1 ohm resistors-measurement problem

[wireful3]

The results with the four resistors is a puzzle and at first sight looks as if they are not making good contact in the series connection. How does it perform with other resistors ranging from 10 ohms to 100 ohms say?

I don't know how digital Multimeters are arranged to measure resistance. My understanding of digital meters is that they are essentially voltage measuring devices as opposed to the older analogue meters that are current measuring devices.

[ENGLISH VICTOR]

Please allow me to play devils advocate for a moment. First what range are you setting your DMM to when measuring your 1 Ohm resistors ? I believe the Tenma you mention has as it's lowest range 600 Ohms max count with an accuracy of 1.0 % not really an ideal instrument for measuring low values of R. I presume you are using the lowest range. I don't know the instrument but presume it is not auto-ranging.
I have noted in my not inconsiderable time in electronics that nearly all cases of seemingly "silly measuring results" are due to the wrong technique being employed, or the method of inerpretation of the results being in error. Reason, plain, simple, understandable lack of experience. We have all been there.
I am suggesting you heed the advice given by those more experienced, it is probably the result of many decades of learning.
Finaly and hopefully this may be of some practical help. Don't worry too much about the accuracy of the resistors you employ in your dummy load, 10% would be fine because in a real world you will not be able to define the resistance of your speaker system very accurately, a number of variables come into play such as the type and length of the speaker cables, the type of connectors used, the resistance of any crossover network employed etc. I would simply do as has been suggested and take a resistor of say 10 Ohms 10% tolerance and connect one of your 1 Ohm resistors in series with it, then apply a voltage across the combination (it should approximate 11 Ohms) the voltage need only be quite low say 1.5 volts from a single dry cell and measure the voltage across each resistor. The result should be in the ratio of 10:1 the voltage does not matter but should be 1.36V across the 10 Ohms and 0.136V across the 1 )Ohm resistor for a supply of 1.5 volts. If the results are as stated you have determined your unknown resistors are just fine for the application and will be accurate within about 10%. Make sure all the connections duing the measurments are clean and reliable, preferably soldered.
Good luck.
Victor.

[dave cox]

magnetic, make sure you have really good connections with your test leads, ideally use allegator clips and twist them (and all the connections) around a bit first to ensure its in contact with clean, oxide free, metal. This should stabilise your readings. As already described try measuring 10 ohms then 10 ohms in series with your "1 ohm" power resistor - hopefully you will start to get readings you can put some trust in.

Test gear is something of a pain, unless you have unlimited funds. I have a DMM that *can* reliably measure 1ohm but bought new it would cost more than 20 times as much as i bought it second hand for. On the other hand, a good engineer can do fantastic things with only the most redimentory equipment !!

dave

[Sideband]

Originally Posted by KeithsTV

Are you sure the 1ohm resistors are 1 ohm and not 0.1ohms? What are the markings on them?

Now that's what I was thinking!


Rich.

[Graeme Murphy]

As I work for a famous UK resistor manufacturer I feel that I must add some input here.
Resistors are measured by all manufacturers by having the measurement taken 6mm from the end of the body.
This is vital when measuring low value resistors as the lead length can be higher in value than the resistance element itself.
4 Terminal measurement is the method used to ascertain that the components are within the manufactured tolerances.
Use the voltage method and take the measurements at 6mm's from ends of the body.
I personally would not use a dvm for measuring resistance lower than 30 ohms.

Graeme.

[glowinganode]

Originally Posted by wireful3

I don't know how digital Multimeters are arranged to measure resistance. My understanding of digital meters is that they are essentially voltage measuring devices as opposed to the older analogue meters that are current measuring devices.

I used to repair and calibrate electrofusion welding boxes. Built into these was a resistance measuring feature (of the coupling to be welded) to allow automatic recognition of the type of coupling.
This was acheived with a constant current source, the resistance being directly proportional to the p.d. across the fitting.
I assume DMM's work in the same manner.
I would highly recommend the fluke 110, reasonable priced and still accurate after 5 years field use.
Rob.

[Diabolical Artificer]

Thanks for all your replies.I see this thread is still getting interest,Ive learnt alot.
Ive finally got to the bottom of the problem;they were the wrong R,s in the right package ie 0R1,s in labelled as 1R on the plastic bag,also my two DMM,s read different values,and the older one doesn,t measure accurately under 1 ohm.
Ive let the company know,theyv,e frozen the stock.(had same thing with some 0.1 caps)
I should have looked more closely at the printed value on the R,s;and feel a bit of a plonker as a result.Still lesson learnt,M

[MichaelR]

Well done for pursuing it to find the answer. At least your test kit is ok and as you say you learned a lot.

Mike