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ohmmeter circuit to test speaker impedance

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drewbgil

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Hi all, I'm looking for the circuit to a digital panel meter that measures ohms. I'm actually trying to link 4 of these together to test a multi-componant loudspeaker. I've researched using the IC7107 chip, but it seems a little large and excessive for the project. The meters would need to read between 0-200ohms at .1 resolution. I'd like all 4 meters to run off of one 9V bat. I'd also like to use 7-segment LED for the display.

Any help would be appreciated.

Andrew
 
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Drew
I cant help thinking that what you want to do cannot be done.
You say you want to test 'loudspeakers'.
Firstly, loudspeaker 'characteristics ' are highly frequency dependent and a DPM is a DC instrument; so the only parameter you can measure is the voice coil resistance.
The other parameter of interest is the frequency response, and this is difficult to measure.
Loudpeakers are usually tested by 'listening tests'.
In the case of bass frequencies, the combination of box AND speaker governs the frequency response and sound pressure level with respect to audio power supplied. The speaker, box, and driving circuit impedance governs the pulse response of the system.
For more information, can I refer you to several papers on the subject.
The most modern papers of any significance are likely to be those in the name of Neville Thiele, Dr. J.E. Benson, Richard Small, but these papers are now some 40 years old.
There are probably a host of others too.
Neville Thiele particularly, was the first to analyse the loudspeaker, box, and driving impedance, to describe the bass frequency response of direct radiator loudspeaker systems.
I hope I've understood your question because anyone who wants to use a DPM as an ohmmeter to measure 'loudspeakers, cant be doing too much in the way of loudspeaker assessment.
 
I'm actually trying to link 4 of these together to test a multi-componant loudspeaker.
If you mean a multi-way speaker with typical crossovers you stand no chance of measuring the speakers separately unless you take it apart.
Anyway I don´t see what could this thing be good for?
 
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look in my blog under "AC ohmmeter" and "AC ohmmeter revisited" for an AC current source that can give you a close approximation of the impedance of a speaker system. you will need an AC current source and an oscilloscope (or a good true-rms AC voltmeter). there's also speaker design software that can do the sweep and calculate the impedance curve in a matter of seconds using a PC sound card. or you can use a function generator with a 50, 300, or 600 ohm output impedance and an oscilloscope. i use a 7-step per decade set of test frequencies (i.e. 20, 37.5, 50, 75, 100, 150, 175 hz, and just multiplying these by 10 and 100 for the next two decades, and finishing with 20khz)
 
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I appreciate the responses but think I'm looking for something a little more simple.

so the only parameter you can measure is the voice coil resistance.

You are correct. That is all I am trying to accomplish. I deal with having to check hundreds of loudspeaker components in a very short time. Each loudspeaker contains up to 9 speaker drivers. None of the loudspeakers contain any crossover circuitry. Up till now I just use a multimeter. I'm really only concerned with whether or not the voice coil is open or not. There are 4 sets of connections going to the loudspeaker. The trick is that most of the connections to the loudspeaker involve multiple speakers that are connected in parallel or series or both. So a sweep test might produce sound but all the drivers may not be there. A quick resistance test tells me if all the drivers are making some sort of noise.
So all I'm really looking for is a device that contains 4 DC ohmmeters with LED display run from one 9V and wired to a connector that my loudspeaker uses.

Thanks guys,
Andrew
 
Just buy 4 cheapo DMMs from your local discount store and wire them to a common battery. Set them all on an Ohms range.
 
Just buy 4 cheapo DMMs from your local discount store and wire them to a common battery. Set them all on an Ohms range.
A good solution, but beware, the power supply (battery) in these DMMs is not isolated from the test circuit.
So, in this case, if the loudspeakers have a common connection, this collection of DMMs with a common supply may not give a correct indication.

JimB
 
Hi all, I'm looking for the circuit to a digital panel meter that measures ohms. I'm actually trying to link 4 of these together to test a multi-componant loudspeaker. I've researched using the IC7107 chip, but it seems a little large and excessive for the project. The meters would need to read between 0-200ohms at .1 resolution. I'd like all 4 meters to run off of one 9V bat. I'd also like to use 7-segment LED for the display.
If you use an LED display your battery life will be very short. Better to have an LCD display.

I go with just buying some inexpensive digital LCD multimeters. Use one battery for each meter. That will allow you to go four times as long before the batteries need replacement. (What type of battery life were you expecting?)
 
the power supply (battery) in these DMMs is not isolated from the test circuit.
Good point. Presumably the one battery is intended for cheapness?
 
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