What resistors to use?

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#2
This is the best resistor PPC.22ATR-ND, but it's .33 W which is a usual value. So, your answer is wirewound fusable. 0.33 W and 0.5 W are much more common. Are you sure your decimal place isn't off by a factor of 10? You want something that will handle a short overload, but will open completely under a large overload.

Metal film resistors puddle
Metal oxide vaporize under short overloads
Carbon film resistors increase value under overloads
 
#3
imperfectcircle said:
Ca anyone reccomend a resistor that Digikey carries that will work well for the emmiter resistors in an amplifier?? They are .22ohm 3w but im not sure what kind of resistor works best in this application? Metal film, metal oxide, wirewound...etc
thanks
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imperfectcircle,

You haven't really given enough details to get a "proper" answer.

But, assuming it's an "audio" amplifier, and that it has been properly designed, such that "overload" (as another poster mentioned) does not need to be considered, and that the design assumes no parasitic inductance, for example, in the resistor, I'd look for the smallest non-inductive type that meets the specs, that is actually available and has a reasonable price.

Note that, in general, all "wirewound" resistors have more inductance than similar-valued film (and most other) types, and that parasitic inductance is usually "a bad thing". But the very low resistance values of some types of wirewound resistors might have very low inductance. The Metal Film type is probably the most distortion-free common construction-type that's available. But you might not find the resistance value and power rating combination that you need, in that type.

Why only Digikey? Mouser.com has a fairly broad selection of power resistor types and values. I believe that Mouser's selection of resistors, in general, is much broader than Digikey's (For example, Digikey doesn't even carry ANY 0.1% resistors, while Mouser has at least two different lines of them, some down to 5ppm/degC.)

Browsing through Mouser's current 1828-page free catalog for you, I immediately found the following (shown in page-number order, i.e. not in order of preference):

71-CW5-J-0.22 (Vishay/Dale), 0.22 Ohms, 5%, 5W, wirewound, silicone coated, +/-90ppm/degC, $0.62 each/1

72-RWM410-R22-5 (Vishay/Sfernice), 0.22 Ohms, 5%, 3W, Wirewound (conformal vitreous enamel), +75ppm/degC, $0.46 each/1

594-AC03W0R240J (Vishay/BC Components), 0.24 Ohms, 5%, 3W, Wirewound (ceramic core, "cemented"), $1.83 each/1

280-CR5-0.22-RC (Xicon), 0.22 Ohms, 5%, 5W, Axial Cement Power Resistor (wirewound), +/-350 ppm/degC, $0.39 each/1

684-MP9100-0.22 (Caddock), 0.22 Ohms non-inductive (film), 1%, TO-247 3.5W without heatsink (or up to 100W with heatsink), 0 to +150 ppm/degC, $11.53 each/1 (Note that, with a small heatsink, other/smaller Caddock non-inductive power film models (TO-220 or TO-126 cases) would work, for less than $5 each.)

588-13FR020E (Ohmite), 0.20 Ohms non-inductive, 1%, 3W, molded silicone w/wire element, $1.40 each/1

588-13FR025E (Ohmite), 0.25 Ohms non-inductive, 1%, 3W, molded silicone w/wire element, $1.40 each/1

588-TCH35P-0.24-E (Ohmite), 0.24 Ohms non-inductive, 5%, 35W (w/heatsink),

588-43F-0.2, 0.20 Ohms (Ohmite), 1%, 3W, 50ppm/degC, $1.76 each/1

588-43F-0.25 (Ohmite), 0.25 Ohms, 1%, 3W, 50ppm/degC, $1.76 each/1

588-23J-0.2 (Ohmite), 0.2 Ohms, 5%, 3W, +/-50ppm/degC, vitreous enamel coated wirewound, $1.82 each/1

72-LTO030FR0220JTE3 (Vishay/Sfernice), 0.22 Ohms non-inductive, 5%, 30W (w/heatsink) TO-220, +/-150ppm/degC, $4.99 each/1

71-RH10-0.22 (Vishay/Dale), 0.22 Ohms, 1%, 10W (already encased in aluminum heatsink), $2.89 each/1

72-RTO20F-R2-5 (Vishay/Sfernice), 0.2 Ohms "negligible inductance" (<0.1uH) Thick Film, 5%, 20W (w/heatsink) TO-220, $9.11 each/1

and quite a few others. If you search for these part numbers, at http://www.mouser.com, most of them will have a link for their datasheet.

There are also some possibly-desirable types that aren't listed as being available with values below 0.47 Ohm or 1 Ohm, which could be combined in parallel (with needed ratings of 3W divided by the number of resistors in parallel) to get close to your 0.22 Ohm target value (e.g. qty 2 of 282-0.47-RC 5% 2W +/-350ppm/degC Metal Oxide, $0.19 each/1, to get 0.235 Ohm 4W).

Good luck!

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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#6
And, from Digikey, here are some (mostly) 0.22 Ohm >=3W resistors that MIGHT work for you, that I found by doing a search at digikey.com. I didn't look at their form factors, or any datasheets, etc. So you'd have to examine each one more-fully.

Digikey part numbers:

MRA05-.22-ND, RES AUDIO .22 OHM 5W 1% CERAMIC, $2.37

TAH20PR220JE-ND, RESISTOR .22 OHM 20W TO220, $7.40

TCH35PR220JE-ND, RESISTOR .22 OHM 35W TO220, $7.98

MP915-0.20F-ND, RESISTOR 0.20 OHM 15W 1% TO-126, $3.24


And there are too many 3W 0.20 and 0.24 Ohm wirewound resistors to list.


You could also try two of any of these, in parallel:

PPC.47ECT-ND, RES .47 OHM 3W 5% MF FUSIBLE, $2.14, Min 5

0.47W-2-ND, RES .47 OHM 2W 5% METAL OXIDE, $0.23

P0.47W-2BK-ND, RES .47 OHM 2W 5% METAL OXIDE, $0.28

P0.47W-3BK-ND, RES .47 OHM 3W 5% METAL OXIDE, $0.51


If you want to get closer to exactly 0.22 Ohm, you could use the free "Resistor CAD" program to find the combinations of parallel values that would get you within, say 1% of 0.22 Ohm (Sorry, I don't have the URL for downloading it. But it's by Terry Harris, Vader Systems.)

Some of the parallel combinations that would get you within 1% of 0.22 are:

R1 R2 (%Error)

0.30 0.82 (0.16%)
0.24 2.7 (0.19%)
0.27 1.2 (0.19%)
0.36 0.56 (0.40%)
0.39 0.51 (0.45%)
0.33 0.68 (0.99%)

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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#9
EchoWars said:

The dual resistors are about worthless, but I appreciate the effort. If you find some singles, I'd love to know where.
Click to expand...

I'm curious. What is special about them? What spec(s) do they need to have that you can't find? i.e. Why won't any of the 0.22 Ohm 3W resistors listed previously, in this thread, suffice?

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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