[Noisebridge-discuss] DC-DC Power conversion - Cheapish finds!

[Garrett Mace]

On Jan 8, 2013, at 5:27 AM, Jake <jake at spaz.org> wrote:

> For me, when i want a DC stepdown regulator up to an amp or so (maybe two amps) i just grab a cellular phone car charger.  They are all the same, a tiny circuitboard inside with a DIP version of the 34063 buck converter.
> 
> up to 14 or 15 volts input, 5 volts output, just drop it in.  If you need a different output voltage, just add a resistor across one of the two resistors in the resistor-divider network.  If you need more than 15 volts input (up to 40v) just replace the capacitor with a higher voltage (unless it is already rated at least 10% higher than your working voltage).
> 
> If you need more current, and/or want to be able to make buck converters from scratch, the LM2576 is great.  There is a fixed 5v-output version (so the only parts you need are inductor, diode, and two capacitors) or a variable output version (add two resistors to select voltage).  It's simple enough that you can put it together without a circuitboard if you're in a hurry.  Just ask Zach!
> 
> The LM2576 goes up to 40V input, and the LM2576-HV goes up to 60V!
> Both can put out THREE AMPS of current.
> 
> http://www.ti.com/lit/ds/symlink/lm2576.pdf
> 
> Garrett, when you mentioned the 7805 you missed that we were talking about PWM-style buck converters, which are up to 90% efficient.  A 7805 linear regulator is basically just an automatic resistor which burns up the extra voltage, times the current you're using.  So, powering something with 5V from a 15V power source, you're literally burning off twice as much power as you're making use of (33% efficiency).
> 
> -jake
> 
> Garrett wrote:
>> This is better/cheaper for 5V 500mA from 6.5-36V:
>> http://www.digikey.com/product-detail/en/V7805-500/102-1709-ND/1828602
>> And the big brother for 1000mA:
>> http://www.digikey.com/product-detail/en/V7805-1000R/102-2178-ND/2352137
> 

Jake,

I didn't miss that, the part numbers I linked are actually buck converters. They happen to have 7805 in the part number because the manufacturer is implying they are a drop-in replacement. They are good enough to use that way until you redesign the power input section. I've spent many hours designing switching regulators (classic buck, synchronous buck, high amperage boost, and lithium chargers). One of the rare tasks that forces me to drag out Excel these days. Did also mention the LM2576 ("Simple Switcher"), I have a box full of all the necessary parts in case I need a quick step down supply. They are just bulky.

Car chargers are definitely good, more likely to be 5V if you make sure it's for USB. Still a bit bulky and odd to fit to a project.

--- Garrett