buliding a power supply question

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Paully's5.0

Limp Gawd
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Jun 18, 2002
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Well I am thinking of building a simple/crude power supply. I am majoring in EE and it would be nice to have a power supply laying around that I can use at home on class projects. I have been searching a lot and seem to have found what I am looking for. Basically its a circuit with a transformer stepping down to about 24VAC with ac/dc bridge going onto a LM317 and I can then adjust the voltage accordingly. (1-24ish VDC) I am wondering how many amps should I think to have idealy? This is basic projects for class, and I am thinking 1.5 - 2 amps will be more then enough. And the circuit schematic that I have found seem is at 2amps. (Althought that is with a L200)

Now, I am wondering.....what if I get crazy and decide to hook things up like my car stereo (which requires some more power) would I be able to make the stereo come to life? (24v*2A = 48watts) And I know stereos will say 4*40watts, but is that wattage give actual wattage?

This is more of a curiosity question then anything, also if you have recomendations about a better power supply idea let me know!
 
There are several power supply kits out there that you can build if you look around, electronics123.com and electronickits.com are some places you can buy them. I've personally dealt with electronics123.com, and have had no problems with them.

epanorama.net is a website with links to a bunch of project websites. They have a Power Supply section you can look through, you'll probably want the low voltage linear section.

As far as the radio is concerned, you'll have to see what your power supply is rated for and make sure the radio is under that. As long as you get one that puts out severl amps, you'll be fine. I'm fairly sure car radios run off of 12 volts, since most cars run of of 12 volts, yours may be different, but you might want to check it.
 
Sure, 2A will power up your 12V car stereo, but don't expect to drive 4 speakers at full blast. The rating 4x40W is actually
≈true if you match both the impedance of the amp and the load (speakers + cable). You'll get max power transfer,
where half the power will drop across the source (amp), and second half across the load.
About PS, either go with the transformer or build yourself a switching-mode PS.
I guess the first one is easier to do and troubleshoot, but it's up to you.
good luck
 
I forget the model, but there is a higher power version of the LM317T. Its a can type. Put a fatty heatsink on that, and look around for cheap high capacitance capacitors to use as filters. And don't get too cheap on your transformer or you'll saturate it quickly.

If you want to get fancy, you can sometimes find multitap transformers. Use a couple switches, and optimize for certain voltages (like 12V) so you can pull alot more out of those.

Switch modes are nice, but quite noisy unless they're of good quality. Another thing you might think of doing, is using a PC power supply. Buy a few power resistors to load it down a bit so the rails settle down, then run an LM317T off that. 12V is really all you need, and a computer power supply is VERY economical. You'll get lotsa power for cheap. Then go find a case to put it in and mount some small gauges and stuff to make it easy to carry. Slowly over time I'm building up my bench with alot of custom stuff like that for my experimentations. Have fun! And visit your local electronics surplus store often. I love $.99 items. Sometimes you can get really good stuff!
 
well at my job I have the hookups as far as transformers go, voltage regs and heat sinks so I'll probably built it from that stuff. So this will be on the cheap!

Thanks a lot for all the suggestions!!

it helped alot!
 
watch out for heat issues!

24VAC rectified with a full wave rectifier is 32 volts DC. Suppose you want to make your power supply put out 5V at 2A, this means the power supply has to drop 32-5 = 27 volts, which at 2A is 54 watts of heat. You'll need an awfully big heatsink, or a CPU cooler or something, to keep the regulator cool.

It might be worthwhile to use a center-tapped 24VAC transformer, with a switch to select between 12VAC and 24VAC taps. This way if you're using an output voltage less than 12V at high current, you can switch to the lower voltage tap and substantially reduce the heat that the regulator is putting out.
 
2 amps will be more then enough. for most digital projects i use a 5v 500mA power supply.
 
You can also use a "pass" transistor and control it with the LM 317 and have more current capacity however if you short the LM 317 or the Pass transistor typically a 2n3055 then you'll be appling the output of your rectifer to the device your powering up and that could mean "fry city" you might want to use a design that has a crow bar circuit in it if you'll email me at [email protected] I should have some different designs I'll send you.


73's
Dan
 
Tangent's easy power supply makes is about painless as possible to build a very nice single voltage adjustable PSU.
Check it out.... http://www.tangentsoft.net/elec/teps/
The coolest part is he even sells "professional boards" for them. :D
If you want a very nice dual power supply check out the Welborne Labs PS1. http://www.welbornelabs.com/index.html
Be warned....... either of these power supplies will cost you at least 75$ to build.
 
If you just want something inexpensive to start with. A local shop sells 12V 85W power supplies for CHEAP. Like $2.50 apiece. jkelectronics.com if you're wondering. They're pretty well conditioned switchers. I use them for all sorts of things. Then I modified them to use spring clips so I can easily hook onto them, and I solder an AC cord onto the other side of them. Then from that 12V source, just run it through a regulator to get the voltage you want.

If you want BEEFY power, I'll sell you a kickass 48VDC 8A linear power supply I have laying around. Shipping will be insane, but you should see this thing. I think it originally was out of ham radio gear. But it is ridiculous. Gigantic heatsinks, monsterous caps. I love it. I'll post a pic of it one of these days.
 
for around $100 bucks canadain you can get all the parts you need to build a function sweep generator and a variable 12 vdc ps. Perfect for school projects...
 
I can see if i can dig up all my old circuit layouts if you wish.. may take a week or so to find tho..
 
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