Hi.
I want to make a power supply for my own uses for experiments... and I need a variable output voltage between about 1v to 15v with current about 3 or 4A, i mean i need it efficient with good current and voltage range as i mentioned.....i may use the LM317 voltage regulator with power transistors (maybe two ones are enough).....I got this circuit below and it's very good for my need but the problem with it is that it's source is transformers while i need to give it a supply source from a 12v car battery. I hope you help me. Thanks

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Hi,

You're going to need at least a 60W transformer for this. You know that the LM317 needs 1.2V above your intended output voltage? You're going to need to get about 16.2V at least in order to do this.. I believe a 12.6V transformer might work, since the output would be about 16.2-16.4V under load.

For cheap parts, I would check out ebay. You can pick them up pretty cheap at Digikey, http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=237-1092-ND for example

Steve

 

You can use the battery, but will need a charger for it. Also, as scubasteve_911 pointed out, the LM317 can't output more than 1.2 volts less than the input. With a fully charged automotive battery, that will be no more than 12.4 volts (battery at 13.6).

Transformers are a bit expensive, but don't give off corrosive fumes.

 

I know that transformers are better here, but here (in Lebanon) it's better to use a battery since electricity is max. 12/24 ON....we don't have electricity all the time. i have a good 12V car battery with a charger and so i may obtain a variable voltage 1.2~12V and it's good; but what i need to know is how to connect the input of LM317 and emitters of 2N3055..etc to replace the transformers with the battery

 

Actually, the voltage drop of an LM317 is unfortunately closer to 1.8V. The minimum input voltage for the regulator is specified as 3V. It is an ancient design. I suggest that you would be far better off selecting a more modern IC.

Look at switching regulators. They are far more efficient.

 

I think the problem will be the current and voltage range requirements; otherwise, he might consider the MC33167/MC34167 ICs fed from two batteries in series, which would get him close.

Alternatively, to meet that current need, he might also consider two batteries in series and something like the TI PTN78020H. The latter should meet his current needs down to about 7V. A second regulator running off a single battery would get him down to 2.5 V.

John

 

It's hard to find any IC or transistor here where I live, I was just thinking to do it using available parts as 2n3055 and voltage regulator as LM317, what you said John is right, but I'm sure I'll not find that parts you are talking about

 

Get the data sheet from National Semiconductor. It shows how to use an external pass transistor for additional current handling. You'll need to find a PNP transistor, too. It calls for a 2N2905, but any PNP with similat votage and DC gain specs will substitute. The passer is some National device - the 2N3055 will work. 3055's are the universal NPN power transistor if you can live with low gain.

 

I am really a second year electronics engineering student and still don't know how to design circuits with transistors, but i have some experience....I got this circuit above from a website and my aim is to build a simple power supply for experiments and testing I do at home....I have never designed a circuit or part of a circuit with transistors....so I hope you help me. I have to use parts easily found anywhere and a source of 12V DC

 

Well, if you look on National Semiconductor's website, in their datasheet for the LM117/LM317, they have an example of a switching power supply, which would be a bit more efficient than using the LM317 and associated transistors in a purely linear mode. The output will have more noise on it than a linear supply. If you are going to be using the supply mainly for digital circuits, that would be OK.

 

Ok. It's evening here, and tomorrow's morning I'll visit my friend in the capital Beirut...and I'll buy some parts. It's a long way to Beirut.... I have no more time. SgtWookie, i searched a lot in National Semiconductor's website and in that datasheets, and searched about switching power supply with voltage regulator....etc. I think you understood what I want: a variable power supply DC from a 12V DC source with output voltage variable from about 1V till 11 (or more if possible) and good current ( about 1A min. at max. voltage/ 1A at 12V) or more. the values are not restricted, they may vary a little from what I mentioned. I hope you give me a suitable and easy circuit or tell me what to do with easily available parts. Thanks.