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toonttm
20-08-2009, 05:36 PM
I want to build a (cheap) varaible DC power supply.

I want it to run from 0V to 12V max with enough current for hobby electronics.

I can source a DC PSU from an old computer but want the output to be variable.

I am not worried about a display as I can hook a cheapie multimeter to the output.

I have googled and found a few circuits based on the LM723 IC but would appreciate any info that you can supply.

Terry Porritt
20-08-2009, 05:58 PM
If you live anywhere near to Upper Hutt I could let you have one.

0-15 volts, 500 mA max stabilised output with switchable current output limits from 50mA to 500 mA.

toonttm
20-08-2009, 06:38 PM
Many thanks for the offer, kind sir!

But...I actually want to build several supplies and the best price I can get is around $230 each...

(They have nice digital displays, variable volts and amps, overload protected etc...viable if I wanted 1 but no go for several)

Zippity
20-08-2009, 06:48 PM
Cheap Auto (or whatever they call themselves) sell such an animal :)

Otherwise, Trinity in the USA make a beautiful item for about US$170 :)

Terry Porritt
20-08-2009, 06:56 PM
Perhaps you could explain a bit further on what sort of information you are looking for, then we may be able to help.

The LM723 is limited to about 150mA. Another IC is the three terminal LM317T, that will give variable output 1.2v to 37v max. and has a nominal maximum working current of 1.5A.

R2x1
20-08-2009, 08:41 PM
Or the LM338 K will handle a bit more - happy enough at 4 amps if it is not dropping too many volts

toonttm
20-08-2009, 09:01 PM
Sorry, after a circuit I can build that will allow me to put something like a PSU from an old computer (so DC, maybe 12V - 15V) and then dial in the output voltage. Not worried about displays as I can use a multimeter to set/check the output voltage and certainly not looking for anymore than 1 amp. Using for hobby electronics so want to use for powering pic/picaxe and 'other' general circuits on a breadboard so really no great load.
Think I am looking for a complete circuit I can build around the LM723 (or similar) IC.
I just came up with LM723 from google, never heard of it before.

toonttm
20-08-2009, 09:03 PM
Oh, but would like stable voltage once I have checked/set it with the multimeter. Guess later on if it is a successful circuit I would add some analogue displays (or even LCD through picaxe adc to get a display). At the moment just looking for a few different circuits I can build and try out. Thanks for input...

Terry Porritt
20-08-2009, 09:33 PM
Right. I will be out all day tomorrow, but I'll have a look around tomorrow night or Saturday to see what I can find, scan any circuits I find and upload them for you.
With the LM723, if you ever need higher current then an external power transistor can be added to increase current capacity.

decibel
20-08-2009, 09:54 PM
Have alook at these -

http://www.instructables.com/id/Convert_A_Computer_Power_supply_to_a_Bench_Top_Lab/

http://www.elecfree.com/electronic/l200-variable-power-supply-3-to-15-volts-2amp/

Terry Porritt
21-08-2009, 05:02 PM
Decibel's websites would seem to provide most of the info you need.

This site gives all the data on the LM723 if you decide to go that way.

http://pdf1.alldatasheet.com/datasheet-pdf/view/9021/NSC/LM723.html

I'd have thought though if you want power supplies that only deliver the current of a LM723, ie 150mA max, then using a PSU from a PC is somewhat of overkill to say the least.

Dick Smith is now worse than useless for componentry, but I see Jaycar stock LM723, LM317T, LM338K etc, also small transformers if needed.

Edit: I do DSE a slight injustice, I see they stock LM317T at$2.98 ea (I hate that sort of pricing), a bit cheaper than Jaycar

My preference would be to use a LM317T three terminal, minimum number of components required (and a year ago or so you could have picked them up for $1 each at DSE on clearance :) )

toonttm
21-08-2009, 09:21 PM
Hey cool, thanks for that! I did find a few kits from both Jaycar and Surplustronics which I think will do what I want - will try them at $9.50 each with all components and nice, small printed circuit board.
Just thought I could grab a DC supply from an old ATX or similar as they have a transformer, rectifier, overload protection, regulated voltage levels (great big fat capacitors) etc and can be bought for like a $1! lol
(I remember paying many thousands for an ATX in the day - makes you cry when you think how much our quad core computers are gonna be worth in 5 years!) - what 4Gb ram! Windows 73.7 needs at least 1Tb to run!
haha, i remember buying a 4Gb HDD drive in 1995? [166Mhz CPU - rocket ship then] It was called 'bigfoot' and was advertised as more storage than you can ever use - my $30? flash drive on my keyring is 4Gb...hahah??