Inductance of wirewound resistors?

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Lee_M

Well-known member
Joined
Nov 23, 2013
Messages
309
Location
Dorset, UK.
Hey guys,

I've picked up some nice enamelled wirewound resistors (the green "W21" type) for use in DIY builds.
I was aware they would have measureable inductance due to their construction, So I tested some out on my cheapo LC meter....Which showed some resistor values having over 20H of inductance!?  :eek:

The meter seems to work ok for most inductors and transformers I've used it on, I've got various BBC transformers which measured correctly for primary L (according to BBC specs) and all of the inductors I've tested match the labelled values, so it seems to work properly for the most part.

Is this a normal thing for vintage wirewound resistors or is my LC meter acting crazy?
It seems excessively high to me...But what do I know?

More importantly, What kind of problems could this cause in circuits and what purposes should I avoid using these wirewounds for?
 
I would suspect your LC meter. Wirewound resistors have very low values of inductance. Many are wound such that they have little or no inductance. Try your LC meter on a regular resistor of the same value.

Cheers

Ian
 
You were correct, I tested a carbon comp of the same value (390R) and got 21H.

Good to know it's only the meter, I had a feeling that was the case (what can you expect for £20?) but figured it couldn't hurt to ask.

Thanks for the help.
 
You should use a series RL model to measure this, since there is no core parallel losses aren't present leaving you with the winding resistance in series. In that case your meter should indicate a resistive value about the resistor intended value, depending on the freq, at high freq should be higher because skin effect. Together with that should show the inductance which should be much smaller than  your 20H.

If you don't have a proper meter to do so you could use a scope and a signal generator, add a resistor in series, 1k maybe. measure the voltage across the resistor (1000x the current on the DUT) and the voltage across the DUT (your WW resistor) and you have your impedance module and with the phase shifting it shows you get how much of it is inductance... The data you need to do all the math is freq, DUT current (voltage across 1k resistor), DUT voltage, phase relation between voltage and current, current should be late compared to voltage since you are expecting inductive impedance.

JS
 
Wow, a lot of that went straight over my head!

I have minimal test equipment (no scope or signal generator) and what I do have is cheap...Basically just a maplin multimeter (with no inductance setting) and a cheap LC meter I got from ebay.

In what kind of situations would the slight inductance of wirewounds be an issue?
 
Lee_M said:
Wow, a lot of that went straight over my head!

I have minimal test equipment (no scope or signal generator) and what I do have is cheap...Basically just a maplin multimeter (with no inductance setting) and a cheap LC meter I got from ebay.

In what kind of situations would the slight inductance of wirewounds be an issue?

You could try out with the sound card of the computer but it probably won't show much at audio frequency...

JS
 
Also note that not all wire wound resistors have significant inductance. Some WW resistors are built of two separate coils arranged so that the flux from each half cancels, or (to over simplify!) they are wound in one direction for half of the resistor, then the other direction for the other half so that the magnetic fields generated will cancel out, resulting in a low inductance. Wirewound resistors can be absolutely awesome if built well, but you have to dig into the details.

As for measuring the inductance, a simple handheld meter is going to give you grief more than anything else! Since the inductance is going to be pretty small compared to the resistance, you need to measure it using a somewhat high frequency test signal, basically to make the inductance 'stand out' compared to the large resistance. The simplest answer is that "it might not matter"… maybe try to find other ways to see if this part is messing up your circuit? Something as simple as a frequency response test done at a few frequencies?
 
It was actually seeing non-inductive wirewounds advertised online that made me think to myself "if those parts are advertised as non-inductive, There must be inductive wirewounds too!".

Sure enough, A bit of google research confirmed that some WW resistors do have marginal inductance...Which is what lead me to testing my resistors and then starting this thread - I figured as my Welwyn W21 resistors are pretty old, There is a chance they aren't wound in a non-inductive pattern.

I'll try to rig up some way of testing frequency response. It will likely be software (so will be limited in bandwidth by the sample rate on my computer) but its gotta be better than nothing.
 
All WW resistor try to minimize the inductance at some level, the non inductive ones are much better for HF but also much expensive as using the construction McGuire said they take 4 times the wire (and are 4 times the power) or need a wire 4 times thiner which is harder to wind.

It always depends on the frequency, the actual size of the component will introduce some parasitic inductance even in SMD resistors or capacitors, will eventually start to act as inductors, usually we don't think about that in audio but working in the GHz region you can't ignore those effects, even for small SMD parts.

JS
 
joaquins said:
usually we don't think about that in audio but working in the GHz region you can't ignore those effects,
I intend on staying well within the Khz area!  ;)

Seriously though, Thank you all for the info. Some of the things mentioned in this thread are beyond my current skill level and understanding, But it's all food for thought and is leading me onto further learning.
 
Lee_M said:
joaquins said:
usually we don't think about that in audio but working in the GHz region you can't ignore those effects,
I intend on staying well within the Khz area!  ;)

Seriously though, Thank you all for the info. Some of the things mentioned in this thread are beyond my current skill level and understanding, But it's all food for thought and is leading me onto further learning.

Just to be clear, GHz concern was for small surface mount component, bigger components means problem at lower frequencies, at audio we usually wouldn't concern about that. In audio you could see skin effect on thick conductors in some inductors but usually nothing to really worry in DIY every day application. I don't think parasitic inductance is a problem for you at all, at least at this moment of your life, stick to concentrated parameters and "ideal" components, just avoid crappy ceramic caps (there are some really good ones) and fake semiconductors and your path should be smooth.

JS
 
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