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Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I got a couple of small (6mm dia) ferrite toroids from a networking device of sort, the toroids were near the RJ45/RJ11 (the thing is a somekind of a specialized ISDN modem or something) connectors.
How can I measure if these are suitable for my application (class-E SSTC gate drive transformers) using common measuring equipment like a function generator/oscilloscope?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Wind a single layer of turns of thin wire. Measure the inductance. If (like me) you don't have an inductance meter then put 1nF to 10nF in series with the inductor and use a signal generator to find the resonant frequency and determine the inductance. Or make a colpitts or clapp oscillator and measure the frequency.
In any case, divide the inductance by (number of turns - squared) to get Al - the inductance factor.
Go to Amidon or Ferroxcube or Magnetics inc. website and look for a core of the same size with a similar Al.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
One problem with measuring ferrite toroids is that the inductance can be very non-linear. For the power ferrite grades, it's typically very high, but saturates easily. So the answer you get will depend on the drive level you choose for the measurement, and the behaviour of an oscillator can be chaotic.
I'd be inclined to make the gate drive transformer, pump RF through it at the intended voltage and frequency, and see if it overheats. If it does, use more turns to reduce the flux density. If you end up with so many turns that the leakage inductance is unacceptable, but the core is still overheating, you know you need a better core.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
What about identifying iron powder toroids from ferrite toroids? I'd need a small iron powder toroid for an RF-choke. I have two "candidates", this energy saving fluorescent lamp has a small SMPS which has a E-core ferrite transformer and a small toroid which is greenish-blue in colour and has two windings on it (other one has ~10turns and the other around 5), could it be an iron powder toroid? How can I identify it? I dont need any exact specs for the core, just knowing if its iron powder or not would suffice. Then I have this small toroid which has a "rough/coarse" feeling (kinda like thick neoprene or something) and a dark grey color.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If the toroid is lime green on all surfaces except for one face that is blue then it is probably micrometals type-52 iron-powder because their colour codes are *supposedly* patented. It could still be a cheap chinese copy though. (Type 52 GREEN/blue is a slightly more expensive and lower loss version of the common Type 26 YELLOW/white Fe powder toroid commonly found in cheap SMPSUs, light dimmers, PC motherboards etc.)
I don't know of an easy way of telling random iron powder toroids apart from the NiZn and MnZn ferrites just by looking. Eventually you just learn to spot the common iron-powder grades by their colour codes and what they were doing in the application you ripped them out of.
In general Hydrogen reduced iron powder is used for DC carrying chokes in buck derived supplies where AC current ripple is relatively low. Carbonyl iron poweder is used for RF chokes and high power HF baluns where the AC component is big. MnZn has high perm for gate-drive transformers, broadband pulse transformers and power applications up to 3MHz or so. Above this frequency NiZn is the material of choice for HF gate-drive transformers and matching networks to reduce core loss.
As Mr Conner said, if you have a mystery toroid the best thing to do is put some test windings on it for the intended purpose and then test it to see if it's performance is good enough for the application. The basic rule is that if you can't get an acceptable combination of magentising inductance, leakage inductance and core loss with any combination of turns, then you need a different core! (Beware some EMC "suppression toroids" are intentionally made lossy in certain frequency ranges so often aren't the best choice for GDTs!)
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Measuring ferrite toroids
