Step-down voltage regulator

[daniel reese]

How do I make a micro-sized, stepdown voltage regulator to bring 8vdc down to 5vdc?

Variable input would be nifty but really, I only need a fixed output voltage.

Is this something that can be done by a techno-dummy with caveman tools or do I need a degree in electrical engineering? (LOL)

[swatson144]

It's definitely a bronze age job. You didn't mention how much current or how good a regulation. Since it's only a couple volts I'd use a LM2931 regulator IC the spec sheet (http://www.onsemi.com/pub/Collateral/LM2931-D.PDF)even shows the circuits for 5V fixed and variable. component count can be as low as 5 parts for an adjustable regulator. That one is good to about 100mA.

Steve

[swatson144]

LM1084 (http://www.national.com/mpf/LM/LM1084.html) is not as small but good for 5A. 1085, 1086 are smaller at reduced currents.

Steve

[daniel reese]

I think maybe Ive not developed Bronze Age technology...certainly not that level of wisdom anyway.

Looking at that spec sheet was, for me, like trying to read sanscrit...

What Im trying to do is use a 7.4v LiPo (8.4-7.4v 900mah) to run a wireless camera (5v 180ma) and transmitter (5v 330ma)...so, to my way of thinking, I need 5v at 410ma or 5a.

So, in caveman speak (pictures help) what bits are needed and how do they go together? Im assuming that a PCB (pre-printed circuit board) would help ease assembly but just soldiering the legs of the bits together would probably work fine...I just cant make sense of the codes and symbols on those spec sheets. I need a shopping list and a picture or drawing of what the finished piece looks like would help tons.

Steve, as always, your expert assistance is greatly appreciated

[swatson144]

hmm 500mA is .5A.

There are products out that will meet your needs. The align "bec" http://www.allerc.com/product_info.php?cPath=80&products_id=2447&osCsid=7f37ec22af4150f98fe9e3cd8bcafa20 for one the camera won't mind 6V or for peace of mind use the diode that comes with it. That'd be the easiest way and probably cheapest but not as much fun.

If you want the fun you could "dead bug" it but I like perf board for ease. Really it don't get much easier. The attached pic is an adjustable V using LM1084IT-ADJ for upto 5A. R2 is just a 5K trimmer pot. That would get you the adjustable part.
fixed 5V if you want I'll figure the values but I' normally would use the adjustable and afix the pot after trimming for my use.

My bench supply is from a "simple switcher" circuit but I can find no source for small qty.

Steve

[daniel reese]

The diagram shows a "C1" bridging the Volt In and Ground...is this a Capacitor #1? There is a note that it is needed if device is "far" from filter capacitors...what are filter capacitors? What do they do? How far is "far" and what happens if I DONT put one of those in?

What is a "10uF Tantalum"?

R1 and R2 are resistors.... Does the diagonal arrow and "5k" in connection to R1 mean variable resistance up to 5,000 mA? What is the significance of 121(upside down U) at R2? Ohms resistance?

The three horizontal bars stacked on top of eachother Im pretty sure are Ground...so the supply voltage would connect to that and "Vin"...but on the "Vout" side there are both a volt out line and a ground line...Why is there a specified line for output + and - but only an input +? Or am I confused again?

Im still struggling with this schematic....guess I need to school myself on some basic wiring diagram symbology....

The Align BEC and voltage regulator may be the more practical solution....

Would be a neat project tho!

[swatson144]

The diagram shows a "C1" bridging the Volt In and Ground...is this a Capacitor #1? There is a note that it is needed if device is "far" from filter capacitors...what are filter capacitors? What do they do? How far is "far" and what happens if I DONT put one of those in?

Yep C1 is a capacitor. You can think of filter capacitors much like a surge tank if voltage decreases they'll discharge to keep it up and if voltage spikes they'll absorb some of the spike and release it more slowly thus filtering (voltage can be thought of as water pressure). Coming from a battery source leaving it out wouldn't have dire results as a battery is pretty much a big filter. Cheap and easy enough though I'd use it for future proof who knows you may later want to use it off an 9V wall wart.


What is a "10uF Tantalum"?

It's a specific size and type of capacitor. I'd probably just use a common 10uF electrolytic unless it was hanging next to one. pronounced "10 micro farad"

R1 and R2 are resistors.... Does the diagonal arrow and "5k" in connection to R1 mean variable resistance up to 5,000 mA? What is the significance of 121(upside down U) at R2? Ohms resistance?

5K Ω is a 5000 ohm variable resitor (potentiometer or POT) it would be connected with the center tap to ground and the rightside tap to the ADJ pin (pin 3). 121Ω is simply 121 ohm resistor, odd size I'd just use 120Ω which is brn, red, brn, color code.

The three horizontal bars stacked on top of eachother Im pretty sure are Ground...so the supply voltage would connect to that and "Vin"...but on the "Vout" side there are both a volt out line and a ground line...Why is there a specified line for output + and - but only an input +? Or am I confused again?


Ground is negative and common to both sides of this schematic Vin would be your +7.4V, Vout would be +5V output (after adjusting) Ground (the 3 bars) is negative connection for both the lipo and camera in your case.

Steve

[daniel reese]

It all makes sense!

Thanks again for you help

[swatson144]

8-) glad that circuit only has 5 parts lol

Gotta start somewhere.

Steve