Run 3V (2 x AA cells) cam from 12V car or SLA battery - Hotwire! Hardware Mods, Accessories and Insights - CHDK Forum

Run 3V (2 x AA cells) cam from 12V car or SLA battery

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Run 3V (2 x AA cells) cam from 12V car or SLA battery
« on: 22 / June / 2009, 20:30:43 »
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Hi there,
I made a prototype 12V to 3V at 2A switching regulator using easy to find components for < US$20.

Efficiency is 71% at 300mA, 77% at 1820mA, load regulation is rock solid at 3.05V (resistor values
are for 3.075 and the reg IC has 4% tolerance).  See:

Circuit: LM2576T-ADJ-12V-to-3V-at2A-circuit
Layout: LM2576T-ADJ-12V-to-3V-at-2A-prototype

Notes: May need a heatsink for high ambient operation, it's Winter here ;)  It's important to use proper PCB layout, or ground plane construction as I did.  Note too that there's no switching currents flowing through LM2576 pin 3 ground, just the voltage feedback network 0V reference, the package tab is connected to pin3 and is used for power ground.  Next one I build would have the diode on a shorter connection to pin 2.  The feedback network and LED drive take 20mA from 3V output, this should be enough load for the regulator to be stable with no external load.  Regulator runs at 56kHz and cycle skips on light loads.  Resistors are 1% 1/2W metal film.  Inductor is prewound on iron powder core.

Of course there are higher effiency ICs around, problem I found was sourcing the ICs plus the more specialised inductor and capacitors required for the modern higher operating frequencies.  For low output voltage like 3V, one really needs a Reg IC with MOSFET and synchronous rectifier instead of the catch diode.  Technology is used in PC power supplies for 5V and 3.3V rails, PC mobo regulators.  But hard to find in small quantities from hobby electronics stores.
« Last Edit: 22 / June / 2009, 20:46:42 by scattered »

Re: Run 3V (2 x AA cells) cam from 12V car or SLA battery
« Reply #1 on: 23 / June / 2009, 00:09:10 »
A little more info: No load voltage (on board 20mA load only) is 3.07V.  At 3.05V and 2130mA external load the IC, Diode and coil (coil might be picking up heat from diode and PCB) were making a little heat, but I could hold a finger on everything, input was 13.5V.  Only thing I've not tested yet is input dropout voltage.  

You might want low voltage shutdown, the IC has a shutdown terminal you could use with a simple zener + transistor circuit to shutoff regulator to not take the battery down too far (~10.5V) -- I think this is described on the LM2576 datasheet from http://www.national.com/ .
« Last Edit: 23 / June / 2009, 00:11:18 by scattered »

 

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