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simulating center tapped transformer

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Member level 1
Jan 23, 2016
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I want to simulate this transformer in Proteus :

1K CT 8.jpg


Please help me to calculate the missing values :


I'd suggest you measure, rather than calculate, inductances.
If you have a 'scope and pulse gen and a resistor you can
do that straightforwardly (LCR meter, OK, but less common
in hobbyists' possession).

At least The Shack gave you the resistances before they

If you made the primary and secondary inductors have
the proper ratio, times a scale variable, and you burdened
the secondary with 8 ohms, you might be able to "back into"
the right(-ish) inductance by getting the frequency response
to match. Loop that variable and watch the plots.

But what sets the upper end, is probably wanting
another element to make it happen - intrawinding capacitance,
core losses, ??? - which I'll leave to some magnet-head to

Now, since it is (or was) a fairly common audio xfmr, maybe
you could find one on one of the broad line component vendors
which would lead you to a better datasheet, or if you're lucky
a LTSpice subcircuit already done at the LTWiki file pile.

For a rough parameter estimation, you can assume that 1k/8ohm is equal to the inductance ratio and lower corner frequency of 300 Hz is set by the nominal impedance and inductance, omega = R/L.

Raw math suggests a few parameters for the transformer .

450 mW to 8 ohm speaker gives you volts and Amperes. (W = V*V / R) (This ignores 0.7 ohm secondary resistance.)

Likewise 450mW through 100 ohms DC resistance.

Step-down ratio 12:1. Depending on how your design uses the center-tap, you need to figure out how current flows in each primary winding, at what times and in what direction. Or to make it easy, assume there is no center tap.

Spec says it offers 1k reactive impedance to incoming audio frequencies. The formula is XL = 2 π f L. Play with primary's Henry value until you obtain satisfactory performance. Try different values, say 100 uH, 1mH, 10 mH.

DC resistance is parasitic. Its effect is to reduce coupling coefficient. Estimate loss at 15 percent.

I want to simulate this circuit :




I found a datasheet for LT700 and used its parameters but no sound is generated in Proteus simulation.


  • LT700.pdf
    89.6 KB · Views: 230
  • Doorbell.pdf
    286.1 KB · Views: 105

If this is meant to be a self-oscillating circuit (which
appears likely) you may need to apply a "kick" to get
it off its DC-stable solution-point. Or, make the switch
close at a timepoint after t=0 in the transient run.

That said, I foresee some excitement if you really want
to put 220VAC (rectified) onto a 25V electrolytic cap.

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