Power Supply purchase

If you're serious about electronics, you'll never have too many power supplies.

You'll want some fixed voltage, some variable voltage, various maximum currents, some brute force unregulated that you don't worry about overloading, and many others. If you buy decent name brand power supplies, they will all be "keepers".

What types of electronics do you think you'l be emphasizing?

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How about you start out by building one? It can be something simple, say a

5v regulated supply or a variable one capable of an amp or so. It won't be the last power supply you'll ever need but as Sam said you can never have too many. It's a great first project too, simple, lots of circuits out there, readily available parts, relatively safe voltages, it's what I started out with.

Your first power supply for simple bench use should be your own construction. Quite simple. A 12 VAC transformer, line fuse (or circuit breaker), IEC power cord connector, diode bridge, some electrolytic (and maybe some tantalum) capacitors, a 7805 voltage regulator chip and a 723 variable voltage regulator. Fixed +5V and variable outputs are through banana plugs mounted in a plastic box. Schematics for both the 7805 and 723 voltage regulator chips are part of the manufacturers data sheets.

There is no way to appreciate what is and is not a good power supply until you have built one yourself. Furthermore, I am still using my supply built 30+ years ago. That basic power supply works great AND I know in advance everything it will and will not do.

Later, as you learn what you want from a supply, then c> Last Spring I took Intro to Electronics and this Fall I'm taking

If this is the case, you might want to stay away from Ebay. Ebay is great when you know exactly what you are buying, but if you're not sure, you can lose in a hurry.

If you find the power supplies at school satisfactory, you might write down the make and model and see if you can Google up a price.

I have five different power supplies on my bench right now. I built two of them and bought three of them. They all do different things, and I'd probably need a couple more to consider it a well-rounded selection.

For basic electronics stuff, you want at least one output that is adjustable to 24 to 30 V at at least an amp. If you're doing a lot of digital stuff, a fixed 5 V 1 A output might be convenient. Adjustable current limiting is nice, but not required. On the low-dollar route, a big string of D-cell batteries will supply an amp or two, is adjustable in 1.5 V increments, is very well isolated, and is portable, non-fattening, and does not promote tooth decay.

Hard to say, based on those descriptions.

If you had a device that needed different voltages and currents at different times, and wanted to control the power supply with a computer, you'd use a programmable supply. It could also be used for automated testing. As a simple example, suppose you are making 12 V light bulbs. You could have a computer tell a robot to put the bulb in the socket, and then have the computer tell the power supply to turn itself up to

12 V. The power supply will report the current drawn by the bulb. If it's zero or way too high, the bulb is no good and you can have the robot toss it in the trash.

For basic electronics stuff, don't worry about this too much. If the supply you buy has meters on the front, you might hook up a multimeter that is known to be reasonably good to the output of the supply, and compare the reading on the multimeter to the built-in meters. If the built-in meters are way off, you might look for an internal adjustment, or just disconnect the internal meter. (Often, no information is better than bad information.)

You might consider building one. Either design it yourself or buy a kit from somebody like Velleman. If you understand normal precautions for working with the 120 V line, this can be a good project.

Whether you build it or buy it, get some extra fuses of whatever size it takes. When you're starting out you'll blow a few fuses and it's nice to be able to get back up and running quickly. As you gain experience, you will blow even more fuses. :)

Matt Roberds

That's very difficult to assess until you know what you want to do. For most stuff you'll not need high current supplies, IMHO. +/-15 volts at say

500 mA would be useful, as would 5 volts. And a couple of 0-40. With perhaps a highish current 13.8 volt one for car type stuff. All with overload protection.

However, you can never have too many. ;-)

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

You might also want to consider a variable dual tracking type, as you're sure to get into opamp circuits on your electronics course. My suggestion on this would be 0 to + / - 30v @ 1 amp.

Arfa