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2 Watt 1% Resistors

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SgtRock:
Greeting EEVbees:
--I need 2 watt 1% resistors in the following values: 2.2 megohms, 3.3 megohms, 4.7 megohms, 6.8 megohms and 10 megohms. I am rebuilding a couple of resistance substitution boxes so they can be used in series and parallel.
--Since my best meter is a Fluke 87 (which I love), I am unable to confirm these values to 1%, considering the+- and added counts of the F87 accuracy specifications. Until I am able to afford a Fluke 87, I shall be forced to rely on the resistors themselves for accuracy.
--Your ideas and comments are humbly solicited.

EEVblog:
The Fluke 87 has a 0.2% basic resistance spec.

You'd need 2098V to max out a 2.2M 2W resistor. I doubt you'll ever get that high!
And 741V to max out a 1/4W 2.2M resistor, so I think 1/4W resistors will do you just fine.

Dave.

SgtRock:
Dear Dave:
--Thank you for your helpful and informative response. As I under stand it the Fluke 87V does indeed have a basic resistance spec. of 0.2%. I do not as yet have a Fluke 87V and am forced to rely on (as I said in my original post) a plain old Fluke 87. I meant to say until I can afford a Fluke 87V, sorry. Looking in the manual for the F87 I read that the accuracy in the 40 megohm range (which is used for 2.2 megohms to 10 megohms) is +- 1% plus 3 counts. So you can readily see it is pretty much useless to confirm these resistances to within 1%.

--I readily agree with your Ohms Law calculation that I would need "2098V to max out a 2.2M 2W resistor". Emerson said that "A foolish consistency is the hobgoblin of little minds." It would appear I was being foolishly consistent, and that indeed, I need not use 2 watt resistors for 2.2 megohm and above. I can easily obtain the resistors from Digikey in 1/4 and 1/2 watts.

--One of the reasons the res. sub. boxes I am rebuilding (Eico and Heathkit), used hefty 1 watt resistors may be mechanical. The main conductor bus is supported on the resistors. Thanks again for your gentle remonstration in pointing out the obvious. Best Regards
Clear Ether

gregariz:

--- Quote from: SgtRock on August 17, 2011, 04:15:04 am -----One of the reasons the res. sub. boxes I am rebuilding (Eico and Heathkit), used hefty 1 watt resistors may be mechanical. The main conductor bus is supported on the resistors.

--- End quote ---

I had one of the Heathkit IN series years ago. The one I had came out in the early 60's. Its worth remembering that for anyone breadboarding with tubes in those days (or still these days), a common HT supply may well be 1500Volts. So the 1Watts made sense in those days... but as was mentioned for transistor work its not really needed.

EEVblog:

--- Quote from: SgtRock on August 17, 2011, 04:15:04 am -----Thank you for your helpful and informative response. As I under stand it the Fluke 87V does indeed have a basic resistance spec. of 0.2%. I do not as yet have a Fluke 87V and am forced to rely on (as I said in my original post) a plain old Fluke 87. I meant to say until I can afford a Fluke 87V, sorry. Looking in the manual for the F87 I read that the accuracy in the 40 megohm range (which is used for 2.2 megohms to 10 megohms) is +- 1% plus 3 counts. So you can readily see it is pretty much useless to confirm these resistances to within 1%.

--- End quote ---

Then you can use a lower and more accurate range to measure it.
If you can measure a lower value more accurately then you can put the DUT in parallel with the known resistor and calculate it's value.

Dave.

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