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Old 20th April 2011, 11:10 AM   #11
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with my cheapo DVMs substracting the shorted leads reading works very well.
Even bouncing 0.2 ohms would result in less than 5% error of a say 5.6 ohm Rdc.
There is no reason for better accuracy. 'Accurate' TS-measurements are an illusion. You would only lie to yourself.
For example try to measure TS specs at different room temperatures and be surprised how rubber surrounds react on a few degrees of enviromental temperatur changes.
Thiele-Small parameters are small signal parameters only valid at a given operating point. As soon the driver is playing some serious level, the voice coil heats up, suspension changes etc.
@buggson:
I suggest testing your DVM on a 2.2 and 5.6 ohm resistor or some similar values. 5% 2-5W metalfilm or MOX resistors are cheap and easy to get.
regards
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Old 26th April 2011, 09:50 AM   #12
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Default Now I feel more comfortable relaxing my measurements

Now I've got lots of suggestions and hope I will manage. I would benefit largely if I coud adhere to KISS more often than I do.
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Old 26th April 2011, 11:58 AM   #13
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My sears 82140 DVM ($29) will resolve 0.1 ohm but won't actually go to zero. What the low number is has a lot to do with how recently the battery was changed. Besides shorting the lead, I measure a 5% 1 ohm resistor before testing under 10 ohm. Then I add or subtract the reading the meter has on the 1 ohm. You can also use 2 or 5 ohm resistors for reference if that is closer to what you are measuring.
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Old 3rd May 2011, 08:38 AM   #14
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Quote:
Originally Posted by Bksabath View Post
Constant current methode works a treat

LM317 and one resistor to set the constant current
Drawing in the LM317 spec sheet
with 100 mA I have measured a batch of MPC74 0.22 homs and go the meter reading volt across them in milli volts.

There is here a tread about a LCR meter designed for speakers that may be worth looking at
An awesome ZLCR meter


How do you post links to treads is what get me cornered
so if you could help me on how to do this I will find it for you
I made exactly the same: but using a 78L24 voltage regulator, and set that to give a 10,0 ma current: I have two 79,6 ohm 0,5% resistors, and could find the appropriate resistors: 7.500//5.100 ohms gives a better than 1% precision. It measures as low as the DVM goes, in practice 0,1 ohm is no problem.
albert
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Old 12th August 2011, 04:31 AM   #15
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Look around for a Fluke 8012. Had a devoted low ohm scale with a front panel lead zero adj. Should sell for pretty cheap if you can find one. Doc
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Old 12th August 2011, 07:33 AM   #16
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Hi Doc,
Actually I myself just bought a new meter, 4 1/2 digits, VC8045 bench-top, mains-fed DVM, that has a resolution down to 0,01 ohm on the 200 ohm range. I was not satisfied with my own breadboard.
It cost me $ 99 plus shipping and tax; imho a bargain. Others have a lowest range of 600 with the same resolution, but have a 50-100% higher price tag.

I have seen meters with four pole plugs (that is two current emitting and two voltage sensing) meters, that overcome the problem of the leads themselves being measured.

Why there is not a kind of 'tara' function, like on the common household weight, I do not understand. Short the leads, press the tara button, and the setting becomes zero, the resistance is then shown as-is.
Look, I am inventing here on the spot!!!

albert
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Old 12th August 2011, 05:50 PM   #17
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Hi back, Albert. Took a look at the VC8045 and in my opinion it is indeed probably a bargan at $99. Over the years I've used quite a few four terminal equipped bench meters and even at one time had a Datron 1082 8.5 digit DMM resident on my bench (along with a maxed out Tek2465 scope). As to the tara function, it's already incorporated in many DVM's (including going back to that Datron 1082). Both my Fluke 87 and my HP handheld have it. I know HP (now agilent) incorporated it on some of their meters as far back as the seventies.
Anyway, I can atest from my cal lab days that virtually all high precision ohms measurement are made using a 4 wire system.
Doc
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Old 13th August 2011, 10:32 AM   #18
gyro is offline gyro  United Kingdom
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Default Maxim App Note 106 works well

If you're looking for a good general purpose milliohm meter I've used the circuit from Maxim Application Note 106 with good results. Quite an elegant circuit that will work with any sensible reference, op-amp and low-Rds MOSFET. I'm still on my original batteries after 2-3 years use.

If you substitute a dual op-amp you can wire the other half as a comparator to light an LED when the FET gate voltage gets near the +9V rail - ie. Over-range indicator (resistance too high to maintain constant current).

I can't find the App note on the Maxim site anymore but there are plenty of hits on a Googe search, eg. http://www.wentztech.com/radio/Proje...sets/AN106.pdf
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Old 13th August 2011, 07:24 PM   #19
Ron E is offline Ron E  United States
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Quote:
Originally Posted by Juergen Knoop View Post
Even bouncing 0.2 ohms would result in less than 5% error of a say 5.6 ohm Rdc. There is no reason for better accuracy. 'Accurate' TS-measurements are an illusion. You would only lie to yourself.
5% is probably about as accurate as t/s measurements can get, but that is no reason to settle for low accuracy measurements. for a nominal 4 ohm driver, the error increases to 10%, and for a 2 ohm, 20%. There are many other sources of error in t/s measurement and this is a simple one to eliminate. Next is to decide whether Re=Rdc or not, for inductive drivers, probably not. Also for inductive drivers the shape of the peak changes...and the FR around resonance...
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