measuring impedance

Well, the easy answer is, you didn't pay enough money. Often the real part is 1/100 or even 1/10,000 of the imaginary part, making it difficult to measure. (If we're talking about components, inductors, capacitors) The real part can also vary (a little) with the frequency. Many LCR meters measure at a single frequency. What are you trying to do? Mikek

If you can stand to make measurements at a minimum frequency of 1.8 MHz, the MFJ-259B antenna analyzer

Hi,

I am trying to make an RCL resonant circuit with a small transformer as the inductor. From I can see, I need Z and requires real and imaginary part for proper calculation. But if it is only 1/10000 th of the inductance, then I suppose it can be ignored? K

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My previous response is not showing up. Hope this wont be a repost. If the R is only 1/1000 of the impedance, does that mean I can discard it, and make my calculations without the real part?? I all text book I seen , the parts ( L and C) are always complex numbers, but when you buy a part its never a complex number...

Lets see, mine has dissipation factor switch in it, that can help in giving you the ESR of the unit. of course, you may need to look at the manual to see how their reading is calculated.

The R option uses 1Khz to measure resistance. with that, you can tell the difference between the true R and reactive.

at least this is how a cheap B%K unit I have does it.

Have you ever thought of simply doing the math to get the other component to match your resonant circuit?

Jamie

"captoro"

The other variable with a resonant circuit, besides frequency, is Q - a measure of how sharp the resonance is.

Look up "Q factor" on Wiki.

What it boils down to is that if you want a sharp resonance, the Q factors of both your cap and inductor need to be high at the frequency concerned and for some range either side.

... Phil

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Phil, when you mention: 'Q factors of both your cap and inductor need to be high at the frequency concerned' does that mean each component have a Q factor. Taking a different AC type capacitor (keeping the same Farad ) will change its Q ???

"captoro"

Phil,

Hello Lerameur - long time no see.....

when you mention: 'Q factors of both your cap and inductor need to be high at the frequency concerned' does that mean each component have a Q factor.

Taking a different AC type capacitor (keeping the same Farad ) will change its Q ???

Eg, a 10uF electro and a 10uF film cap will have very different Qs at 1kHz.

Cos the resistance ( or ESR) of the electro is about 0.5ohm and the film cap about 0.01ohm giving Qs of 32 and 1600.

A 1000uF electro has a Q of about 2 at 1kHz.

... Phil

The Q of the transformer will be lower than a capacitor, which can be 10,000. The R of your transformer will have minimal affect on the resonant frequency of your circuit. Mikek

Inductor--- Q = Inductive Reactance / Resistance Capacitor--- Q = Capacitive Reactance / Resistance

Yes, Capacitors made from different material will have different Q's. Capacitor Q does become important in some circuits. Your capacitor Q will be much higher than your inductor. The way you are asking the questions makes me think having high Q's for your capacitor and inductor will not be important. Mikek

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Ok great.. I need a 20uF capacitor, with the lowest resistance. what would be the best type capacitor to use in this case: I have bought this capacitor:

Ken

"captoro"

Hi Phil,

I am using a 120v/120v 100VA transformer, primary in 120v, the secondary I want to make it resonate at 60hz, the inductance reading is 0.352H, so from calculation I am suppose to use a 20uF cap. So I got the motor run caps, and a few t 1uF to make adjustments, I am not getting any resonance..

ken

"captoro"

I am using a 120v/120v 100VA transformer, primary in 120v, the secondary I want to make it resonate at 60hz,

the inductance readingis 0.352H,

You sure you are making the measurement with no load on the tranny ?

The real value should be 10 to 100 times higher.

BTW iron core transformer primaries are VERY poor inductors.

.... Phil

I measured it with my genrad 1657 and another cheap brand reader and they gave me the same reading. what type of inductor should I be using then? I will verify all the values tomorrow if you want. I heard that ferro resonant transformer are the best , but I cant find anywhere....

Ken

Hi Phil,

I am using a 120v/120v 100VA transformer, primary in 120v, the secondary I want to make it resonate at 60hz, the inductance reading is 0.352H, so from calculation I am suppose to use a 20uF cap. So I got the motor run caps, and a few t 1uF to make adjustments, I am not getting any resonance..

ken

Assuming you can get it to resonate, expect some irregular behaviour if your peak current goes much above rated. The core will start to saturate, and the circuit may flip out of resonance unpredictably.

Correction: In fact, if the non-resonant winding is not loaded, saturation is likely to start to show not far above magnetising current, which is going to be somewhere below 10% of FLC.

Some of them do...give L and R at the same time, or C and G at the same time. Probably Wane Kerr bridges were tops i this, with the "newer" HP bridges next in line.

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Ok I took the measurements again, I got 0.254H , so not too far off. I got a number mixed... but still, not even a volt more, actually I am getting less then my input frequency (14vAC). I am getting may be 4v more when I add a

Ken