switchmode supply LM2596

M

marks

Senior Member
Build a switchmode supply for your picaxe based on the LM2596-adj
Have been using this for a while but finaly got arround to doing a drawing,
so thought I would share as surprising these work really well providing a nice clean supply and no heat lol!
and handy too able to use those header leads to power projects.

The design is really based arround a 16v ac plugpack I aquired and other parts I could scrounge.
But other dc plugpacks could also be used.
The voltage is adjustable from 1.25v dc to 5.35v dc basically the range of our picaxe!
testpoint 1 is for when we are using the ac plugpack providing a 50hz signal we can use in future projects lol.
Building your own does allow your to fine tune things to your own requirements
and provides a learning step to next bigger project in mind.
 

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M

marks

Senior Member
I'm not very good at question time but I'll try lol.just reading the datasheets like every one else...

I did say I scrounged the parts I had was actually
a 150uH ring inductor core 13.1mmOD X 8.7mmH 49 turns of 0.6mm copper winding wire.
to create a 68uH inductor I reduced to 32 turns.
A normal 68uH ring inductor (3amp) would have 40 turns of 0.72mm copper wire core about 13mmOD x 5.3mmH

for the filter core 20mmL x 6mmD was a 55uH radial inductor 0.5mm copper wire
I reduced this to 10 turns creating a 4 uh inductor.
those small radial 3.3uh (2amp) 6x8mm inductors they have on ebay would be ideal.

Using a slighty higher inductor value reduces swith current allowing me to use the In5822 (3amp) catch diode and supply which I had.
I thought I'd better road test it a bit
set to maxmium 5.35v when a 1amp load applied volage drops to 5.32v inductors remain cool and noise free filter works extremely well
the only thing I didnt like the bridge rectifier gets warm suprising as i'm only feeding it with 16 v and about 500ma
not really a problem as i usually only use 5v 500ma at most time ideally seperate diodes or larger bridge rectifier would be more ideal.

Because the lm2596 operates at a higher switch frequency of 150 khz smaller inductors and filter components can be used due to reduced losses.

Inductor basics lol!
Use of torroidial inductors contain the magnetic flux better generating less Electro Magnetic Interferrence.
Selection can be difficult as different inductor types and sizes all have completely different characteristics.
If we exceed our inductors energy limits it begins to saturate inductance decreases causing switch current to rise rapidly.
Manufacturers usually quote Idc at this point we have already passed saturation by 20 % and Irms has caused temperature to rise by 40 degrees C.
A safe operating value to select would be say 50-75% from 50 % upwards temperature will start to rise and inductance will reduce.
So we would hope to get at least 2 amps from our 68uH (3A) inductor lol.

If I noticed this before I'd would have bought one the additional filter will provide a nice clean supply it really does make a big difference
http://www.ebay.com.au/itm/DC-DC-Buck-Converter-Step-Down-Module-LM2596-Low-Ripple-Power-Supply-/130640142499?pt=LH_DefaultDomain_0&hash=item1e6ac260a3
Almost Identical I do wonder how it would perform ideal input supply would be 10-20v giving 5v out should be good for 2 amps I would think.
Although I prefer a lm2596t something I can bolt a heatsink too
 
B

booski

Senior Member
To be honest, personally, I use an old Trafo that I pulled from work and slung it with a little bit of circuitry into an old knitting box drawer.

Trafo outputs 24V with rectified and filtered offers up +-15V. Put that through a pair of 7912 and 7812 and you've got regulated and unregulated bench PSU.
Combine that with an LM317 with adjustable output and I've got 1.25 to 15V selectable output. If I really wanted to I could have slung in an L7805 or an ST l4940V5 for a high current 5V supply.

No messing with inductors or the likes.

OR, simply buy one of those cheap LM2596 boards of ebay :p
 
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