How do I safely build a variable DC power supply for my electronics projects?

[ckaiser813]

I'm taking an intro electronics class and I'm really enjoying it. I want to be able to work with electronics in my garage, I have a proto-board and but I would like to build a power supply that can go from 1vdc to 15vdc, this is what my class has. I've looked on the net for instructions, but they usually are instructions for a set voltage, I would like a variable setting. Just looking for some help, tips, or instructions.

Thanks Chris

 

[Pumblechook]

What current? You could step up with a DC to DC converter to a highish voltage and then down again with a variable regulator.

 

[berkeman]

I'm taking an intro electronics class and I'm really enjoying it. I want to be able to work with electronics in my garage, I have a proto-board and but I would like to build a power supply that can go from 1vdc to 15vdc, this is what my class has. I've looked on the net for instructions, but they usually are instructions for a set voltage, I would like a variable setting. Just looking for some help, tips, or instructions.

Thanks Chris

I'd suggest starting with a 15Vdc output "wall wart" transformer (since it is safe already and UL approved), and then adding a small post-regulator circuit based on the LM317 adjustable linear regulator. Check out the LM317 datasheet to find the right package size, and also pay attention to the minimum input-output voltage differential that it can tolerate. That will determine how high you can adjust your output voltage. You may need to start with more like 18V to be able to adjust the final LM317 output up to 15V.

 

[Proton Soup]

yeah, this actually makes for a nice little design project. i made myself a +/-15VDC_variable and +5VDC_fixed supply as diversion during my senior design project. some things to think about here.

what are your specs? that is, how much current? this will be a huge limiter on your design, so consider carefully how greedy you want to be. what is your max voltage? and what is your min voltage? that 117/317 is only going to go so low.

now, like Berkeman says, your input voltage needs to be a volt or three above your max output voltage. err on the high side because line voltage varies. subtract min output voltage from input voltage. as you can see, this is going to be at least 15 volts or so across the device. at 1 amp, that's 15W of heat you have to dissipate. at 5 amps, 45 watts. this will determine what package you use (TO-3 for high power, e.g.).

once you've picked a power and package (don't forget a heat sink), if you don't just pick an appropriate wall transformer, you'll need to design it. which for me meant a full wave rectifier. that is a design project in itself. high current means big honkin' step-down transformers, big ole diodes, and big capacitors. this probably isn't a lot of help design-wise, except to say that being greedy on the initial specs can get big and expensive on the design, especially if you're buying parts instead of salvaging them. if you're more interested in the product than the process, you can probably buy an equivalent PS for less than you can build it.

as for design help, it's usually better to have the whole databook, but i imagine those are harder to get hold of now. within the databooks, they have application notes, which are design templates. and not that I'm promoting this supplier (actually, i don't think i prefer them, it just googled easily), but this page has some notes in addition to datasheets. http://www.national.com/mpf/LM/LM317.html for notes on rectifiers, see notes in a diode databook.

 

[berkeman]

To build a little on a comment by Proton -- if you are considering building the whole power supply from scratch, including the AC Mains connection and step-down transformer, you will need to be familiar with the UL regulations that apply to AC Mains connections (including grounding, switching, fusing, isolation, ventilation, etc.). It's much safer and easier to use a pre-made "brick" or "wall wart" of some kind that is UL approved already, that gives you your DC starting voltage(s), or a stepped-down AC voltage that you then run through another transformer and rectifiers to give you +/- voltages that you then post-regulate with adjustable linear regulators.

The first power supply that I built from scratch was a +5V and +/-12V DC output supply, powered directly from the AC Mains. But I was working a summer job at Tektronix at the time (I built the PS in my spare time after work hours), and was pretty familiar with all the UL safety rules for AC Mains connections for instruments (plus had lots of EE and technician mentors to answer my questions). The hobby supply that I built could have gone through the UL approval process and passed. That's important, since if you do things wrong, you can easily cause a shock hazard or start a fire on a single fault, which is a very bad thing in the real world.

So hopefully you see that using a pre-built, pre-approved front end for your first hobby power supply is a good idea. There is still plenty to be learned in designing the output regulator stages, and building a solid enclosure to hold the whole project together in a nice, solid way. Have fun!