measuring impedance

I have three of these transformer at 100VA, my third one measures

Ken

?

And how are we determining that we are not getting any resonance?

If I were you, for a simple basic line test. I'd have nothing connected the secondary and place a non inductive series R device in series with the primary to connected to the 120 Volt source.

Most transformers of that size should have enough current in the primary to allow for lets say a 50 watt incandescent lamp to be used as a resonance test.

Also beware that some transformers have a shunt in the core design to force some saturation, this is to reduce the effects of resonance with capacitor loads. Commonly found in microwave ovens and such. Maybe you're dealing with this?

Jamie

Get an HP4194. Little LCR meters aren't meant for serious work.

that night be a little expensive for me right now. I am using a variable AC input. with a lamp in the primary, its not lighting up but my input is 10vAC and the output is 30Vac, so I guess I am getting some resonance now. I will play around with the parts. thanks for the help, anymore feedback will be appreciated. and happy new year to all.

ken

"captoro"

I have three of these transformer at 100VA, my third one measures

** What is the resistance of each winding ?

... Phil

Is that the inductance of the primary or secondary?

Disregard. I just found earlier where you said it was 120/120.

"John S"

Quote from OP:

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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..

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... Phil

Anyone attempting to resonate the primary of a microwave oven tranny is a clear candidate for a Darwin award.

"Bruce Varley"

He is failing and will continue to fail.

.... Phil

I measured a 50VA transformer I have here with an LCR meter at 1kHz and it was 442mH. I then measured the unloaded line current to be 72mA at

My conclusion is that I cannot rely on my LCR meter to give the correct inductance for a 60Hz transformer. My LCR meter still read only 538mH at

If he really wants to know the primary inductance, he should measure the current when the transformer is plugged into the line and make the calculation. That won't necessarily resolve is resonating problem, as you say.

John S

the

What frequency is the measurement being made at?

?-)

If it were a toroidal tranny, L would be about 50H.

.... Phil

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yes phil, my reading in the last post were erronated (.. not good) I understand after the load , the inductance changes. I then decided to use commercial inductor like a 330uH from delavan and a 3.3uF cap. still not getting any resonance at any frequency . I was using for this test a frequency generator.

--- Well, something's definitely wrong then.

From: 1 f = -------------- 2pi sqrt(LC)

If you're using a signal generator, you should be able to see resonance a couple of different ways: (View using a fixed-pitch font)

First, since the reactances of the coil and the cap are low:

Xl = 2pi f L = 6.28 * 4825Hz * 330µH = 10 ohms

If the signal generator approximates a voltage source with a very low output impedance, this will work:

. +------+ . | | . | [330µH] . | | .[GEN] +--------+ . | | | . | [3.3µF] [DETECTOR] . | | | . +------+--------+

That is, the voltage peak at resonance should be discernible at Vout, and the higher the impedance of the detector (scope, DVM, whatever) the higher the peak.

However, if the signal generator has an appreciable output impedance this will work better:

. +------+--------+ . | | | . [Rg] [330µH] | . | | [DETECTOR] .[GEN] [3.3µF] | . | | | . +------+--------+

What you'll see at resonance won't be a peak, it'll be a dip, and the sharpness of the dip will be determined largely by the generator's output resistance, the DC resistance of the coil, and the ESR of the capacitor.

Here's the LTspice circuit list so you can play around with the circuit:

Version 4 SHEET 1 880 680 WIRE 192 48 48 48 WIRE 192 80 192 48 WIRE 48 96 48 48 WIRE 48 208 48 176 WIRE 192 224 192 160 WIRE 48 336 48 288 WIRE 192 336 192 288 WIRE 192 336 48 336 WIRE 48 384 48 336 FLAG 48 384 0 SYMBOL cap 176 224 R0 SYMATTR InstName C1 SYMATTR Value 3.3µ SYMBOL ind 176 64 R0 SYMATTR InstName L1 SYMATTR Value 330µ SYMBOL voltage 48 192 R0 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value "" SYMATTR Value2 AC 1 SYMBOL res 32 80 R0 SYMATTR InstName R1 SYMATTR Value .1 TEXT 56 360 Left 2 !.ac oct 1024 1000 10000

If you don't have LTspice you can get it, free, at:

-- JF

On Mon, 02 Jan 2012 05:56:24 -0600, John Fields wrote: . . .

Here is the circuit:

I tried 10 different capacitor values, different inductors including some transformers. I am not getting any resonance. I put the oscilloscope probe across the inductor and across both inductor and cap. There is something else that is not told. I do the exact circuit that is shown here in this java simulation:

if this continues I am going to start that resonance is a myth, and everyone is conspiring against me LOL Ken

Here is the circuit:

I tried 10 different capacitor values, different inductors including some transformers. I am not getting any resonance. I put the oscilloscope probe across the inductor and across both inductor and cap. There is something else that is not told. I do the exact circuit that is shown here in this java simulation:

if this continues I am going to start that resonance is a myth, and everyone is conspiring against me LOL Ken

uit.jpg

HI John,

I installed LT spice, but just tell me where to paste the Circuit list. I browsed the menus . I see Spice Netlist in the view menu, but I cannot edit it.