0-30 v Variable DC Power supply!!!!

SgtWookie

Joined Jul 17, 2007
22,230
OK.
If you want to use the LM338T regulator at 5 Amperes current, if you limited the regulator to 5A current and shorted the output to ground, even if you had it frozen into a block of ice, the regulator would burn up with only 6.5v input.
 

R!f@@

Joined Apr 2, 2009
9,918
why does anybody want to short circuit a supply.
It's one thing we would avoid ever doing.
OP has no idea what he wants, the vast availability of supply options are clouding his judgment.
Surely OP does not understand power dissipation and it's relation to heat.
And thinks a unit in the size of 4" by 4' cube can contain everything that even I am trying hard to fit neatly in a 650VA APC UPS housing. Which by the way is a 0 to 30V Variable at 12 amps, with both CV and CC and digital Meters.
OP does not realize the capacity and size relations.

First of all I thing OP or we have to decide a baseline for his intention on the PSU capacity. Find out what type of project he is using to power up, and what does he want to built as a learner. With a 500W PSU on the bench for doing just 555 timers and LED flashers is a total waste of money, space and power.

What are your project plans OP, cause Sgt can advice a lot for you, and you will end up with a headache and question mark in ur head. The more info you get, the more options are there, and since you donno the basic of typical a supply fundamentals you are not going to able build anything useful.

Determine ur requirements and stick with it.
If just building a supply than first find out what are the components you can find.
Mainly the power transformer is your primary concern, not the Schematic, cause you can get any sort of schematic here.
 

studiot

Joined Nov 9, 2007
4,998
Very very few 5 amp circuits need the level of regulation provided by an integrated regulator.

Why don't you follow the path of many who have well regulated 1 amp ± supply and a fixed 24 - 30 volt 5amp or even 10 amp supply? These could be from the same transformer and bridge or a different one.

I have seen many an amateur bench furnished with this arrangement. It works very well.
 

Thread Starter

farhan89

Joined Feb 13, 2010
34
Very very few 5 amp circuits need the level of regulation provided by an integrated regulator.

Why don't you follow the path of many who have well regulated 1 amp ± supply and a fixed 24 - 30 volt 5amp or even 10 amp supply? These could be from the same transformer and bridge or a different one.

I have seen many an amateur bench furnished with this arrangement. It works very well.
Actually I want a regulated one ,
i am going to make the ckt that given in post no 32 of this thread.
I want to attach the small cooling fan [ like the one in laptops] with supply , is this idea good or this will decrease more current :p???
also how to connect the Digital meter with that ckt which would show me voltages???
 

SgtWookie

Joined Jul 17, 2007
22,230
At maximum voltage output, power dissipation in the regulators will be very low.

At minimum voltage output, power dissipation in the regulators will be very high.

Just for instance, let's say you had 30v available at the input of the regulator, ignoring ripple voltage on the filter.

If you had the regulator set to output 28V with a load of 100mA, then power dissipation in the regulator would be (30v-28v)*0.1A = 2*0.1 = 200mW.
Power dissipation in the load would be 28v*.1 = 2.8 Watts.

Now let's look at a load that requires 100mA at 3.3v.
This means that the regulator will have to drop 30v-3.3v = 26.7v. With 100mA current, it will be dissipating 2.67 Watts (2,670mW) and the load will dissipate 3.3v*100mA = 330mW.

If you're trying to get 1A output at 3.3v, your regulator power dissipation will be 26.7 Watts.

The TO-220 packaged LM317T has a package thermal resistance of 4°C/Watt.
It will shut down if it's internal temperature reaches 125°C.

With no heat sink, the junction-to-ambient (room temp) thermal resistance is 50°C/Watt.

You tell me how many Watts you think that the regulator can dissipate before it shuts down.

Then tell me what kind of heat sink and airflow it will need to keep it from shutting down.
 
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Audioguru

Joined Dec 20, 2007
11,248
The LM317 1.5A, LM350 3A and LM338 5a adjustable regulators are smart. they reduce their max output current to protect themselves when the voltage from input to output is high.

The LM338 has a max output typically of 8A (minimum is 5A) when its input to output voltage is only 10V or less. When its input to output voltage is 30V then its max current is only 1A with no guarantee.
 
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