Simle Power supply project

[maltese001]

Hello guys, I'm fairly new to electronics so this might be a straight forward question. I was planning to build my own simple power supply using an old printer power adapter and an LM317 as a positive voltage regulator and an LM337 as a negative voltage regulator. The printer supply has 3 outputs.. 32V, 15V and GND. The power supply is only about 20W and the two outputs are rated to supply about 500mA.

My plan was to consider the 15V output as my reference/ground.. therefore i would have a +17V supply and a -15V supply (hence i'll be using the LM337 so the "new ground" line would be unaltered). I started off by testing my approach by building the +ve supply using the LM317... I built it using the datasheet and many guidelines found on the internet.. the designs are all the same and I have altered nothing. Using this configuration with the 15V as GND and the 32 V as my +ve the supply does not work properly. The voltage drops on load and sometimes it also seems to be oscillating. At first i suspected that i was misusing the LM317 but then i tried the same setup, this time connecting to the proper GND and +32V and the supply worked fine, even on load.

I am therefore suspecting that the problem might be coming from the printer power supply itself and was wondering what it might be and if there might be a solution to it. Thanks a lot : ) This is the link of the circuit diagram... its all the same using (the .1uF capacitor is ceramic and the 1uF is electrolytic) : http://www.electronics-lab.com/articles/LM317/

[KMoffett]

Not knowing the internals of the power supply, I would say that it will never work as if they were two independent supplies. Best to forget that and go find two of wall warts and arrange them for a +V/Gnd/-V supply.

Ken

[eng_mido1]

hi ..it can be build too easy beacuse you have 2 power source and the ground using 2 reg. lm317 . tomorrow i'll upload to you the circuit when i'm on my pc

[eng_mido1]

[pebe]

I don't think that's going to work, because the two voltage supplies are both positive with a common negative terminal. They are both outputs, so the only way you could get it to work as you want would be to reverse the +15V section to give -15V, if that were possible.

I think it is an HP supply with the two halves of the case screwed together and it may be possible to get into it to see. But this is mains voltage stuff and it could be DANGEROUS to tamper with unless you know what you are doing

[pebe]

Hi maltese001,
The power supply you have will only give up to 500mA. If you intend to experiment with electronic circuits you may be better of setting your sights a little higher and getting something more versatile. With these bits you do that quite cheaply.

You need a basic DC supply and I would suggest this one. It’s a laptop power supply and will give 19.5V at up to 3.3A. It only costs £7.

http://www.ebay.co.uk/itm/Laptop-Charge ... 5d503e8dfc

Add to that a switch mode voltage converter to give an adjustable voltage output and adjustable current limiting. It would give you an output of 0.8V to 17.5V at a set maximum of 100mA to 5A. This one costs just £3.26

http://www.ebay.co.uk/itm/5-30V-to-0-8- ... 3a8900d308

Alternatively, you could use this voltage converter with built in voltmeter and ammeter, for £5.89

http://www.ebay.co.uk/itm/DC-Converter- ... 2589b65615

So for an outlay of just over £10, or £13 with meters, you can have the makings of a good variable power supply.

A switch mode converter changes the voltage without changing the power, so with 19V 3.3A max input you can get 12V at about 5A max output.

I hope that information may be of use to you.

[eng_mido1]

i think you understand what i mean ..
you must open your power suppply and modify the bridge rectifier inside see this photo

[eng_mido1]

this another circuit i creat it

[I_Daniel]

It would be better if you use a dual regulated power supply. That is a positive and a negative regulator must be used.
As your design now stands your smoothing capacitor is way too low and should be at least a couple of hundred Mfd and you will also need some RF decoupling. This could be the cause of the instability.

A circuit diagram for a 12V dual power supply can be found at the following link.

http://electronics-lab.com/projects/power/011/

It should be remembered that at an absolute minimum the input voltage must be at least two volts higher than the output voltage. Check your 15 volt line. If it is 15V AC then after rectification the DC voltage, after smoothing should be about 21 volt. If it is only 15V then 12volt 7812 and 7912 regulators should be used. You may of course use variable output regulator IC's.
It is also so that the full amps will be available from the 0 V to the 32 Volt line but only half the output amperes will be available on each leg.