Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on February 17, 2022, 11:25:48 pm
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Another question for you, is there a way to test a choke for it's impedance without using an impedance meter?
I have a few chokes with unknown ratings, but no tester to check them with. The coils seem good when I test them for resistance.
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Do you mean inductance? Usually it’s inductance(henries) and resistance(ohms) that are spec’d on the sheets.
I wouldn’t know how. Do they have any model # markings that you could research?
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There are these 20€ testers that also test inductance but if you know the DC resistance and the size you can look up the inductivity in the hammond catalog.
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Amazon ;
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... test a choke for it's impedance without using an impedance meter? ...
Do you mean inductance? ...
"Impedance" is only known when applied frequency is known:
Inductive Reactance = 2 x π x f x L
A 4H choke has a reactance at 120Hz of 2 x π x 120Hz x 4H = 3016Ω.
But that's 301.6Ω at 12Hz, and 30.16kΩ at 1.2kHz.
Impedance is a vector-sum of resistance & reactance, but resistance is small compared to reactance at frequencies where the choke is doing anything useful.
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WRT the meter linked above…
As I understand it, inductance is dependent on current (am I mistaken?).
If this is true, how does the meter take current in to account to give a good measure of inductance?
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inductor "resist" the change in current, they are not "dependent" on current.
look at HBP's formula, current isn't there
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this thread also covers chokes, aka, inductors, pretty well;
questions on choke selection... (el34world.com) (https://el34world.com/Forum/index.php?topic=28486.0)
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As I understand it, inductance is dependent on current (am I mistaken?).
... they are not "dependent" on current.
look at HBP's formula, current isn't there
Brewdude is right though, there has to be some current through an inductor for it to exhibit inductance. That's not a part of the reactance equation. It is, however, a part of other fundamental equations describing inductors (https://en.wikipedia.org/wiki/Inductor#Description).
With respect to the question about meters: the companies making them are using proven measurement techniques in the construction of their products. It's up to us to know the limitations of the meter/tool we use, but at first let's trust it works.
What I've noticed using an Inductance function on a meter is that they output an AC signal voltage, and drive some amount of current into the item being measured. I found this out measuring inductance of speakers, because I heard the AC tone applied to the coil. What's more, when you change ranges, the frequency of the AC changes.
My vague recollection is that a DC bias is really only needed for incremental inductance measurements. But I might be wrong & perhaps the meter supplies a DC bias as well.
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It is, however, a part of other
:laugh:
I didn't want to go that deep.
You are correct, without a induced magnetic field, it's sorta just a resistor.
current also plays a big role when the coiled wire can't handle the current running through it :icon_biggrin:
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This is the test measurement I use for my power supply chokes with unknown DC current ratings:
https://www.dalmura.com.au/static/Choke%20measurement.pdf (https://www.dalmura.com.au/static/Choke%20measurement.pdf)
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I'm not sure a coil "has" to have current to have inductance, but otherwise makes no sense too.
An IRON-core coil will *change* inductance with current. Some H at "no" current. More inductance at some higher current, then falling. Well-shown in trobbins' paper.
So you can't get an EXACT inductance without knowing more than you want to measure.
But we are practical people. "Enough" inductance will do.
And the basic notion of impedance tells you how to get a rough idea.
This method has a number of errors but will be closer than darts.
Your most available test-tone is line frequency, 50Hz or 60Hz as you like it (or as you get it).
Assume an inductance value. Say you'd like a 10H choke. Do math (sorry). 10H at (my) 60 Hz is 3,800 Ohms reactance. If you series the choke with a 3,800 Ohm resistor, you might expect "half". No, there's a phase shift also, like crossing a baseball diamond, so the real result is 0.707, the notorious "-3dB" all over audio.
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A variation on PRR's setup is a common way to measure capacitance as well as inductance (at no DC) - and that involves connecting a pot/rheostat in series with the choke and applying any ACV across that series circuit. Use an AC meter to measure the voltage across the choke, and then across the variable resistance - vary the resistance to make the voltage measurements the same - then measure the resistance value with a DMM and use the impedance equations to calculate the inductance that would cause the same voltage drop.
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Cute meter. Even cheaper in the USA...
https://www.amazon.com/Proster-Multimeter-Capacitance-Resistance-Inductance/dp/B071WNNYQT/ref=sr_1_3?keywords=proster%2Blcr%2Bmeter&qid=1645187044&sprefix=proster%2Blcr%2Caps%2C93&sr=8-3&th=1
i have one of these, handy little meter. my radioshack multimeter never really measured capacitance very well but this one fills the gaps that ol' faithful doesn't do
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Lots of good information here!
It would seem that my easiest option would be to buy the LCR meter.
I'm guessing that there's really no way to tell how much current a choke is rated for without having the specification. I guess it would be a guess based on the size of the unknown choke as compared to a known one, but that might not be very reliable.
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I'm guessing that there's really no way to tell how much current a choke is rated for without having the specification. I guess it would be a guess based on the size of the unknown choke as compared to a known one, but that might not be very reliable.
The inductance drop with DC current is one valid indicator. The part size and external surface temperature rise of the inductor with DC current is another valid indicator. The internal temperature rise of the winding is another valid indicator (that can be calculated from copper tempco and measurement of dc resistance at ambient and at raised temp. All valid and reliable methods, especially when grouped. The actual spec rating made by the manufacturer could have been made based on just one, or two, or all of those measurements - and made by an engineer who had made those measurements. All those measurements take effort to make.
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Or go by DCR and weight. These are tightly determined by inductance and current. Look in the usual choke catalogs like Hammond. Iro and copper have changed little if at all since the rise of electronics.