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Needing help designing 5V PSU with LM2596

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nzoomed

Member
Im wanting to build a variable voltage input power supply so that my project can run on either 12v or 24v operation.

Im looking at using the LM2596 like in this datasheet.
http://www.ti.com/lit/ds/symlink/lm2596.pdf

I see its specs say it can handle up to 40V input, but im unclear if the design has to be modified much in the datasheet's schematic because it quotes 12V and im unsure if it would have to be changed if I am using different input voltages.

The inductor value seems to be confusing also, since to get the full 3A output, I could either use a 22uh or 68uh inductor, which are significantly different.

I see there is a link to a PSU designer and simulator in the datasheet, but it does not work when you enter any data.

TIA for any help. :)
 

ericgibbs

Well-Known Member
Most Helpful Member
hi nz,
Can you confirm that you have the adjustablle version of the LM2596.?
The d/s circuit is for the 5V version with a 12v input voltage.

I have used the adjustable verison with a 22uH inductor and they work OK.

Look at this image.
E

BTW: The designer will not work me .

A005.gif
 

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nzoomed

Member
hi nz,
Can you confirm that you have the adjustablle version of the LM2596.?
The d/s circuit is for the 5V version with a 12v input voltage.

I have used the adjustable verison with a 22uH inductor and they work OK.

Look at this image.
E
View attachment 104587
I thought the Adjustable version was designed to change the output voltage by means of a potentiometer?

I was going for the 5V version, because thats the voltage I was working with.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
Look at the notes on the image I posted , R1 sets the Vout, use a pot with a series resistor.
The 5Vout version is fixed.

E
OT: really sad news about all those beached pilot whales...:(
 

nzoomed

Member
hi,
Look at the notes on the image I posted , R1 sets the Vout, use a pot with a series resistor.
The 5Vout version is fixed.

E
OT: really sad news about all those beached pilot whales...:(
OK, I think I understand a bit better.

basically, you are saying that you have to design the unit to work with the voltage being fed in, so in other words, if i wanted to change to 24v from 12v, I would have to trim the pot to get me back to 5v?

Im sure ive seen buck converters out there that you can connect to virtually any voltage between 6-35v and out comes 5v without any tweaking.

If its too difficult to design this, I may as well just leave it for 12v operation, but it would have been nice if I could have simply connected the unit to any power source and still got 5v.
Ive already ordered a lot of the 5v fixed regulators anyway, so i may as well use them up lol.

And as far as the whales go, this is a regular occurrence here in New Zeland, not sure what causes it, but that sandspit is a common grounding place.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
With the 5V version the Vout will be fixed at 5v.

If you drive it with say 24v it will give 5v=Vout, but the heating of the LM will mean heat sinking the LM when operating at high current outputs.
 

nzoomed

Member
hi,
With the 5V version the Vout will be fixed at 5v.

If you drive it with say 24v it will give 5v=Vout, but the heating of the LM will mean heat sinking the LM when operating at high current outputs.
OK, thats good to know, this is what I suspected under load, but TBH - I dont think that this will be a problem.
Im only running an arduino UNO, an LCD screen and 2 small sugarcube 5V relays, I doubt this would even use 1A, going by the current these small relays draw.
The LCD probably uses more than the arduino and relays combined.
I could heatsink it to the metal box anyway.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
For such a light load current, I would try it without a sink and measure the TAB temperature, check if it needs sinking.
E
 

nzoomed

Member
hi,
For such a light load current, I would try it without a sink and measure the TAB temperature, check if it needs sinking.
E
OK, will be interesting to test.

As long as it works, ill be happy anyway, I probably will still allow for a heatsink as i intend to make this well engineered.
 
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