follower based Sallen-Key two-pole low pass filter in
Since it’s an active filter, the gain is one. So, the
signals stay nice and large, and you can also cascade
them without loss. In a passive filter, amplitude is
generally lost (really, it’s power) with each successive
pole that is added. A four-pole filter would take two
of these Sallen-Key elements; a six-pole takes three.
The voltage doesn’t matter much, as long as there is
enough excursion to cover the signals. The transistor
can be any garden-variety NPN.
To design the filter, one must consider the pole
frequency and the Q. The pole frequency should be
where we want our roll-off, but the Q (or quality factor)
is a bit more mysterious. I’m going to recommend
1/√ 2, or ~0.707, giving us a maximally flat Butterworth
The governing equations are shown in Figure
2. Sometimes it’s convenient to set the two resistor
values (and even the two capacitor values) to the same
value to simplify solving for the right parameters. I also
found an online calculator for these filters at www.
Let’s assume for a moment, though, that you
actually meant PWM, or Pulse Width Modulation.
Unfortunately, there doesn’t seem to be a clear spec
for RC PWM base frequencies, or pulse-to-pulse
period, if you prefer. One reference (Wikipedia) seems
to indicate a period of 20 ms, so let’s go with this
for now. We can do the calculations for a different
frequency/period using the same principles. A 20
ms period would be a frequency of 50 Hz (1/0.02
A way to think about how to do this integration
is that the pulse width is almost like a pulse code
amplitude, as discussed above. In the case of PCM,
there are numbers, but here we have voltage integrated
over time. Low pass filters do a kind of integration,
using the physical characteristics of reactive elements
to do this. This is a bit inexact, but a filter similar to that
used for PCM Nyquist Criteria enforcement should
work well enough. For the PWM case, try a four-pole
Sallen-Key filter with the poles set for 25 Hz ( 50 Hz /
I know this was a bit of a long answer to a
seemingly simple question, but there is actually a lot of
depth to this subject. Let me know how you make out
Fixing a Dead Power Supply
QMy son has an XBOX 360 and the power supply is dead. I’d like to fix it for him. Any ideas on where to start? Lisa Creger
ALooking on the Web, there are many replacement XBOX 360 power supplies available. If you want to do it as a construction project — which is always
fun — we have to start by finding out what voltage and
current are required. Let’s take a look.
I found an Instructables that was doing the reverse:
repurposing the power supply as a general-purpose
12V supply for lab use. That gave me these specs:
12V at 12.1A and 5V at 1A. You can see it at www.
into-a-12v-lab-PSU. That seemed promising.
However, the 5V was actually labeled Vsb, and
that’s when I became suspicious that this might not
be a simple thing. Being mainly a Mac user, I wasn’t
familiar with that designation. It turns out to be a PC
power supply reference to a “standby” voltage that is
present even when the other voltage(s) are turned off.
I found another Instructables that was similar to the
first, but actually had the pinout for the power supply’s
connection to the power cord to the XBOX 360. You
can see that one at www.instructables.com/id/XBOX-
The pinout shows that we have the 12V supply
being enabled by another pin. It was then that it
became clear to me that this was no simple matter
of finding a similar power supply and attaching the
existing cord. While it is certainly possible to design a
circuit to enable the 12V supply, the project is getting
complex requiring two voltages; with one of them
controlled by an input voltage, and being prone to
failures in the control circuit, given the current.
Looking on Amazon, I see that the average price
for a replacement is around $25, with some as low
as $20. At this point, I’d recommend simply buying a
replacement, unless you’re really motivated to tackle
the dual supply, circuit, and an enclosure. NV
QUESTIONS and ANSWERS
Post comments on this article and find any associated files and/or downloads at
n FIGURE 2. Sallen-Key low pass filter equations.
November 2017 7