Output Transformer as PSU Choke - Measuring Primary Inductance

[Matt C]

I'm wondering if anyone has any tips for measuring the primary inductance (not the leakage inductance or reflected impedance) of an output transformer (from a single-ended power stage). 

I have a few sitting around unused (from Akai M8 units I'm working on) and I'm wondering whether it would be possible to just use the primary side (open secondary) as a power supply choke.  Am I wrong to think that would work?

I tried measuring it using a wheatstone bridge with an AC signal applied, but I'm not sure if my methodology is appropriate, and I'm not sure what range of values to expect (I think I measured something like 35H).

any help would be great, thanks.

 

[abbey road d enfer]

My preferred method is using an oscillator and voltmeter. Create a voltage divider where the series element is the inductor and the shunt is a resistor. This gives you the possibility to submit the inductor to a large-ish voltage, which can be made close to normal operating parameters.

However, in your case, the main factor is the DC current. The inductance value with DC current may be 1/5th of the value without. You could hook a battery in series with the oscillator, but I would worry about the risk of frying the oscillator. Anyway, I think you need a ballpark figure before actually experimenting. But you don't really need to measure anything.
I believe the xfmrs you're referring to are those in the anodes of the 6BQ5's.
The 6BQ5's there operate at about 50 mA. They should present an impedance of about 5-7k, which computes to about 15-30H. Most of the times, a SE xfmr primary makes a better choke than a choke - better than needed - the only penalty being that the DC drop may be higher.

 

[Matt C]

thanks, sounds easy enough.  Can't I just crank up the DC offset on the oscillator to more closely approximate the conditions it will see as a PSU choke (instead of trying a battery)?  I mean that DC voltage will be pretty low but at least it's something.

 

[CJ]

just stick the xfmr in the pwr supply and measure ripple at both ends to see how good it works in circuit.

here is the test setup, you could also just insert a current meter in series with the generator instead of measuring the voltage drop across the resistor,

 

[CJ]

here is the math for finding a good resistor value, skip step 1 and just crank the generator all the way up, unless your generator puts out 600 volts, 

use 120 hz for f,

step #7 is Inductance=XL/2 pi f,

 

[Matt C]

well i don't know, I ran some tests based on what you guys posted and got some huge result like 120H of primary inductance.  I'd still like to be able to measure it accurately but at this point I think I've established that it's "big enough", at least for use in a choke-input PSU for a mic preamp.

 

[fpliuzzi]

Hello,
I found the following URL about building a filter choke analyzer a few years ago. It allows the testing of power chokes with DC applied to them. You may find it interesting...

www.angelfire.com/electronic/funwithtubes/Filter_Choke_Analyzer.html

 

[abbey road d enfer]

Hello,
I found the following URL about building a filter choke analyzer a few years ago. It allows the testing of power chokes with DC applied to them. You may find it interesting...

www.angelfire.com/electronic/funwithtubes/Filter_Choke_Analyzer.html

This article is interesting, because it proposes an ergonomic solution for evaluating chokes under real-life conditions, without having to do a complete build.
However, I think there are several flaws in the design:
[list type=decimal]
[*]Measurement is done at 60Hz. In most cases, rectifier is full-wave, resulting in 120Hz fundamental.
[*]DC current source must be floating relative to the test jig. Can be a problem sometimes.
[*]Proposed test jig does not provide adequate current measurement.

[/list] These points are mentioned in the text but require particular attention IMO.

 

[abbey road d enfer]

well i don't know, I ran some tests based on what you guys posted and got some huge result like 120H of primary inductance.  I'd still like to be able to measure it accurately but at this point I think I've established that it's "big enough", at least for use in a choke-input PSU for a mic preamp.

120H is a very credible value. Makes the impedance about 15k at 20Hz, an adequate value for a low power SE stage.