer is particularily appreciated.
Ed
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