Servo - June 2010

Enhance Your Plain Vanilla

Benchtop Power Supply With A PICAXE

There are several essential tools on any robotics experimenter bench. From an electronics standpoint, there are four: a programmer for the processor of your choice; a soldering iron; a multimeter; and a benchtop power supply. The latter is often substituted with either batteries and/or wall-wart adaptors. However, this is both an inflexible and unreliable arrangement, and eventually one finds a need for a "real" power supply. To do any serious design and development work, you need a supply with more features than an on-off switch. Otherwise, how do you test changes in performance with voltage fluctuations? Reliable power is required for work on vision systems, navigation, IR and US sensors, a wireless communications link, and more, and each system typically presents different power requirements. To get what you need, you can roll your own, purchase a full fledged and expensive lab grade unit, or build one of the many kits found on the Web. Usually, the end results will be a no-frills unit, with either fixed or variable voltage, and from one to three outputs. Voltage and/or current metering is a big plus, but rapidly becomes quite expensive. This article presents a cost-effective alternative.

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This project is not exclusive to the Elenco kit; it can be used on any other plain vanilla power supplies, as long as the following criteria are met: • Common ground for all the outputs.

• An unregulated DC source around nine to 12 volts. This usually will be the unregulated source that feeds the +5V fixed regulator.

• A maximum of 1.5 amps on the variable outputs; three amps on the fixed one.

If your supply only has two outputs, you’ll only leave the unused monitoring channel unconnected.

I originally purchased an Elenco Precision power supply kit shown in Figure 1. This is a triple output job built around the ubiquitous LM317/LM337 for the variable voltages and a PNP-enhanced 7805

By Fernando Garcia

for the fixed one.

It all comes in a sturdy and attractive steel case with all the necessary hardware, and can be purchased for around $60 US. This is one simple kit, reminiscent of the old EICO kits — functional but otherwise bare bones — that just begs to be upgraded. With an upgrade in mind, I designed a board based on a PICAXE 14M and a few other components which will add the following features:

FIGURE 1.

• Simultaneous voltage reading of the two variable voltage outputs and current reading of the fixed voltage output via an LCD screen. You don’t need to tie up your multimeter every time you need to make an adjustment.

• At power-up, the supply starts with all outputs disabled. When enabling them, they are all enabled simultaneously — to prevent latch-up in multi-voltage applications.

• Audible alert when enabling/disabling the outputs.

• While the outputs are disabled, they still remain monitored to allow proper voltage adjustment before enabling them. This avoids accidentally frying the circuit with an incorrect voltage.