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-   -   Low-ohm meter, or what? (http://www.diyaudio.com/forums/equipment-tools/187293-low-ohm-meter-what.html)

buggsson 18th April 2011 09:33 AM

Low-ohm meter, or what?
 
I know nothing about electronics, and in another forum:
http://www.diyaudio.com/forums/subwo...ml#post2542298

there was suggested to use a low-ohm meter to measure speaker driver Re as you would then get more reliable results.

There was one suggested (Low Ohm Meter - Measures 0.001 up to 1.999 Ohm), but its range is only 0.001 - 1.999 ohms, is that range common for low-ohm meters? I guess drivers have higher ohm values as well? You would also need a reliable ammeter to calibrate the device. There was also some issues for causion (on a native forum) about thermal effects, etc.

There was also suggested these parts:
Kjell & Company - Nordens bredaste sortiment av tillbehr fr hemelektronik
Kjell & Company - Nordens bredaste sortiment av tillbehr fr hemelektronik
(in swedish, but I guess you will know what it is anyway)

which I don't really understand what you could do with for my inteded purpose.

The good thing about the low-ohm meter is its four point measuring for example. A bench multi meter has also been suggested, but then I guess that prices really go up for reliable ones?

It got much more complicated than I first expected. I intend further on start building power amp. kits, but would I be better served just go measuring Re with a DMM?

richie00boy 18th April 2011 10:56 AM

Yes that is a normal range for low-ohm meters. They are not appropriate for measuring speakers, they are intended for things like measuring resistance of earth cabling etc.

All you need to do with your meter is short the probes to obtain the lead resistance then subtract this from your reading.

Note that in the thread you linked to, Ron suggested BUILDING a low-ohm measuring device using a current source, not an actual low-ohm meter.

AndrewT 18th April 2011 11:26 AM

Using a current source and calibrating your DMM & source with a high accuracy resistor, gives very good amateur results at very low cost.
A "normal DMM set to lowest resistance measures 199.9ohm at full scale.
The resolution of this scale is 0r1. Not nearly good enough for low ohm measurements.

With a range of 10mA and 100mA and 1A CCSs, you can use your 199.9mVdc scale to measure to milli-ohms (0r00x) accurately.

Henry8 18th April 2011 04:01 PM

Heres my PRACTICAL METHOD 1. you need to get a constant current diode, ccd 2. let say you get a nominal 1.8ma ccd 3. connect it up to a 9v supply and measure the current flow, say its 1767uA 4. now connect in series with ccd, your driver and measure the voltage across it 5. Re=Vdriver/1767uA (check the accuracy of your results by measuring a 10R 0.1% resistor)

Andrew Eckhardt 18th April 2011 05:47 PM

I'm not sure I understand the reasoning behind the high tech resistance measurements.

On a 5 ohm copper coil at 20C, you get 5.4 ohms at 40C. If you can resolve to .1 ohm with your measurement you're doing pretty good.

Getting the accuracy from a regular DMM on the lowest range will probably require that you zero the leads before you measure.

.1 Ohm will not significantly affect the response of an 8 ohm nominal speaker unless it's in a bandpass box or something, and even then, it's going to go out of tune by more than that if you run it to max power.

joesch1999 19th April 2011 03:53 AM

A Idea
 
Just buy a fluke lol

Bksabath 19th April 2011 10:44 PM

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

AJT 19th April 2011 11:02 PM

easy way is just copy the url address on your browser pane and paste it on your posts...

Ron E 19th April 2011 11:25 PM

Quote:

Originally Posted by Andrew Eckhardt (Post 2542743)
I'm not sure I understand the reasoning behind the high tech resistance measurements.

This thread came from a discussion about crosschecking T/S measurements. I suggested a (low tech) way to build an add-on to get more precision in DCR measurements (helps with inductors too ;) ) Some of my inexpensive meters bounce ~+/- 0.2 ohms or so with the leads shorted, so what should I subtract?

I wouldn't suggest buying a milliohmmeter unless you have another use for it. ;)

AndrewT 20th April 2011 09:25 AM

Quote:

Originally Posted by Ron E (Post 2544450)
Some of my inexpensive meters bounce ~+/- 0.2 ohms or so with the leads shorted,

Don't use the ohm-meter function unless there is absolutely no other way to get your measurement. It's really a very last resort.

Use the voltmeter function. One of the scales of the voltmeter will be the most accurate of all the functions with the DMM. It might be 1.999Vdc or just possibly 199.9mVdc, check the specification.

Use a current and voltage to measure resistances accurately.


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