Maker Pro
Maker Pro

Modifying SMPS to work off automotive "12V"

B

Boris Gjenero

Jan 1, 1970
0
How hard is it to modify a computer switching power supply that
normally works off 120V AC to work off "12V" automotive power. I
guess I'd want it to at least work of 11.5 to 14.5V DC.

I'm assuming that I could keep the secondary windings and
secondary-side components, including the control circuitry, but would
need to rewind the primary and change practically all of the
primary-side stuff.

Could I use 160 to 12 as the ratio for decreasing the number of turns
and increasing the wire thickness in the primary? Does the wire
thickness have to be exact or will thicker wire be ok if it fits?
What about if the power supply has multiple transformers, just rewind
all the primaries?

I suppose at the much higher new primary currents a different sort of
switching transistor would be needed. What would you recommend? A
low Rds MOSFET?

I suppose I should put some sort of filtering on the input. Is it
necessary for reasons other than RFI? (ie. will something be damaged
or not work if the power supply draws it's current in pulses) What
sort of filter should I use, just L-C?

Any other concerns? Has anybody done something like this?
 
S

Spehro Pefhany

Jan 1, 1970
0
How hard is it to modify a computer switching power supply that
normally works off 120V AC to work off "12V" automotive power. I
guess I'd want it to at least work of 11.5 to 14.5V DC.

Just go buy one like that.

Best regards,
Spehro Pefhany
 
J

j.b. miller

Jan 1, 1970
0
Ahhh...let him have the fun of unwinding the transformer, counting turns and
wire sizes, then wind a new primary, then put back the secondary
windings.....
Seems like a nice way to spend a friggin' month or two !!!!

OR

Like Spehro says....

go buy one....maybe $100 at most !!!

even CTC has then now !!
 
B

Boris Gjenero

Jan 1, 1970
0
Ahhh...let him have the fun of unwinding the transformer, counting turns and
wire sizes, then wind a new primary, then put back the secondary
windings.....
Seems like a nice way to spend a friggin' month or two !!!!

Rewinding transformers isn't that hard. You're massively exaggerating
there!

Building stuff can be fun and you can learn somerhing in the process.
What's wrong with that?
 
G

Guest

Jan 1, 1970
0
Boris,

The computer SMPS usually runs from doubled and rectified 120Vac which gives
around 320Vdc. So the ratio is actually 320:12 = 26. So, the input
circuitry has to handle roughly 26 times the current when powered from 12V.
THe Power Mosfets and other primary circuitry couldn't handle this well.

Why not use an inverter that converter that converts the 12V to 115Vac?
 
M

mook johnson

Jan 1, 1970
0
he is correct, you will have to completely redesign the primary to handle
the higher current. Even the PCB will need to be modified because the
traces were probably sized for much lower current that you'll need at 10
volts input.

I'd suggest buying on that fits your needs or building one. I suggest going
to http://power.national.com and looking at the LM3478. This is 1MHz design
work so it's not for the meek.

You could google around and most likely find a design that already exists
for this application.
 
F

Fred Bloggs

Jan 1, 1970
0
If you want to retain as much of the SMPS as possible, then you can
build a simple boost circuit 12VDC->300VDC or so to keep the SMPS input
filter capacitors charged under load.
 
Top