Electronics > Beginners
How do I measure various parameters of a ferrite toroid
ZeroResistance:
--- Quote from: beowulfenator on September 30, 2018, 09:17:52 pm ---
--- Quote from: ZeroResistance on September 30, 2018, 09:21:19 am ---Can these be measured with a simple DC source / Oscilloscope ?
--- End quote ---
I've come across this simple device:
http://elm-chan.org/works/lchk/report.html
You can wind a few turns and measure saturation current.
--- End quote ---
This is pretty amazing, its a similar setup to what T3sl4co1l mentioned.
However how do we convert saturation current to Saturation density? I
chickenHeadKnob:
--- Quote from: T3sl4co1l on September 30, 2018, 07:22:04 pm ---A definition: inductors are intended to store energy, transformers aren't.
Thus, a flyback transformer would be more descriptively called a coupled inductor.
I don't know how common this definition is, but I think it is helpful. If you adopt it, do make sure to specify what you mean, in any case.
A non-ideal transformer encompasses both, having magnetizing and leakage inductances specified. Theoretically speaking, it's just different values of the same general component.
F.. snipage...
Ferrite of course isn't useless for inductors, you just have to gap it so the average permeability drops into the sweet spot. Typical ferrite inductors have a good Q, but they can be more expensive.
Tim
--- End quote ---
If a clueless noob like myself wanted to experiment with a sine output royer oscillator operating at say 40-50 kHz what core to choose? I would think energy storage is required for the oscillator yet I also want it to function as a step down transformer.
ZeroResistance:
--- Quote from: T3sl4co1l on September 30, 2018, 04:16:33 pm ---1. Set up a pulser circuit. Easiest to do this in time domain.
2. Measure the core dimensions: OD, ID and height.
l_e = (OD + ID) / 2
Ae = (OD - ID) * H / 2
(Actually a little bit less because of geometry, and rounded corners and coating thickness if any. Adjust if you like.)
3a. Estimate flux based on known properties. Ferrites saturate at 0.2 to 0.45T. Flux is:
Phi = Bmax * Ae
Flux is in units of Vs/t, so multiply by number of turns to get the in-circuit value, the flux on the total winding.
3b. Measure it with a pulser.
Make a basic flyback circuit: transistor pulling to GND, inductor from +V to drain, diode from drain to output cap, load resistor from cap to +V. Drive transistor with a func gen, square wave, +10V/0V levels (verify with scope, don't trust the generator's dials!), low duty cycle (5-10%?), variable frequency.
Ensure the supply is well bypassed (a few 1000s uF local to the circuit). Add a series current sense resistor with the inductor or transistor source. Typical value say 0.1 ohm, and an IRFZ46N or the like for the transistor.
We use a square wave because, for a constant voltage, flux increases linearly with time. So we read off time on the oscilloscope as flux.
Watching the current waveform, we expect it to rise linearly with time (current proportional to flux, i.e., constant inductance), until saturation occurs, at which point current shoots up rapidly. This is why we use a variable frequency, and start on the high side and adjust downwards until saturation is observed.
One catch: note that a flyback circuit is unipolar. It doesn't apply reverse flux to the inductor. Ferrites tend to have some remenance, meaning the core remains somewhat magnetized between pulses. This reduces your measurement somewhat. If you were going to use them for half-wave applications anyway (flyback or 1 or 2 switch forward converter), that's actually more accurate, but a full-wave application (inverter, forward converter), you'll get somewhat more than twice the measured flux. This is particularly exaggerated in magamp cores, which you will find have very little flux relative to their size. Use this to identify them, and test them with another method (a half bridge inverter, maybe).
Now we can determine A_L, mu and Bmax.
L = V * dt / dI = A_L * N^2
A_L = mu * Ae / l_e
(Note that mu = mu_r * mu_0 by convention, where mu_0 ~= 1.257 uH/m, or nH/mm if you prefer.)
If you find mu is anomalously low, and Bmax seems to be awfully high (or it's not saturating at all because DC resistance is dominating -- in which case, use more turns and heavier wire, and try again?), it's probably a powdered iron, not ferrite. Powder is used for inductors (energy storage), ferrite for transformers (power transfer).
4. Loss: harder to measure, you'll probably not get a reasonable figure with this setup (unless it's a shitty #26 powdered iron, in which case the turn-on step might be enough to read as parallel resistance, YMMV). Instead, set up a resonant circuit with sine wave excitation, and use this method to find it:
https://www.seventransistorlabs.com/Calc/RLC.html#frq
Losses generally depend somewhat on level, so keep that in mind. You may not be able to drive more than a few volts with a function generator, so a real at-power test has to be done with an inverter.
Also, if it's a powder core, you may be able to identify it by color. If it can be positively identified, the datasheet will give useful info. They don't usually give Q unfortunately, but it can be derived from the material curves (loss at frequency). Here are some examples: https://www.seventransistorlabs.com/Images/Powder_Core_Q.png
Tim
--- End quote ---
This is tremendously profound! Many thanks for your generosity and your valuable time!
So as as summary of your process
First determine L, then AL, then the absolute permeability of the core.
All this is brilliant! :-+
Then using all these, determine the saturating flux density of the core?
Er! But how to calculate the max flux density in the core? :-/O
This is an excellent process that you have suggested and does the job with simple equipment. However can this done more accurately with a sensor like a hall sensor.
BravoV:
--- Quote from: ZeroResistance on October 01, 2018, 06:07:30 am ---
--- Quote from: beowulfenator on September 30, 2018, 09:17:52 pm ---
--- Quote from: ZeroResistance on September 30, 2018, 09:21:19 am ---Can these be measured with a simple DC source / Oscilloscope ?
--- End quote ---
I've come across this simple device:
http://elm-chan.org/works/lchk/report.html
You can wind a few turns and measure saturation current.
--- End quote ---
This is pretty amazing, its a similar setup to what T3sl4co1l mentioned.
However how do we convert saturation current to Saturation density? I
--- End quote ---
In this forum, we have similar inductor saturation tester circuit with energy recycling, designed by respected member here Jay_Diddy_B -> Inductor Saturation Tester
And successfully built by other member here too
T3sl4co1l:
--- Quote from: chickenHeadKnob on October 01, 2018, 06:28:15 am ---If a clueless noob like myself wanted to experiment with a sine output royer oscillator operating at say 40-50 kHz what core to choose? I would think energy storage is required for the oscillator yet I also want it to function as a step down transformer.
--- End quote ---
Yes, you need magnetizing inductance to set the resonant frequency, in that case you would have a "coupled inductor", probably one with tight coupling.
Tim
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version