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 Forum index » DIY Hardware and Software
Matching resistors
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OW



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PostPosted: Fri Jul 06, 2012 7:59 am    Post subject: Matching resistors Reply with quote  Mark this post and the followings unread

This may seem like a really stupid question but how do you match resistors?

On all of my meters the value seems to fluctuate and doesn't remain on a single resistance value, so what would be the best approach?

Thanks!
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Dave Kendall



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PostPosted: Fri Jul 06, 2012 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Hmmmm..... that doesn't seem right.

A Digital meter should settle down after a second or two. Occasionally it seesaws a bit, but usually settles on a value after a few more seconds.

Are you pressing quite hard with the probes either side of the body of the resistor? If you use too light a touch, the DMM probes aren't making good contact with the resistor leads, and then you can get what you describe.

cheers,
Dave

Last edited by Dave Kendall on Fri Jul 06, 2012 9:17 am; edited 1 time in total
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PostPosted: Fri Jul 06, 2012 8:53 am    Post subject: Reply with quote  Mark this post and the followings unread

One possibility might be that you are touching the leads with your fingers on both sides of the resistor. This puts the body's skin resistance in parallel with the resistor and can cause such fluctuations due to movement, breathing, etc. Always measure resistance without touching the leads, or if you must then touch only one side.

Les

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elmegil



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PostPosted: Fri Jul 06, 2012 11:06 am    Post subject: Reply with quote  Mark this post and the followings unread

I use the pin clip attachments on my meter. (the ones in the bottom of this picture http://ecx.images-amazon.com/images/I/41921IJd4yL._SL500_AA300_.jpg)

this actually makes it very easy to walk through a bandolier of resistors, which also keeps them in the same order so I can find the best matching pair when I'm done Smile
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diablojoy



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PostPosted: Sat Jul 07, 2012 2:01 am    Post subject: Reply with quote  Mark this post and the followings unread

try a new battery in your multimeter
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Uncle Krunkus
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PostPosted: Sat Jul 07, 2012 2:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree with elmegil, I always use clips, and then hands off when matching resistors.
Keep in mind too that often when matching resistors, it's not that they have to be spot on the value, it's more that they are the same as each other. I get a sheet of paper with a grid drawn on it. Write the nominal figure in the middle square, decimal increments in each box. Then you just use the box that fills up first. Smile

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OW



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PostPosted: Sun Jul 08, 2012 1:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for all the great advice, it's all very helpful! Turns out that the meter is not functioning as it should. I was thinking of purchasing this:

http://www.maplin.co.uk/ut-801-bench-multimeter-589599

But I don't know whether it would be that useful to match resistors, capacitors and transistors as I don't know how many decimals it goes to; can anyone comment on it, or something better in that price range?

Thanks again!
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elmegil



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PostPosted: Sun Jul 08, 2012 2:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

http://www.uni-trend.com/manual2/UT801%20%20802%20Eng%20manual.pdf

The 801 has 3 1/2 digits per the manual. I'm not the one to say whether that's enough or not. The 802 has 4 1/2 if the price is not a big jump....
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OW



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PostPosted: Sun Jul 08, 2012 2:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

This may be a better choice then, as it has 19999 count display and 4 1/2 digits. What do you think?

http://www.rapidonline.com/Tools-Equipment/Uni-T-UT58E-series-Modern-digital-multimeter-400417
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OW



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PostPosted: Wed Jul 11, 2012 9:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Anyone?
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Dave Kendall



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PostPosted: Wed Jul 11, 2012 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi.

FWIW I went for an Maplin Academy PG017 DMM - pretty similar to the one you describe. Battery usage is really good - it doesn't. Well *nearly* doesn't - a 9V battery lasts a year or more ...
Downsides - PG017 doesn't have auto-shutoff - you have to remember to switch off. It also doesn't use the banana sockets for capacitance - it uses a pair of slots. Using regular sockets for capacitance would be more useful IMO.

A transistor tester is v. useful to have - to check pinout and whether it actually works in one go.
Audible continuity beep is dead handy, as you can look at what you're doing, not the display. Less risk of a slip and PPPPZZZZZZZZT!
Some DMMs don't go up very high in capacitance - it should measure at least up to 20uF, for those common 10uF bypass caps. Some guys use 22uF, so bigger range (200uF) is even better.
Diode test is common and useful.
For assembling kits and building modules, current readings are less common, but most DMMs seem to allow that, and it does get used. Some features are pretty vital for design, but less so for building.

Your choice looks good to me - just my 2 pennies worth..... Smile

cheers,
Dave

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diablojoy



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PostPosted: Wed Jul 11, 2012 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
This may be a better choice then, as it has 19999 count display and 4 1/2 digits. What do you think?


look at the data sheet for it. resistance = +/-0.5%+10 best accuracy

if your main aim is getting it for matching resistors its not really accurate enough but then few meters are unless you are willing to pay lots of $
and then they need regular calibration to be certain , look into building a good wheatstone bridge circuit instead. you will need 2 very well matched resistors for R1 and R2 to begin with though

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OW



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PostPosted: Wed Aug 15, 2012 10:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Catch 22, how can I build one without the capabilities of matching resistors before hand? Unless someone could send me two, I would pay of course.

Thanks!
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diablojoy



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PostPosted: Wed Aug 15, 2012 2:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Catch 22, how can I build one without the capabilities of matching resistors before hand?

You would have to buy the 2 resistors of course
lots of places sell very close tolerance resistors down to .01% even, they are just very expensive but not quite as expensive as a meter with the required accuracy or the pain of recalibration every so often
and from then on you can match as many resistors as you like
from standard 1% ones

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marvkaye



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PostPosted: Wed Aug 15, 2012 4:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

I asked kind of the same question over at Muff's... See this thread for a great description on how to build a WHeatstone bridge along with how to calibrate it if you don't already have matching resistors.

<marv>
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