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DSP_Geek 26th December 2020 03:33 PM


Originally Posted by trobbins (
Press Simulate button, using something like : for 300ms after 5 secs delay.
And click the parameter for the voltage across your load.

PSUD has a table which shows the maximum, minimum, and difference for each parameter, quite handy if all you want is a number. trobbins' tip will get you to the steady state of your filter after it's settled down from power-up.

Marco audio 13th June 2021 06:04 AM


i use PSU designer 2 (thanks Ducan Munro) and i'm begining to build amplifiers.
This power supply is for a phono preamp.
But i'm wondering why the current is higher at the secondary (168mA) than at the load (88 mA).
Someone could help me please ?


DSP_Geek 13th June 2021 06:28 PM

Compare the curves for I(T1) and I(I1): I(I1) will be flat, indicating smooth DC current flow, whereas I(T1) will be a train of narrow spikes. RMS stands for "Root-Mean-Square", which means you take the square of the value over a time interval, divide by the time, and then take the square root. For DC values this is equivalent to the mean, but the mean for AC should be zero, which is what you see for I(T1). However, taking the RMS value of a sinusoid yields a number giving the equivalent energy of a DC value.

This assumption falls down for signals with a high peak-to-average ratio, where the square term blows up and predominates over the time interval. For example, say the signal is 1000 volts for 1 millisecond, recurring at one second intervals while being zero elsewhere. The average would be one volt, but the RMS would be sqrt(1000*1000 * 1 millisecond / 1 second), or 33 volts!

So why use RMS at all if it's subject to this madness? Power determines heating in circuit elements and power goes as resistance times the square of current, so RMS is useful for computing how much stress a component will undergo, such as the transformer equivalent resistance at about 78 ohms in your circuit.

Since you have a capacitor-input filter, the current going through T1 would be highly spiky (that's a technical term, be careful where you use it :D ) so it would have the problem mentioned above. Choke-input filters are better behaved vis-a-vis peak-to-average power ratio so the RMS value would be much closer to the DC average at I1 in your circuit.

I'm a big fan of choke-input filters since the above shows they're much easier on transformer and rectifiers and the subsequent capacitor sees a happy little sinusoid atop DC instead of a train of narty looking spikes. However, choke-input filters really want a steady load: they're not the best choice for something like a class-AB or class-D amplifier, with low idle currents and high maximum currents, since the output voltage would rise to that of a capacitor-input filter at idle and collapse to a choke-input filter at high power demand. Class-A amps, on the other hand, are at their best with choke inputs.

Marco audio 13th June 2021 06:33 PM

Thanks a lot for explanation DSP-Geek.
What a reply !

DSP_Geek 14th June 2021 05:38 AM

You're welcome :) I never would have known about PSUD if it weren't for someone here telling me about it years and years ago so, y'know, carry the information forward.

As a point of interest, PSUD also works well simulating low-voltage solid state supplies. One preamp design (LCRC) showed 20 volt rails with 200 uV ripple before the regulators, and by golly throwing a scope (AC coupled of course) on +-Vcc showed that within the limits of error.

And, yes, full credit to Duncan Munro for a really useful program. It does one thing, one thing only, incredibly well.

srdjant 14th June 2021 11:36 AM

I downloaded PSUD last night, and struggled to add two C Filter sections. Kept saying it was an illegal combination to do that. But my PSU design is using 2 caps of 4700uF in parallel due to space / parts restrictions. Anyone know if this is possible with the software?

trobbins 14th June 2021 11:42 AM

The software is simple in concept. 2 capacitors in parallel is the same as 1 capacitor. If you think it is still worthwhile trying to simulate 2 separate capacitors, then use an R-C section for the 2nd capacitor and make R a very low value (ie. the resistance of the wire connecting the 2 caps).

Marco audio 14th June 2021 11:46 AM

Maybe common mode inductors will arrive ...

trobbins 14th June 2021 11:59 AM

If you really need to explore more than base level mains frequency rectifier and filter circuits - such as the influence of common-mode inductors, then LTSpice is the appropriate simulation software to start your learning curve on.

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