Tonight I unsuccessfully played around with this tank circuit to see if I could see oscillations between the C2 and L1, with the goal of measuring the inductance of L1.
I’ve picked up various tips from YouTube videos. C1 is a blocking capacitor I believe this is to hide the DC 50 Ohm resistance of the signal generator from the circuit.
I’ve added the diode because I want an edge rather than a square wave (input voltage). The square wave hits the diode which then cuts off for the lower half of the cycle, this makes observing any oscillations easier.
I probe the circuit at the top side of the inductor with probes set on 10x.
Well, I could see no oscillations at all!
It’s bedtime, but my next step will be to mock this up in LTSpice and see where I’m going wrong.
C1 = 220pF
C2 = 10uf
L1 = 100mH (guess)
I expect oscillations around 150Hz with these values.
I configured my square wave frequency low enough and duty cycle so I have multiple 1/150Hz seconds gaps between pulses. If there were oscillations I should be able to see them.
Anyone see what I’m doing wrong or could clear up my understanding a bit?
The inductor is actually the primary tap of an Autotransformer. But don’t think it makes a difference?
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You should remove the diode and increase C1, maybe to 22nF. However, it is preferable to use a sinewave and sweep the frequency until you see a maximum, it will be more accurate.
The only addition I would give is, when testing cores inductors, especially mains and output transformers, you need to aim for high voltages and low frequencies for high excitation of the core. Otherwize you'll be measuring the core in zero crossing, low permeability region.
This is why I measure with a stabilized high-voltage sinewave, up to 120V 15-50Hz, via a resistor to measure the current and the voltage value across the inductor. Within an excell sheet it is a piece of cake.
This is why I measure with a stabilized high-voltage sinewave, up to 120V 15-50Hz, via a resistor to measure the current and the voltage value across the inductor. Within an excell sheet it is a piece of cake.
make a few wire windings and connect them to a generator and scope to monitor them. hold the windings close to the core to excite the core magnetically, and see at what freq the voltage dips due to the LC tank resonating. this is a contact free test. works best at high resonant frequencies
Thanks for this, I just want to understand a little better.
For this transformer I did something similar because I was curious when the core would saturate (in frequency) and at what voltage. This is an audio Autotransformer (for impedance matching), I just wanted to know if it could cope with guitar frequencies without saturation.
For measuring inductance what is the advantage of operating the core close to saturation? Or is there a problem operating in low magnetisation region? What do you mean by zero-crossing in this context?
Thanks!
Regarding zero-crossing, is it related to this kind of hysteresis plot? I want to measure the magnetising inductance of the transformer winding. I’m trying to understand what problem using small voltage would have in this context?
In order to arrive at a significant induction, you need a sufficient excitation level to overcome the hysteresis. This is the amplitude permeability (~=inductance).
When you make a low-level measurement, you measure the initial permeability (~=inductance).
If the amplitude becomes very large (more precisely the time*voltage), saturation kicks in and also reduces the inductance.
Morality: you need to make the measurement for a voltage and frequency similar to the application you have in mind.
Ferrite materials have a relatively low hysteresis, vaccuum has none, but iron and other metals have a significant hysteresis: iron is the worst, some alloys are much better, for example mumetal, but they are not perfect, and they suffer from a low saturation threshold.
When you make a low-level measurement, you measure the initial permeability (~=inductance).
If the amplitude becomes very large (more precisely the time*voltage), saturation kicks in and also reduces the inductance.
Morality: you need to make the measurement for a voltage and frequency similar to the application you have in mind.
Ferrite materials have a relatively low hysteresis, vaccuum has none, but iron and other metals have a significant hysteresis: iron is the worst, some alloys are much better, for example mumetal, but they are not perfect, and they suffer from a low saturation threshold.
Thanks, I think I understand better. This B-H loop might show it better. To overcome the hysteresis we need to go from the origin of the graph to the loop.
B/H has units of henries per meter (H/m), or magnetic permeability. So it makes sense that once we are on the loop we have a constant inductance (until saturation), but need a minimum amount of distance along the x-axis to get away from the zero-crossing region.
I managed to measure the inductance tonight, changed the circuit to a series tank and it worked. I guess I setup the parallel tank circuit badly.
In the graph in post #7, the dotted line is somewhat misleading. A more correct line would approach the locus formed by the tips, points 1 and 4, of the static level curves as the level is changed. As signal level is decreased the slope of this locus passes from the flattened slope in the saturation region through a constant slope (constant permeability) region until, approaching zero crossing, slope again decreases. Inductance varies linearly with permeability.
All good fortune,
Chris
I should be able to confirm this with the tank circuit. Vary the input voltage until the oscillation frequency becomes constant with frequency. Thanks all, very helpful.
To arrive at a sufficient amplitude for large signal effects to become detectable, you will need more than a standard signal generator. For the amplitude inductance, you might get away with a series resonant configuration, provided the autoformer is low-power, but to detect saturation effects, a lab amplifier will be required
I’ve done the large signal measurements already using a power amplifier. What was news to me was the small signal stuff.
Inductance varies with both level and frequency, and independently, just to keep things interesting. If you're really interested in the gory details, you might even spring for an impedance bridge that accommodates an external signal generator and external DC source. The General Radio 1650B is really beautiful, and not completely unaffordable, but wait for a pretty one. I sprung for an ESI 250DE, newer so not as beautiful, after foolishly waiting a lifetime.
All good fortune,
Chris
All good fortune,
Chris
I'm using a very simple setup:
1. Frequency generator
2. LM1875 amplifier
3. Mains transformer connected backwards to increase voltage
4. A selection of series resistor (100R, 1k, 10k)
5. Two AC voltmeters. One across the resistor and one across the DUT
6. Both voltage values will give you the inductance value using an excell spreadsheet.
7. Entering the turns, core and voltage values in the spreadsheet will give you the flux density where you're taking your inductance value.
A second method is using your inductor as a filter choke after a rectifier. Easy way to also have DC across it. You can calculate the inductance by measuring the ripple voltage. In case you're lazy with calculations, you can use LTSpice or Duncan PSUD2 back and forth untill you get the inductance matching your ripple.
1. Frequency generator
2. LM1875 amplifier
3. Mains transformer connected backwards to increase voltage
4. A selection of series resistor (100R, 1k, 10k)
5. Two AC voltmeters. One across the resistor and one across the DUT
6. Both voltage values will give you the inductance value using an excell spreadsheet.
7. Entering the turns, core and voltage values in the spreadsheet will give you the flux density where you're taking your inductance value.
A second method is using your inductor as a filter choke after a rectifier. Easy way to also have DC across it. You can calculate the inductance by measuring the ripple voltage. In case you're lazy with calculations, you can use LTSpice or Duncan PSUD2 back and forth untill you get the inductance matching your ripple.