Output impedence in spice

[RajkoM]

I did not "restricted". I'm just sorry that I can not get/simulate good LC PSU with out impedance less than 400 ohms 🙂

 

[euro21]

I'm just sorry that I can not get/simulate good LC PSU with out impedance less than 400 ohms 🙂

Hmmm....Hmmm... I don't understand....

 

[RajkoM]

Tnx euro.
I simulate your PSU with GZ34 and my power transformer (applied from my PSU for SET amp).

From my simulations, your PSU "ringing like mad."
And Zout@50Hz is about 500 ohm.

 

[RajkoM]

...and identical results got form PSUD2 with 5U4GB.

 

[euro21]

Rajko, the ringing is now irrelevant, the question was the output impedance.

At 50Hz the last 47uF capacitor impedance is 67.7R (Z=1/2*pi*f*C)
Everything else impedance is parallel this, ergo reduce it.

 

[RajkoM]

My calculations for Zout:
Look at the results from PSUD2. When I load increases 5mA, the voltage drop is 2.5V.
d (U)/d (I) = 2.5 / 0.005 = 500 ohms @ 50Hz.

This is my calculations and I believe in it.

 

[cnpope]

My calculations for Zout:
Look at the results from PSUD2. When I load increases 5mA, the voltage drop is 2.5V.
d (U)/d (I) = 2.5 / 0.005 = 500 ohms @ 50Hz.

This is my calculations and I believe in it.

I think your calculation is for the voltage drop after having turned on a 5 mA step load? in which case your 500 ohm result is essentially the DC output resistance, not the output impedance at 50 Hz.

Chris

 

[RajkoM]

Saw! Thanks Chris.
I thought my PSU is not so good 😀

 

[peufeu]

Yeah, if you sim it in AC mode, then the simulator computes a linear equivalent of the circuit. If the operating point calculation has the rectifying diodes turned off, then they will be off for the whole calculation, even at 1 Hz. This is of course not realistic, as we would expect the rectifier to impose a voltage on the caps, and therefore a rather low impedance, as the test frequency drops.