Non inductive resistor

[ostie01]

Hi, How can I know if a resistor is non inductive if I want to use it in crossover set up. is there a way to measure it, Thanks

 

[Jennice]

What kind of measurement equipment do you have?
Do you have an oscilloscope or a True RMS voltmeter with sufficient frequency range (see its datasheet)?
Do you have a signal generator of some sort?

Jennice

 

[ostie01]

HI, yes I have everything you have listed

 

[matjans]

check the resistor's datasheet. Most of the time the data given in there is within 10-20% of measured data.

that is, if you know the resistor's make or brand

 

[Rtop84]

I think that inductance measurement in range below 1uH will be very difficult at home. You can measure inductance of wirewound resistors(without non inductive windings), which is high above 5-10uH.
Better way is looking in datasheets.

 

[Jennice]

Agreed. Data sheets are clearly the number one choice, but my guess is the brand/type is unknown.

My suggestion would be to make a voltage divider with the unknown resistor and a well known, non-inductive type in series.
Using a sine generator, a sweep should show a change in the divider ratio between the known and unknown type.
The needed sweep range will depend on the inductance.
If the sweep goes way beyone audio frequencies before it reveals any change, the resistor should be fine for a XO-design.

Jennice

 

[peranders]

Hi, How can I know if a resistor is non inductive if I want to use it in crossover set up. is there a way to measure it, Thanks

Check how wirewounded resistors usually look like, compare datasheets, ask us!

In most cases you can see if a resistor is wirewounded or not.

 

[AndrewT]

Hi,
let's assume my resistor (0r1 or 0r22) is wirewound (I know it is).

I also have a high value metal film resistor (1r0) that may have a spiral track ground into it.

Now wire the metal film and wirewound in series and measure the inductance of the wirewound.
HOW?

Can we assume the inductance of the metal film is zero?

Some can of worms!

 

[Jennice]

AndrewT,

You seem to have too many worms in your scot'ch drinks.

One thing is that you don't agree, but there are many ways to disagree. Maybe your location on an island isolated from the european mainland was God's way of saying you don't behave too well?

I took the liberty to assume some knowledge on Ostie01's behalf, which you have clearly demonstrated to lack yourself, or hide behind equal portions of arrogance and ignorance.

Anyway...

My suggestion would be to make a voltage divider with the unknown resistor and a well known, non-inductive type in series.

As I pointed out, the reference resistor should be well known, i.e. by a datasheet which would also indicate any inductance, if applicable.

I did not say I wanted to MEASURE the inductance of the wirewound resistor.

As the frequency changes, so does the complex impedance of the unknown resistor, if it is inductive.
If Z_unknown is sqrt(R*R + Inductive_part*inductive_part), and the inductive_part is zero at DC, then the changing ratio (over frequency) of the voltage divider (compared with the ratio at DC) should make it possible to calculate the complex part, and hence the inductance of the unknown resistor.

Jennice

 

[AndrewT]

Hi Jennice,
nice of you to come back and illuminate.

Could you take the time to turn your description into a formula?

i.e. I measure the volts drop across the metal film and wirewound at DC and again at an AC frequency. How do I convert that into useful answers?

Do I need to parallel the high value metal film to approach the resistance of the wirewound?

 

[matjans]

this should help you with the basics:

http://zone.ni.com/devzone/conceptd.nsf/webmain/DE66F98D9A37869586256EF600648527
and
http://www.iee.or.jp/honbu/back_number/journal/index_back_number/2005/2005_06a_07.pdf

-- viva google!--

it might be easier to order resistors from rs-components or farnell which provide datasheets though

 

[Jennice]

Here's my math suggestion for you all...

Isolate the part (sumscript number 1 or 2) by moving the rest to the other side of the equation and lifting it to the power of two to remove the sqrt of the remains.

I hope it's readable. It's limited by file size and that I don't have a program to write formulas nicely.


Jennice

 

[nunayafb]

this might be kind of a lame Mechanical engineers answer but resistors are cheap arent they? Throw them away and buy new ones with datasheets. What kind of quantities are we dealing with here?

 

[Jennice]

this might be kind of a lame Mechanical engineers answer but resistors are cheap arent they? ....

Yes, I agree. That's far easier, but beside the point.

The question was, if its possible to find the inductance of an unknown resistor.
In my voltage-divider based suggestion, the optimal result would be to see no change in Vi/Vo ratio over the audio band, as this would render the resistor virtually non-inductive.

 

[nunayafb]

How can I know if a resistor is non inductive

Actually the question was is it inductive.

In my voltage-divider based suggestion, the optimal result would be to see no change in Vi/Vo ratio over the audio band, as this would render the resistor virtually non-inductive.

As an M.E. even I know that you are right about this, and it seems like a logical and practical way to measure the inductance, however, I dont of any application(statement of ignorance) where a resistor of known incuctance is use for its inductance, I think he wants a resistor that is non-inductive and needs to confirm whether the one in question is.
Am I right ostie01?

 

[ostie01]

You're 100% right, In reality, all I want to know is -- is it a non inductive resistor that I can use in a crossover or other application--
Maybe by connecting it to a sine wave generator at a certain frequency and measure the inductance. Thanks

 

[Jennice]

Actually the question was is it inductive.


As an M.E. even I know that you are right about this, and it seems like a logical and practical way to measure the inductance, however, I dont of any application(statement of ignorance) where a resistor of known incuctance is use for its inductance, I think he wants a resistor that is non-inductive and needs to confirm whether the one in question is.
Am I right ostie01?

That just makes it all the much easier.
The main reason for my thorough explanation was to show AndrewT and his "What a can of worms" statement, that it was possible to make the measurement and do the math to actually get a number, rather than saying either "inductive" or "non-inductive".

I can't think of an application either, where a resistors inductive characteristics are used on purpose. However, the frequency at which the characteristics change significantly are important to know for some applications. At low frequencies, a slight inductance may not be a problem for a cross-over, whereas the same resiator may cause catastrophic results in the output stage of an amplifier, causing it to oscillate.
No resistor is completely non-inductive. It's just a matter of frequency before things start to act funny.

 

[lndm]

Maybe you could 'suck it and see'. In practice at audio frequencies, resistor inductance is rarely a 'problem'. For crossovers I think any resistor will work, and inductance probably won't be the larger difference between types. I think larger values of resistance will likely swamp their own inductance in typical uses.

I have used common inductive resistors in amps and crossovers, and find them to be generally well behaved. I believe common sense combined with trial and error will quell oscillations in amps, and a well designed amp won't oscillate due to any normal speaker.

This phenomena does exist, but I am thankful that we live in the world of AF (most of the time).

 

[nunayafb]

Maybe you could 'suck it and see'

At first glance this sounds like the kind of statement that leads to a fist fight, is this some "inside EE" joke/phrase, would you be so kind as to elaborate.

I have used common inductive resistors in amps and crossovers, and find them to be generally well behaved.

At minimum it adds to complication in x-over design, however based on an article from linkwitz it would seem that small values of inductance are fairly negligable. He tested the inductance of some Ratshack megacables and found that they would attenuate a particular tweeter .17dB at 20khz, pretty trivial indeed. I dont recall the actual inductance but could anyone (indm, you sound like youve used few) recall what order of magnitude of inductance is found in inductive resistors.

 

[rpapps]

Don't know what the origin of the expression was but it loosely means
"Try it and see what happens".
Cheers
Rob