The heart of my circuit design for this Halloween display is
the PIC16F690 microcontroller from Microchip. The 16F690 is a 20-pin, 8-bit
flash micro that comes with a 4MHz internal RC oscillator and a 10-bit
analog-to-digital converter across 12 channels. I have generated the schematic
in DesignSpark so I can easily talk about each piece and its function. This
entry will entirely focus on the power piece of my design, hence the modified
schematic in the picture below.
First and foremost, every circuit needs power in one form or
another. In my post “Power Your Next Project” I took a high level look at how
best to power each of the micros in the system and decided to use a 9V battery
with a 5V voltage regulator. Like I stated in that post, I will do a full blog
on voltage regulators in the future if for now you can accept that its purpose
is to turn higher voltages into lower voltages. However, selecting the right
regulator proved to be harder than I initially anticipated. One of the good
things about being rooted in power electronics is that I know more about the
ins and outs of various regulators than the average electrical engineer.
Unfortunately, this can sometimes make it harder to sift through the noise and
choose the right part. My biggest concerns were the package size, the output
voltage, dropout voltage, and the rated current of the regulator. For simple
projects like this those are generally going to be the driving factors. Others
like tight line and load regulation are nice to have but I am not switching any
heavy loads and the micro can operate on a range of 2V – 5.5V so it is not
going to shut down because the power supply is lagging.
I wanted a regulator with a constant output voltage of 5V, a
rated current of at least 200mA, and the lowest dropout voltage I could get in
a TO-92 package to save on size (I will do a blog on IC packages someday). The
standard 5V regulator that meets most of these criteria is the LM7805, but it
has a 2V dropout voltage meaning that if the battery voltage drops below 7V the
entire system will cease to function as originally intended. The LEDs would start
to dim and most likely the timing would start to drift even further out of
whack. On the other hand, low dropout regulators (LDOs) can keep drawing energy
from the battery down to as little as 5.5V, which increases battery usability.
You can see the comparison between the two in the annotated picture from my
last post to the right.
After spending a few hours on FindChips, Octopart, and
Digi-Key’s parametric search I came across a part I thought would do the job. I
chose the LT1121CZ-5 from Linear Technology. The regulator has a dropout
voltage of 0.4V and outputs a stable 5V signal with an input voltage up to 30V.
It also sports a shutdown pin which can lower the quiescent current consumption
from 30µA to 16 µA. The regulator is only rated up to 150mA, but after checking
the maximum current the microcontroller would consume in my application I felt
comfortable with that figure.
Another downside is that the regulator was about $2.60. LT
has a reputation for making quality but expensive parts so I was not surprised
the see the cost come out so high. While I know this does not sound like much,
it is quite a lot to spend on a single component that doesn’t have any
programmable intelligence. Still, I decided I was going to splurge on this part
because it has some interesting properties that could be useful in future
applications and it provides a stable output with only a single 33µF capacitor
on the output. I also needed a 33µF capacitor for an LED driver I am working on
so ordering this regulator did save me a little money on capacitors. After the part showed up, I did a quick test with my power supply to confirm that it worked the way I expected. You can see the results in the Figure above.
After I placed my order for this voltage regulator I decided
to keep looking for something else that might have worked in its place and
almost immediately found an equally matching part for far less money. The
Microchip MCP1702 provides a 5V output with a dropout voltage around 0.1V at my
loading conditions. It comes in a TO-92 package and operates with input
voltages up to 10V (though the datasheet says it can go up to 13.2, roughly a
12V battery + float voltage). Since I only planned on using a 9V battery for
this project, I figured the 10V limit would be enough even with the batteries
inherent float voltage. Like the LT1121CZ-5, this regulator is stable with an
output capacitance of 1µF to 22µF with higher values possible for electrolytic
capacitors. The best feature of this other regulator…I was able to get it from
Allied Electronics for about $0.48 – roughly 1/5th the cost of the
LT1121CZ-5. I bought a bunch of these as well to use in throw away projects
like this one. It definitely was not worth splurging on the extra special
regulator when this one can do the job for a much lower price. I like putting
in the effort to find the lowest cost solution even on one-off boards because
it gets me in the right mindset for consumer product design. Look for the full
schematic and the software in my next post.
1 comments:
I am impressed by the amount of work and thought that went into this.
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