Guys I am trying to figure out what resistor to use to drop the voltage, from my 12 volt field battery to 7.2 volts, so my JetCat field box fuel pump can be powered using my 12 volt battery. Somebody has bound to have figured this out allready.

Hornet Driver, if you are listening, you are absolutely, 100 percent correct (of course you knew that ) when you stated that powering the 12 volt pump using 12 volts is waaaaaaay too much power. My dual Muellers were never in danger because of the restriction from the 2 dubro tanks ahead of it would slow the flow as not to harm them. My figuring was correct as they didn't expand at all during fueling but the Dubros were screaming.

One thing I almost goofed on though, was during defueling, I had my overflow tank about a 16" below the mains( I had the plane hanging in a cradle to balance it checking CG shift). When I started to defuel it started to collapse the mains. I caught it in time but know I know why you should put your overflow on the wing if you are going to defuel.

Dave Rigotti did such a thing with his Fuel pump system:

http://www.rcuniverse.com/forum/fb.asp?m=2651233


Might want to PM him to get the specifics...He's at the Toledo show this weekend.


Cheers,

I'll post the circuit in a little while.

Dave Rigotti

My buddy put an aftermarket automotive fuel filter on the output line from the pump. That plus about 3' of hose dropped the flow down to where the tanks can handle it with no problem...It has worked for 2 years now...

Sorry to volunteer you like this Dave, but it's a great idea and hope you don't mind passing it on...

Cheers,

Here's the circuit. The 2 resistors are 1/4W or larger. The LM338 can be a T0-220 package. I had the lrager package on hand so I used it. Mount to a piece of alum to help with heatsinking. I used a reversing switch for the fill and empty function.
Here's the data sheet
http://cache.national.com/ds/LM/LM138.pdf
If you don't have the exact value resistors use the link below to see what will work:
http://www.electronics-lab.com/articles/LM317/

Dave Rigotti

Thanks Dave,

That is what I was looking for.

You're Welcome!

Here's the reversing switch wiring..

Dave Rigotti

I imagine the same logic/circuit could be used to power a glow system in lieu of using a 2 volt battery.

Have you used a variable resistor with one of these circuits before?

You CAN use a pot to vary the output voltage. I think driving a glow plug from a 12VDC source is a waste of power. ALOT of "head" to overcome. Maybe a 2V plug from a 3.6 or 4.8V pack would be more like it.

Dave Rigotti

lol rcguy,
I had just drawn just a similar circuit, before scrolling down...
oh well!

It was pratingly the same, but with a adjustable voltage reg, with a
set-pot, so you can change voltage if needed.

see this: