Calculating total AC impedance of capacitor and resistor in series

[Deke609]

I'll be adding R-CR C-RC [edit: oops] snubbers to all the secondary windings in the BeePre and Kaiju. The CR RC is a resistor and capacitor in series. The capacitor is 150nF (0.15 uF).

Can I simply calculate the capacitive reactance of the cap at 60 Hz and add it to the resistance of the R? And then calculate the voltage dropped across the R by plugging the cap reactance and the resistance into the equation for volatge dividers? Or do I need to learn something about phase angles and get into complex/imaginary numbers? I'm hoping to defer learning that stuff to some later date.

If I did the math right, the reactance of a 150 nF cap @ 60 Hz is about 17.7 Kohms, making all the r values irrelevant for calculating current (the snubber resistor values range from 8R to 164R). And that means that the peak current through the RC across the Kaiju HV secondary will be approx. (1.4 * 160)/17,700 = approx. 13mA.

Treating the cap reactance as a simple resistance and plugging it and 95R (Kaiju HV snubber resistor value) into the voltage divider formula suggests that approx. 159 of the 160 VACrms will be dropped by the cap, and less than 1 VAC rms will be dropped across the resistor - roughly, 0.85V.  Which in turn suggests that the resistor will dissipate about 11 mW.  So I'll need a heavy duty cap but light duty resistor.

Does that makes sense, or am I way off (once again)? 

many thanks in advance, Derek

[Paul Birkeland]

If I did the math right, the reactance of a 150 nF cap @ 60 Hz is about 17.7 Kohms, making all the r values irrelevant for calculating current (the snubber resistor values range from 8R to 164R).

Yes.

Treating the cap reactance as a simple resistance and plugging it and 95R (Kaiju HV snubber resistor value) into the voltage divider formula suggests that approx. 159 of the 160 VACrms will be dropped by the cap, and less than 1 VAC rms will be dropped across the resistor - roughly, 0.85V.  Which in turn suggests that the resistor will dissipate about 11 mW.  So I'll need a heavy duty cap but light duty resistor.

Yeah, the resistor won't be doing much.  The 150nF cap should be a part specifically made to do snubbing duties.  This would be a good place to have a cap that's more physically large than you'd expect (but a bad place for an "audiophile" capacitor).

[Deke609]

Awesome. Many thanks PB. I will look for heavy duty snubber caps.

cheers, Derek

[Deke609]

Installed the snubbers in the Kaiju tonight. It was a lot more difficult than I expected. There was very little space to work in, and getting all the wires through the small secondary terminals was a challenge. And ... I miswired the 300B heaters the first time 'round, forgetting that one of the heater windings wraps around the other. All in, it took me nearly 8 hours to assemble the board and get it installed - although probably half of that time was spent just staring at the task at hand. I'm a little trepidatious about installing the snubbers in the BeePre b/c I'll have even less room to work with.

Haven't fired it up the Kaiju and tested voltages yet. Will try that tomorrow. [Edit: checked voltages first thing this morning and everything seems A-OK. But I can't listen until I get the BeePre put back together] But here are some pics. I've included one of the miswiring b/c it provides a clearer view of the underside of the board.

cheers, Derek

[Deke609]

And here's a shot of the entire underside of my "FrankenKaiju".

cheers, Derek