[pinkfloyd4ever]

ok, so again, I'm about to build my first CMoy, and I'm wondering if I should get extra resistors and input caps to try to get matched pairs. My meter is a Fluke ET18 (i know, crappy) originally purchased in July '98 that I bought off eBay a couple weeks ago. It came with all the manuals and the orig. receipt, so I'm assuming it has been taken relatively good care of, but still, it hasn't been calibrated in 8.5 years and its only an 18. I believe the current equivalent model is the 114. So, without buying a Fluke 187 (or any other meter for that matter), is it worth it to try to match pairs? I'm using Wima MKS2's for the input caps and cheap Xicon 1% resistors. I dunno if matching is even that important for the power cap, but I'm using a rail splitter, so that doesn't apply to me anyway. My guess is my meter could maybe match the input caps but not the resistors, but I'm not sure or I wouldn't be asking.

Your thoughts, opinions, insults, etc?

 

[tangent]

Did you read this?

 

[pinkfloyd4ever]

ya I did, that's why I thought it wouldn't work for the resistors, but unless I missed it, I don't think you say anything about caps there? Does all the same apply?

 

[Paragon]

Matching should be all relative. If a meter is of by say.. +.5 Ohm, it will be off by that amount on all resistors. So if you match a pair of resistors to 5.5Ohm on the uncalibrated DMM, they will both read 5 Ohm on a calibrated meter.

It is just a matter of relativity.

 

[pinkfloyd4ever]

Quote:

Originally Posted by Paragon /img/forum/go_quote.gif It is just a matter of relativity.

wait, does that mean I have to use E=MC^2 when I'm matching then?

your icon scares me, Paragon, what is it?

 

[Paragon]

It is an M.C. Escher drawing. Mumified Frog.

 

[tangent]

Quote:

Originally Posted by pinkfloyd4ever /img/forum/go_quote.gif ya I did, that's why I thought it wouldn't work for the resistors

It sounds like you are guessing. I pointed you to the article because it tells you how to find out for sure, so you don't have to guess. So go find out.

Quote:

I don't think you say anything about caps there? Does all the same apply?

There are some technical differences, but the principles are still the same. If you understand what it says about how to read a meter's resistance spec, you should be able to translate that into the equivalent procedure for the cap side. All that remains then are part tolerance questions, and that's the same for any passive part.

Quote:

Originally Posted by Paragon Matching should be all relative. If a meter is of by say.. +.5 Ohm, it will be off by that amount on all resistors.

That ignores short-term drift, which is very real. If you search these very forums for posts by bigcat39, you will find where he reports that you can see as much as a 25% difference in short-term measurements with cheaper meters. I don't know whether "short-term" means 24 hours to him or 24 seconds, but the point is that you can't say "this meter is off by half an ohm". It may have been half an ohm off at some point in time, but drift ensures that it's no longer off by half an ohm any more.

That's why you're supposed to calibrate your meter periodically. Even an excellent meter drifts, so you have to drag it back onto to the path of truth and light every so often.

 

[Aguirre]

That is an interesting read Tangent. My own DMM is a cheap velleman with what looks like 2000 counts of resolution. I gave up trying to match resistors because under most ranges it's totally not up to the task. But now I am wondering if I can use my limited understanding of Ohm's law to help my meter do relative matching.

In the 2K range my meter can measure to the nearest Ohm which seems sufficient to perform .1% matching on resistors valued 1K-2K. My next range is 20k. Which can only measure to the nearest 10 Ohms. So for a 3.32K resistor my meter won't work. It seems though that if I take two of these 3.32K resistors and put them in parallel on my alligator clips I would get into1660 resistance and could use the 2K range on my meter. This would give me the needed resolution. And then I could keep one fixed as I varied the other to get a nice .1% match.

This technique sounds like it might work. I wonder if its valid.

 

[tangent]

If you want to get tricky, I wouldn't stop short of building a Wheatstone bridge. Borrow a good meter to calibrate the bridge, then you can go back to your cheap meter to measure resistors.