Concerned with your project’s power consumption but don’t want to constantly leave an ammeter wired in series with your power supply? [Rajendra] feels your pain and has recently documented his solution to the problem: a variable-output bench top power supply that clearly displays load current consumption among other things!

Everything is wired up in a nice roomy enclosure that has front-panel access to ±5V and variable outputs, an adjustment potentiometer, and even an input for an integrated frequency counter. A PIC16F689 MCU runs the show and displays the variable output voltage and current on a 16×2 character LCD. Although clearly useful as is, the PIC has plenty of I/Os and muscle left for future expansion and a capacitance meter has already been hinted at as and addition for version 2!

The power supply itself is pretty straight forward and uses 7805 and 7905 voltage regulator ICs to provide ±5V DC output. A LM350 IC also provides a variable output of between 1.25V and 9V – limited to 3V below the input voltage, in this case a rectified 12V from a standard transformer.

In order to measure current, a shunt of low but known resistance is wired in series with the output. In high-current applications these shunts are typically made of alloys that maintain a fairly consistent resistance across a wide temperature range. Since the currents in this project will be limited to a few Amperes there shouldn’t be too much resistive heating going on, and a 5 foot length of 22AWG wire wound into a coil provides a convenient and low-cost alternative. The voltage dropped across this resistance can then be measured and is directly proportional to the current flowing through it as related by Ohm’s law. This voltage drop reduces the voltage presented to the actual load as compared to the output of the regulator IC where the voltage is being measured in this case, but is accounted for in the code before the value is displayed on the lcd.

Full source code and schematics are provided and plenty of time was devoted  to explaining some of the trickier concepts such as amplifying or decreasing signal amplitudes to levels suitable for ADC input and how to use a prescaler to count high frequencies exceeding the PICs own 20MHz oscillator. Also, although this design limits the current sensing capabilities to 1.2A, alternatives to the op-amp stage are discussed that could increase this limit.

Overall this project should be very approachable to even novice hackers and is a great way to practice many basic electronic concepts. What makes it even better is that the end result is a useful tool for future prototyping.

We still can’t figure out why a standard charging scheme hasn’t been developed for handheld devices (other than greed). Certainly we understand that many devices have different electrical needs as far as voltage and current are concerned, but we still long for the ability to use one charger for many different doodads. [Rupin] is trying to narrow down the number of dedicated chargers he uses by adding a USB charging port to his Nokia cellphone charger. Since the USB standard calls for regulated 5V a hack like this can often be done just by patching into the power output coming off of the voltage regulator in the plug housing of the device. [Rupin's] charger had 5V printed on the case, but when he probed the output he found well over 8 volts. He added a 7805 linear regulator to get the stable output he needed, then cut a hole in the case to house the connector.

Since [Rupin] wants to use this as an iPod charger he couldn’t just let the two data lines float. Apple uses a specific charger verification scheme which requires some voltage dividers to get the device to start charging.

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.