Model parafeed in PSUD2?

[drlazybones]

Can the PSRR effect of a parafeed output be modeled in PSUD2? It would be helpful to avoid over-specifying the power supply filters. It might be tempting to put a huge power supply choke in place, but it is not very likely to be necessary based on the initial results I've seen.

So, I've set up a 300V transformer full-wave rectified through a pi filter (1uF, 10H/102Ohm, 160uF) into a 5k load impedance. The software shows about 288Vrms with about 300mVpp ripple. If I add in a second LC filter to represent the plate choke (50H/550Ohm) and parafeed cap (5uF), then ripple at the load resistor or final cap is reduced to about 1.5mV with just about 260Vrms output.

The problem I see with this is that the load is in parallel with the cap, not in series. I thought I should change the load to a 40mA current source, and increase the 5uF cap impedance to 5k, then measure at the cap. Unfortunately, the software then throws floating point errors and the voltage starts ringing horribly regardless of whether I measure it at the cap or the current source.

I've searched a bit and didn't find much on this here or elsewhere. Has anyone on this board done this test? Is this "good enough" or am I making some bad assumptions? Any thoughts about why it would have such a fit when I change to a current source?

[Paul Birkeland]

PSUD is awesome within its limitations.  It will show you things like how much power supply ripple you're likely to end up with, how much ripple current you'll have across a capacitor, and the actual AC current drawn through a transformer. 

It won't do what you're trying to do unfortunately.  Spice will, but the learning curve is steep!

What you ultimately have with the Parallel Feed output stage is a voltage divider between the power supply ripple and AC ground.  There are a lot of variables to calculate how much attenuation of the B+ ripple you will get here, and it may be easier to build out your circuit with the iron you have on hand, then measure how much ripple attenuation you are getting from the power supply.

[drlazybones]

Thanks. I may go ahead and download a copy of SPICE just to play with it. I had hoped PSUD could be used to get a close approximation, but it doesn't sound like it's the right tool.

[Paul Joppa]

What I do is calculate the ripple attenuation of the parafeed stage by hand; I do a very crude approximation but it's close enough. Since the impedance of the plate choke at 120Hz is very much larger than that of the output stage (tube, parafeed cap, output transformer) you do not need to worry about phase. Attenuation factor is approximately the output stage resistance divided by the impedance of the choke.

Output stage resistance is close the the output tube plate resistance; choke impedance is 2*pi*120*inductance. This will usually get you within a couple dB of the true value.

[drlazybones]

Thanks, I can work with that.