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 Post subject: Attenuator Resistors
PostPosted: Oct Sun 22, 2006 7:39 am 
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Hi,
I have recently run into a couple of instances of blown resistors in calibrated RF attenuators. These are almost always very odd values such as 7.33 or 50.7 ohms and thus cannot be easily replaced with off-the-shelf parts. Will it work, then, to put several standard resistors in series to make up the correct value? If so what kind should be used? Composition resistors are not precise enough. Metal-film and carbon-film have small amounts of inductance and only seem to be available in a relatively limited number of values. Wire wound resistors are easy to get in lots of values and are quite stable and precise, but have quite a lot of internal inductance. Are inductive resistors in an RF attenuator a problem? Any other recommendations?

Thanks,
Matthew D'Asaro

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PostPosted: Oct Sun 22, 2006 11:17 am 
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Matthew, you might consider starting with the next-lower carbon comp value and abrading the resistance higher. I file a little groove and keep measuring the resistance until it reaches the value I want. Then I put a dab of finger nail polish in the groove to keep moisture out.

Brian


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 Post subject:
PostPosted: Oct Sun 22, 2006 1:22 pm 
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Use small film resistors in parallel. Their inductance is negligible. Start with a resistor of the next higher value that you have on hand, parallel it with a decade box and twiddle the dials until your digital ohmmeter reads the right value. Then solder in a carbon or film resistor of that value (its tolerance or stability won't matter if it is much higher in value than the first resistor).

You could figure the values mathematically too.


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PostPosted: Oct Mon 23, 2006 8:48 pm 
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You can use any series and/or parallel combination needed to build up the value, as long as the result fits in the available space :D

I normally work one decade at a time, starting with the highest digit. It depends on what resistors you have and what degree of accuracy is required. I once had to replace a 31+K .005% resistor. It took 13 resistors to do it, but I came up with the exact value.

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PostPosted: Oct Mon 23, 2006 11:34 pm 
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Thanks Leigh. I understand that I can put resistors in series/parallel to make the appropriate value. However, I am still not sure if internal inductance in the resistors I use is a problem. Can I use wire-wound resistors in an RF attenuator? What about spiral-cut metal film? Carbon film?

Thanks,
Matthew D'Asaro

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PostPosted: Oct Tue 24, 2006 12:06 am 
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Hi Matthew,

I would not use wirewound resistors in any RF application... the inductance is too high. And they're normally much too large for the application.

However, modern metal- or carbon-film resistors can be used as they are designed to be non-inductive. I use the 2% 1/4-watt or 1/8-watt parts made by NTE. They have good accuracy and stability at a reasonable price. Of course the 1% military RN55 parts are available from Mouser for $0.10 each in single quantities. They also carry 0.1% parts for about $1 each.

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PostPosted: Oct Tue 24, 2006 12:08 am 
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Thanks again Leigh. That is exactly the answer I was looking for.

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PostPosted: Oct Tue 24, 2006 1:16 am 
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Location: Peru,IL USA
Matthew: Here is a link to a parallel resistance calculator: http://www.computertorture.com/noncompliant/parallel/

Might come in handy while solving your problem.

Regards,
Ron


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