CJ

Primary Inductance Test
« on: August 01, 2019, 01:11:48 AM »
Primary Inductance measured with hand held meters leaves much to be desired.

There is no adjustment for voltage and frequency unless it is an expensive bench model.

There is an easy way to DIY your own inductance meter .

All you need is a signal generator and a DMM or two.

here is the setup>



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


CJ

Re: Primary Inductance Test
« Reply #1 on: August 01, 2019, 01:31:07 AM »
If you just want a rough idea of what your inductance is, then you can just use one voltage and one frequency,

since low end response depends on primary inductance,  20 or 40 Hz will work well for an inductance test.

there are two charts that i like to construct as related to inductance,

one is Inductance vs Voltage Level.  this can tell you the saturation voltage of the primary at say, 20 Hz.

saturation will be the point where an increase in voltage does not produce a significant increase in inductance.

here is a worksheet you can use for H vs Level>

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Primary Inductance Test
« Reply #2 on: August 01, 2019, 01:41:47 AM »
The second graph that is fun to generate using the same test method is Inductance vs Frequency.

Inductance is not that important past a certain frequency. In fact, at high audio frequencies, you do not need the core at all. So we are mainly interested in the frequency range of about 10 Hz to 1000 Hz.

here is a work sheet that you can use to get some data points that might be used in a graphing program. You can set up a spread sheet to do both the H vs Level graph and the H vs Frequency graph.

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

scott2000

Re: Primary Inductance Test
« Reply #3 on: August 02, 2019, 03:11:05 PM »
Thanks for the nuggets..!

Re: Primary Inductance Test
« Reply #4 on: August 04, 2019, 09:16:06 PM »
'Inductance is not that important past a certain frequency. In fact, at high audio frequencies, you do not need the core at all. So we are mainly interested in the frequency range of about 10 Hz to 1000 Hz.'

Is it that above a certain frequency the distortion in the core itself becomes insignifigant ? obviously the lower we get in frequency terms the harder the core has to work to pass a  signal of the same magnitude .

CJ

Re: Primary Inductance Test
« Reply #5 on: August 07, 2019, 03:03:32 AM »
let us say that you have a 600:600 transformer with 40 Henries of primary inductance at 20 Hz,

look at inductive reactance, XL.

at 20 Hz we have 6.28 x 20 Hz x 40 H = 5024 Ohms of Inductive Reactance - XL

does the core have any permeabillity at 20 K?  probably not much, maybe 100 mH at best,  so

at 20 K Hz we have 6.28 x 20 K Hz x   0.1 Henry  = 12,560  Ohms of XL

so even with the core operating at 0.1/40 = 0.0025 of it's original perm, we still have over twice the Reactance,

at 100 K we could use the Permeabillity of air to get enough Reactance?  let's see,

5,000 Ohms = 6.28 x  100,000 x Henries,      H = 8 milli-Henries, 

pick a 50 EI core and figure perm,

for 50 EI,  L=0.2657 x 10^-8 x K1 Stacking x N^2 x U-ac

so     0.008 Henries  = 0.2657^-8  x  0.9  x 600 Turns ^ 2 x U-ac

U-ac = 0.008 / 0.000861 = 9       

relative perm = U/U-o  with U-o = 1 exactly,   so we only need a core with 9 times the perm of air at 100 K with a 600 ohm primary,    copper has enough impurities to supply enough perm to keep the transformer running by itself with no core,

you can try this, look at in/out with a dual trace scope at 100 K Hz and remove the core,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

rackmonkey

Re: Primary Inductance Test
« Reply #6 on: August 23, 2019, 09:37:08 AM »
Thanks for posting this stuff!
Whether you think you can or you think you can't, you're probably right.

rackmonkey

Re: Primary Inductance Test
« Reply #7 on: August 23, 2019, 10:29:11 AM »
The challenge with this method w/regard to output transformers is the relative wimpiness of modern function generators. My Chinese DDM based unit can deliver 7.2V rms max (20v p-p). In terms of power, it can for example provide 7.2V rms into the 5000 ohm primary of an output transformer (terminated with 8 ohms on the secondary just for power testing purposes) at 1.38mA for an output of 0.0099W, or just about +10dBm.

My jurassic HP sinewave generator can do much better.

If you don't have an older, higher horsepower generator, you need to use a signal amplifier between the generator and the DUT to adequately test many output transformers.

I ended up building one and buying another, after a couple of cheap eBay signal amps failed to deliver much more than my modern function generator.

If there's interest I'll create a new post in this forum about what I built and what I bought.
Whether you think you can or you think you can't, you're probably right.

cyrano

Re: Primary Inductance Test
« Reply #8 on: August 23, 2019, 02:14:19 PM »
You betcha there's interest!

I finally found the time to read through this thread. A few times even. It's a godsend. My understanding of audio transformers is finally getting somewhere.

Thanks for your willingness to share your knowledge!
Why is it people love to believe and hate to know?

 

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