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- Thread starter t991731
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How big a difference are you getting?

Does it matter if you're off by a few percent%

When I measured the secondaries with the primary and other secondary

shorted then I got approx 4,4uH for one and 4,9uH for the other.

When the other secondary was open then I got 15uH and 17uH.

I measured this at 1MHz. This is not 100% true inductance since the

phase never reached 90deg, the highest I got was 80(@1MHz).

I think the need for shorting or opening the secondary B during

stray inductance measurement of sec. A depends on whether the secondary B

will be switched on or off at the time when the rectifying diode on

sec. A is closing but I have not looked into that more deeply at the moment...

I forgot to mention, that the transformer itself is 2x115v|2x25v 225VA toroid

from Noratel.

shorted then I got approx 4,4uH for one and 4,9uH for the other.

When the other secondary was open then I got 15uH and 17uH.

I measured this at 1MHz. This is not 100% true inductance since the

phase never reached 90deg, the highest I got was 80(@1MHz).

I think the need for shorting or opening the secondary B during

stray inductance measurement of sec. A depends on whether the secondary B

will be switched on or off at the time when the rectifying diode on

sec. A is closing but I have not looked into that more deeply at the moment...

I forgot to mention, that the transformer itself is 2x115v|2x25v 225VA toroid

from Noratel.

Last edited:

At higher frequencies you are also measuring the effect of stray capacitances too. There will always be resistance as well, from both copper and iron losses.

If your question is "how do I measure impedance in a three winding transformer", there is a very specific procedure and equations in ANSI/IEEE C57. Most of us are familiar with a two winding test procedure, but the 3 winding is more complex. For those of us who perform short circuit studies on 100MVA three winding xfmrs, this is necessary to understand. I can provide you with the specifics if you are serious, but we are only talking about a little 225VA unit.

Don't count on a highly inductive result either; that's reserved for the big boys. A little xfmr like this will actually be mostly resistive.

If you are only interested in the inductance number, you will still have to go through the impedance method. Then again, I may not understand exactly what you are looking for.

for the rectifying diodes.

Now after I ran few simulations it appears that it really doesn't matter

that much if the stray inductance is 15nH or 5nH, with the inter-winding

capacitance of ca 65pF the ringing is damped in 2-3 periods in both of

these situations using the same (510ohm) resistor value...

Why do we need this?

How do we use this information to help with our design?

It will give you the coupling ratio "K".

K= SQRT(1-LLeakage/LMagnetizing)

More germaine for the folks who design switchers.

I find the following model from Basso to work well and faster than using "K"

.subckt TFR_8K_8 1 2 3 4 Params: Ratio=0.0316

LLeak1 1 20 9e-3

RSeries1 20 30 46

RPri 0 2 1e6

C1 1 3 900e-12

C2 1 2 500e-12

E1 5 4 Value={V(30,2)*Ratio}

G1 30 2 Value={I(VM)*Ratio}

RS_Sec 6 3 1e-6

VM 5 6

.ends

Where Ratio = SQRT Zprimary/Zsecondary

for the rectifying diodes.

Now after I ran few simulations it appears that it really doesn't matter

that much if the stray inductance is 15nH or 5nH, with the inter-winding

capacitance of ca 65pF the ringing is damped in 2-3 periods in both of

these situations using the same (510ohm) resistor value...

You should read Morgan Jones' article on snubbers in Linear Audio Vol 5. Revealing!

jan

With switchers, di/dt is much faster so there's the problem of relatively high energy emissions -- you can see it in the Musical Fidelity V-PSUii which radiates into the 10M ham-band.

I got some valuable insights from Hagerman's article, most especially that oscillatory ringing occurs because there's an underdamped LC resonant circuit which is stimulated by dI/dt pulses. So you need to

But I don't bother to measure the transformer L&C, nor do I bother to calculate the snubber component values; I just connect the transformer to my special purpose test jig (link), and adjust the snubber trimpot for optimum damping. It only takes ~ 90 seconds to dial in a snubber that damps out the ringing completely, at the damping factor that I happen to prefer: Zeta=0.8. Now I've got a 2C-1R snubber that optimally damps this particular transformer, for any and all board layouts & rectifier choices.

http://www.diyaudio.com/forums/tubes-valves/207017-rectifier-injecting-noise-into-heater-supply.html

Start with the smallest C that affects the ring, go with the largest R that snubs it out. A little trial and error and it's done in 20 minutes.

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