digital multimeter, checking ohms?
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=98649
Printed Date: May 02, 2024 at 5:57 AM
Topic: digital multimeter, checking ohms?
Posted By: steveskal
Subject: digital multimeter, checking ohms?
Date Posted: November 05, 2007 at 10:57 AM
Question for you guru's, I have a pretty cheap Craftsman Digital Multimeter, I forget what I paid for it but I know it was the cheapest one they sell, anyway I've always used it for just checking 12 volts DC in automobiles and thats about it. Well I am selling a speaker which is 3 Ohms and the guy who wants to buy it wanted me to check the voice coil to see if it reads 3 ohms, so I hook up the meter and its reading 3.5 ohms??? I tired a different 3 ohm voice coil speaker I had laying around and it read the same, 3.5 ohms exactly, do you think the meter just isn't calibrated worth a damn because its a cheapy?? Any ideas?? THANKS!
Replies:
Posted By: DYohn
Date Posted: November 05, 2007 at 11:02 AM
The impedance rating of a speaker and its DC resistance (what you can read with a DVM) are always different. You need to look up the specs for the speaker you are testing and see what its DCR rating is, and your test should be within a half ohm of that. ------------- Support the12volt.com
Posted By: steveskal
Date Posted: November 05, 2007 at 11:28 AM
Well I went on the manufacturers web site and the first speaker I checked which is a JL Audio 12W7 it says the DC Resistance is 2.47 Ohms and the 2nd speaker I checked which was a 10W7 it says has 2.75 Ohms of DC Resistance yet both read exactly 3.5 Ohms with this meter? I've attached a picture of the first test. 
Posted By: haemphyst
Date Posted: November 05, 2007 at 11:46 AM
Don't worry about it. As DYohn says, the DCR will ALWAYS be different than the impedance. 3.5 ohms is close enough that you can tell the guy that they are 3 ohm coils. Did you subtract the DC resistance of the leads? Those are being figured into that circuit as well.
All you can REALLY tell from your test, is that the coils are good. You COULD use the current method, by running an absolutely known voltage, and measure the current through the coil, and using Ohm's law, you can figure it out that way...
Too much work. They're 3 ohms... Sell 'em to the guy.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Posted By: steveskal
Date Posted: November 05, 2007 at 12:08 PM
LOL, thanks for the reply, yeah any other tests would be too much work and probably over my head, ohms law, all that stuff unfortunately is greek to me. I probably should read up on this stuff some more but I am still stuck in the 80's with car stereo stuff, you have positive, negative and a gain control on your amps and thats about it! :-)
Posted By: DYohn
Date Posted: November 05, 2007 at 12:17 PM
The 80's huh? Ohm's Law was first published in 1827...  But as haemphyst says, you are fine don't worry about it. ------------- Support the12volt.com
Posted By: sarcomax
Date Posted: November 06, 2007 at 1:26 PM
He didn't say which 80's...It could be the 1780's...
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Posted By: tedmond
Date Posted: November 06, 2007 at 2:16 PM
3.5 is close enough to 3 ohms. Like haemphyst mentioned, take the two test points and put them together while u trying to read 20 ohms - 200 ohms. If they are good test leads they will show 1 - 1.5 anything higher, try getting better test leads. ALso the value of yout test lead, stubtract that from the sub resistance.
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