Would anyone happen to know a source (for 1, not a thousand) for a 31 ohm (or close) 5W resistor of the thru-hole variety? It is as near as I can tell what the trickle charge resistor used to be in my Porter Cable 8501 battery charger (an oldie). (I may be overestimating what was used on the
5W part of the spec...it only LOOKS like about 2W (which might explain its toasty brown color) but I think 5W is closer to what it should have been.)
Remember all those resistor ladder puzzles you did in elecrtronics
101. Dust off that skill and build a resistor with the values you can find. Three 100 ohm rersistors in parallel will get you close. If you started with 6, and checked them I bet the tolerance would let you get very close.
For odd values, you can use series and/or parallel resistors. For example: You could parallel 2 62 ohm 5 watt resistors - part 62W-5-ND from digikey if you have the space. Or, if 30 ohms is close enough, you could use part # 25J30R-ND. If you must have
31 ohms, you could add a 1 ohm 1 watt # ALSR1J-1.0-ND resistor in series. Another combination: 82 ohms 3 watts # 23J82R-ND in parallel with 50 ohms 3 watts # 23J50R-ND would give you 31.06 ohms at a bit under 5 watts.
How sure are you of the 31 ohms? Hard to figure out why a trickle charger would need such an odd size - only 1 ohm from a standard 30 ohm value.
Hmmm, guess I wasn't clear enough on this... It's not that I forgot how to figure out how to make what I want with 1,000 resistors that are 1/8W and around 1k ohms in value. The problem, as I see it, is that I'd like to have it still mountable to the printed circuit board, fit inside the case, and keep costs reasonable by buying a reasonable number of resistors (preferably one, though two is okay, and three is about my limit (I still have to be able to close up the case and keep everything out of the way of all the case screws)). There also isn't anything sacred about 31 ohms...30, 33, 27, 36 are all fine, I just stated what is in there (I think...it IS hard to read and you can't measure a burned-out resistor).
The big problem that I'm encountering is:
1) almost everywhere, all I find in through-hole resistors are 1/8W and 1/4W (once in awhile I'll even find 1/2W, but even 10 resistors in parallel isn't a very attractive proposition)
2) the higher wattage resistors that I find all seem to be wirewound (even one would be too big to fit inside the charger case)
3) I DID find some current sensing resistors (power handling is fine, the problem is they are values like 0.1 ohm...310 resistors in series doesn't seem like a practical solution)
4) resistors like this USED to be common (they were never common where I live now)...I thought someone might be aware of a stash somewhere...