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OW
Joined: Nov 03, 2009 Posts: 122 Location: London
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Posted: Fri Jul 06, 2012 7:59 am Post subject:
Matching resistors |
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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
Joined: May 26, 2007 Posts: 421 Location: England
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Posted: Fri Jul 06, 2012 8:18 am Post subject:
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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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Inventor
Stream Operator
Joined: Oct 13, 2007 Posts: 6221 Location: near Austin, Tx, USA
Audio files: 267
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Posted: Fri Jul 06, 2012 8:53 am Post subject:
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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 _________________ "Let's make noise for peace." - Kijjaz |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Fri Jul 06, 2012 11:06 am Post subject:
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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 |
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diablojoy
Joined: Sep 07, 2008 Posts: 809 Location: melbourne australia
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Posted: Sat Jul 07, 2012 2:01 am Post subject:
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try a new battery in your multimeter _________________ In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any |
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Uncle Krunkus
Moderator
Joined: Jul 11, 2005 Posts: 4761 Location: Sydney, Australia
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Posted: Sat Jul 07, 2012 2:50 am Post subject:
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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. _________________ What makes a space ours, is what we put there, and what we do there. |
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OW
Joined: Nov 03, 2009 Posts: 122 Location: London
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Posted: Sun Jul 08, 2012 1:15 pm Post subject:
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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
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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OW
Joined: Nov 03, 2009 Posts: 122 Location: London
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OW
Joined: Nov 03, 2009 Posts: 122 Location: London
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Posted: Wed Jul 11, 2012 9:49 am Post subject:
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Anyone? |
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Dave Kendall
Joined: May 26, 2007 Posts: 421 Location: England
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Posted: Wed Jul 11, 2012 1:37 pm Post subject:
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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.....
cheers,
Dave _________________ "Everything in moderation, including moderation" |
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diablojoy
Joined: Sep 07, 2008 Posts: 809 Location: melbourne australia
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Posted: Wed Jul 11, 2012 2:55 pm Post subject:
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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 _________________ In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any |
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OW
Joined: Nov 03, 2009 Posts: 122 Location: London
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Posted: Wed Aug 15, 2012 10:18 am Post subject:
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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
Joined: Sep 07, 2008 Posts: 809 Location: melbourne australia
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Posted: Wed Aug 15, 2012 2:27 pm Post subject:
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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 _________________ In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any |
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marvkaye
Joined: Mar 14, 2011 Posts: 225 Location: Fla
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Posted: Wed Aug 15, 2012 4:19 pm Post subject:
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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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