Measuring DC resistance of small value inductor (~1uH-4mH)

[krazyfencer]

Hello,

I have a number of inductors from 1uH to 4mh that I wanted to use in making colpitts oscillators of various frequencies. I don't know the dc resistance of the inductors (bought off ebay, no specs), and thought I'd just hook up my cheap digital multi-meter to it to figure it out. I was wondering:
1) is that even reasonable
2) when the current from the dmm has established the inductors magnetic field, and you disconnect the dmm from the charged inductor, does the magnetic field collapse, and is it going to mess up the dmm?

Thanks!

 

[barry]

1) I think it's reasonable, but it kind of depends on your dmm, don't you think? What's the resolution and accuracy of your meter?
2) I don't think a 4mH inductance will hurt your meter. Remember v=L*di/dt? Your current is going to be pretty small, your inductance is pretty small, so, you're probably ok.

 

[krazyfencer]

Thanks! The dmm says between 4.5 and 5 ohm for all of them, which I've heard is reasonable (< 10 ohm).

 

[Kripton2035]

humm : the dc resistance of a small inductor should be around 10mΩ
as barry said, it depends on the precision of your multimeter, often does not go below 0.1Ω, rarely 0.01Ω
can you please touch the probes of your multimeter and verify you don't have 5Ω ? :wink:

even my 50000pt fluke displays 0.2Ω when I touch the probes (the probes are NOT fluke ...)

 

[krazyfencer]

well crap, you're right My multimeter is giving me about 5 ohm across the probes. I figured it wasn't great, but I didn't think it would be that large. I believe that I need the dc resistance of the inductors to calculate various parts of an oscillator I want to build (impedance of the LC circuit). If it is too small to show up on my meter, should I just assume some small value, or is there some other moderately painless way to calculate this value more accurately (without buying a Fluke)?

Thanks!

 

[Kripton2035]

here you can build yourself small LC(R) meters or buy a ready made one, in an amateur budget...
I "love" the DerEE lcr meter for it's price/performance ratio : with $80 you get the milli ohm !

**broken link removed**

 

[krazyfencer]

Thanks, that site is pretty cool. If I start making something else, I'll never finish this thing though... Can't blow $80 on it right now either I just got a digital O-scope (Owon PDS5022T, SPEC on page 72-73). Can I make some bridge or something and use the O-scope, or is it not accurate enough?

 

[Audioguru]

I had a Fluke meter that calibrated away the resistance of the probes.
I shorted the probes together and it showed 0.4 ohms. Then I pushed the CAL button and it showed 0.00 ohms.
It also measured capacitance and frequency but not inductance.

 

[Kripton2035]

you can buy a small lc meter ref LC100 (as a kit) or LC200 (in a box) for $25 to $40 on ebay
they work nice and you will measure your inductance precisely.
your scope is certainly less precise then your multimeter
scopes have 8 bits of screen resolution = 256 values, your multimeter has 2000pts at least.

- - - Updated - - -

I had a Fluke meter that calibrated away the resistance of the probes.
I shorted the probes together and it showed 0.4 ohms. Then I pushed the CAL button and it showed 0.00 ohms.
It also measured capacitance and frequency but not inductance.

yes my fluke does that too : you must have a "REL" button for relative measurments
entry level multimeters dont have this.

 

[krazyfencer]

But does an LC meter help me with finding the small LC resistance of the inductor? I'm fairly sure about the inductance (the parts are labelled for inductance), it's the dc resistance I need...

 

[Tunelabguy]

No matter how good a meter you use, there is always a problem making accurate low resistance measurements using only two probes. The contact resistance of the probes changes if the point of contact wiggles the least little bit. You can't zero out something that isn't even constant. A much better way to measure low value resistances is with a "four point resistance measurement". Look it up with Google.

 

[Kripton2035]

But does an LC meter help me with finding the small LC resistance of the inductor? I'm fairly sure about the inductance (the parts are labelled for inductance), it's the dc resistance I need...

oh yes I did not notice that ... then you're stuck with an lcr meter and the $80 of the DerEE5000 !
youalso have milli ohm meters on my web site, you could use a milliohm to dmm adapter
like this one : **broken link removed**
or this one : **broken link removed**