This and That

I started tinkering with electronics again recently and quickly realized the need for a good power supply.  Shopping around, it became apparent that a decent dual-output unit was going to set me back quite a bit.  So I started doing some research on what would be involved in a simple little power supply I could build on my own.  Enter the LM317 variable positive voltage regulator.  Fantastic little IC capable of providing up to 1.5A and 1.2-36 volts.

First iteration - variable power supply

Here was the first schematic I put together from various circuit designs available around the internet.

Working from left to right, we have our 120VAC input, which goes through a switch and a fuse to energize a basic 120/28 or 120/36 step down transformer.  ( BTW, it’s very difficult to find a transformer of that size with the VA rating to handle a couple of these power supply circuits at full output! ).

Now that we’ve stepped down our common household voltage, we rectify it using a standard bridge rectifier B1 ( 2A for safety ), followed by a large capacitor and a small one.  The 1000μF cap is there to provide substantial current in the event of transient demand spikes.  This large capacitor, though, does not respond very quickly, so we paired him up with a 0.1μF cap for higher frequency pulse suppression.  These caps help cut down on voltage ripple in the eventual output.

Next we have the workhorse of the circuit, the LM317.  Note that most manufacturers that produce regulators will have something comparable to the 317.  You don’t have to have the actual National Semiconductor part for this project ( I don’t!  I use an NTE956 because it was readily available at Fry’s Electronics ).  Any 1.2V-36V variable positive voltage regulator with a 1.5A output will do fine.  Double check that pinout!

Anyhow, the 220Ω resistor ties back in to the adjust pin as feedback gain, which you then pull down with the 5kΩ potentiometer.  The more current fed into adjust, the higher the output voltage.  Hence, as you increase resistance on the pot, you increase the output voltage!  All the way up to about 36VDC!  BTW, another fantastic feature about these regulators is that they’re extremely hardy, with built-in overload and thermal protection features.

Finishing up the circuit walkthrough, we have one more cap, C4, rated at 0.1μF, for further noise suppression and to hopefully limit our voltage ripple to an acceptable level.  Once I get a good oscilloscope, I’ll let you know what the actual ripple is.

In later posts, I’ll cover the ongoing construction of this project, along with adding a second output and voltmeter displays.

So I’ve spent a couple of weeks looking for a good tool to handle schematic capture and PCB layout.  At first I tried various free tools from the cheaper PCB fab houses like ExpressPCB and Pad2Pad.  They were ok, but I ran in to limitations pretty quickly.  And the bigger problem was the lack of adequate parts libraries.  I didn’t want to spend hours and hours laboring over footprints that should already be available.

So, after some more research, and high marks from hobbyists and professionals, I started working with Eagle 5.6.0.  I am extremely pleased with this tool.  I’m currently using it as freeware, which is great because for most hobby type electronics, it’s more than adequate.  Plus, if I need more power, I can upgrade the license at a fairly low cost.

After reading thoroughly helpful tutorials from Sparkfun, I got the hang of the software.  I will admit it’s a bit, umm, gangly in some of it’s user interface.  But once you get used to it, it’s fantastic!  Check out this power supply I designed after only a few hours of work:

Simple power supply

I even created a custom library to handle off-board components like pots, LEDs, etc.  Overall, I’m impressed with the drafting abilities of Eagle, and will continue using it for future projects.

Now the next question was the PCB layout software.  How good was it?  Did it have the all-important auto-routing functionality?  For most prototypes and simple one-offs, I like auto-routing.   It provides a good starting point, and can help you solve routing issues in ways you hadn’t considered.

Well, here’s the PCB from the above schematic.

Corresponding PCB

Note that I only spent a few minutes laying out the components.  The silkscreen needs a lot of tuning.    There are certainly better ways to lay things out to economize on board space and simplify routes.  Also note that I currently don’t have dimensions on heatsinks, which are an absolute necessity in power circuits like this.  Still, this simple project should give you an idea of the power available in Eagle.

I’ll start providing libraries, schematics and boards on projects like this and others as soon as I think they’re worth posting.    Anyhow, enjoy Eagle and share your thoughts/comments on it and other EDA tools!