We often look at battery-operated hardware and shake our heads at the wastefulness of throwing away disposable batteries. There are some devices that minimize the waste, like those TV remotes that seem to never need new cells. But the C cells that [Quinn Dunki] kept replacing in her elliptical trainer were only lasting about three months at a time. The manufacturer hadn’t cared enough to build a power jack into the machine, so she built her own AC adapter without modifying the stock hardware.

The first thing she did was to patch in a couple of wires between two of the batteries. This let her measure the current consumption, which topped out at around 200mA. This is good news because that’s easily sourced with a cheap linear regulator. Out of the junk box came a 12V/1A wall wart transformer, which just leaves the need for a fuse and some capacitors to finish out a voltage regulator circuit.

Since [Quinn] didn’t want to permanently alter the exerciser, she came up with a way that it could take the same physical space as the batteries. Two long stand-offs are used as prongs to interface the spring terminals in the battery compartment. They attach to a piece of protoboard which hosts the rest of the circuitry. Now she just needs to remember to unplug this from the wall after each session and she’ll be in business.

Finally, the USB port on the back of your television can be tapped for something useful. [Don] is using this add-on device to automatically cut the power to his Ambilight clone. Initially, he got tired of unplugging the power adapter each time he shut off the television, so he added a switch. But laziness overcame him and he decided he needed an automatic method. After probing around on the connections available, he established that the serial interface (normally used for servicing the device) was not of any use, but the USB port is. He measured the voltage of the power bus to be 5V when the TV is on, and 0.15V when it is off. He whipped up the circuit you see above which uses the USB connection to trigger a relay, connecting power to his Ambilight clone when the television comes on, and disconnecting it when the set is switched off.

Our dream has always been an XBMC capable device that can Velcro to the back of a TV, and be powered from that USB port. Unfortunately the Beagle Board hasn’t yet made it to a stable level when running XBMC. Our next hope is the AppleTV 2, which can run XBMC but would require some hacking to get it working off of the USB port, raising concerns about how much current it would draw at 5V.

Powering your gadgets generally seems like a necessary evil. To help with this [Felipe La Rotta] made a really nice bench power supply using a PC power supply and a LM317 adjustable voltage regulator. PC power supplies are an example of a switched power supply(more on that later).  The LM317 is a type of linear voltage regulator that allows for adjusting the output voltage by varying some resistors. Whats the best way to power your circuits? well that depends…

Usually the first step for powering your product is batteries, they are easy, cheap, and can be strung together to get a voltage close enough to what you need (hey sometimes it doesn’t really matter that much). But What do you do when your super picky sensor only accepts 3.3V? A quick and dirty voltage divider will bring the battery voltage down to 3.3V.

Unfortunately the more the sensor pulls on the divider the farther from 3.3V it will be. This is the basic principle of load regulation. The general idea is that the more current you need the farther off your voltage will be. Well what if there was a buffer in there so that the circuit doesn’t affect the voltage divider at all.  Maybe something like this.

But then after a day or so the sensor isn’t sensing very accurately.  The voltage going into the sensor is now only 2.8V. This is the second problem with a voltage divider; it’s sensitive to the supply voltage. This is called line regulation.  Basically as your battery voltage drops so will your output voltage.  What would be useful is a voltage that doesn’t change, that way the output could be based on that.  Here is where the Zener diode comes in.  The voltage across a Zener is set when it’s made and it varies very little with respect to current (after it gets into breakdown).  So now the Zener can be used as a reference, and then the OP-AMP buffers that to the output.

This is the general idea of how voltage regulators work.  Luckily there is no need to make one of these for every project because companies sell them in nice little 3 pin packages. All you have to do is hook up ground, the unregulated voltage, and it will regulate the output on the third pin.  Linear regulators address both load and line regulation and everybody’s happy, right?  Well maybe not.  Say a regulator takes in 9V from the battery and supplies 3.3V to a circuit and the circuit responds by drawing in 500mA.  This means that the power going into the regulator is 9V*500mA = 4.5W and the power out of the regulator is approximately 3.3V*500mA=1.65W.  What happened to the other 2.85W? It was burned off as heat inside of the voltage regulator. That means only about 57% of the power even makes it to the load; The rest is wasted.

Enter switched mode power supplies (like the one in your pc).  These circuits are made using inductors, capacitors and switches (transistors) in order achieve much higher efficiencies.  They work by constantly adjusting the current through an inductor resulting in higher or lower output voltages.  Switching supplies may be more efficient but they are also more complex, harder to implement, and can be rather noisy circuits.

so generally:

voltage divider: very easy, cheap, bad regulation

Linear voltage regulators: easy, good regulation, poor efficiency

switching power supplies: hard, noisy, good efficiency

[Ken] needed to supply 3.3 volts of regulated power. He started by using a linear voltage regulator but after a few calculations he discovered that 72% of what he put in was lost to heat. The solution to this is a switched-mode power supply. Rather than burn off energy through a voltage divider, an SMPS turns the power on and off very quickly to achieve the desired voltage.

A car charger-type USB regulator was chosen as [Ken's] donor device. He figured that making adjustments to the resistors inside would affect the output voltage and he was right. He adjusted the potential divider and ended up with a steady 3.295V.

We asked him to share the schematic that he put together from studying the board and he came through. See that and get the link to the DC-DC converter datasheet after the break.

Above is [Ken's] hand drawn schematic. After conversing with him about this project he grabbed a jeweler’s loupe and was able to identify the DC-DC converter in the circuit. It’s an MC34063 whose datasheet can be found here (PDF).

[Melanie] had some time this weekend so she whipped up a dual voltage power supply from parts on hand. This design plugs right into a breadboard and, unlike the last breadboard power supply we saw, provides two voltages at one time. 5v is delivered to one power bus while 3.3v goes to the other. Her design uses two linear low voltage drop regulators from the LF00 family (PDF datasheet) to accomplish this. Nice work!

This looks like a great addition to your breadboard. [Nerdz] wanted a power supply that was easily portable and adjustable. He built a custom board that plugs directly into the breadboard’s power rails. It has a pot attached to the ground of a 7805 voltage regulator so the output can be adjusted from 5V to just under the supply voltage. Anything that makes a breadboard less of a rats nest is definitely a good thing.

Every project needs a power supply. As 3.3volt logic replaces 5volt systems, we’re reaching for the LM317 adjustable voltage regulator, rather than the classic 7805. We’ve found four different hobbyist-friendly packages for different situations.

A simple voltage divider (R1,R2) sets the LM317 output between 1.25volts and 37volts; use this handy LM317 calculator to find resistor values. The regulator does its best to maintain 1.25volts on the adjust pin (ADJ), and converts any excess voltage to heat. Not all packages are the same. Choose a part that can supply enough current for your project, but make sure the package has sufficient heat dissipation properties to burn off the difference between the input and output voltages.

Here is a breakdown of the voltage regulators illustrated above:

IC1 LM317LZ 200mA, TO-92 ($0.59)  – This is the smallest common LM317 voltage regulator. The part linked can supply 200mA, but 100mA is more common. The TO-92 package can get searing hot because it doesn’t dissipate much heat.

IC2 LM317T 1.5amps, TO-220 ($0.64) – At 1.5amps, this regulator supplies enough power for most digital circuits. We prefer the surface-mount D2Pack version (IC4) because we don’t like to drill holes. The TO-220 package dissipates a ton of heat, and the metal tab will accommodate a heat sink if you want even more cooling. Use this package if you need maximum heat dissipation.

IC3 LM317MDCYR 500mA, SOT-223 ($0.80) – This is our favorite LM317 package. 500mA is plenty of power for many projects, and the small SOT-223 package fits about anywhere.

IC4 LM317D2T 1.5amps, D2Pack ($0.83) – We design with the D2Pack regulator when a circuit uses more than 400mA of current. D2Pack is a surface-mount version of TO-220 that’s easy to solder.

Footprints for all LM317 packages are included in the default Cadsoft Eagle v-reg (voltage regulators) part library.

Want to learn more about the LM317? Instructables, [ladyada], and SparkFun Electronics have detailed LM317 power supply tutorials.