Need an electricity formula

[NewBlackDak]

I have a source voltage I can't alter, and I want to pull the voltage down to power a device that will burn up at the voltage it outputs. How do I figure the resistance needed to acheive the voltage drop?

I've done a few unsuccessful searches, but I'm sure it's because I'm searching with the wrong terms.

Thanks for any help or direction.

 

[unipidity]

I think you are trying to find the potential divider formula, though im no electrician and am used to hearing things phrased differently.

Anyway, you have 2 resistors in series, R1 and R2, and connect your device across R2. Your output voltage will be your input multiplied by R2 over R1 plus R2.

V(o)= (V(i)*R2)/(R1+r2)

 

[TuxDave]

Originally posted by: unipidity
I think you are trying to find the potential divider formula, though im no electrician and am used to hearing things phrased differently.

Anyway, you have 2 resistors in series, R1 and R2, and connect your device across R2. Your output voltage will be your input multiplied by R2 over R1 plus R2.

V(o)= (V(i)*R2)/(R1+r2)

That may not work. That will generate the voltage that you want (according to the equation) IF you attach it to an open load. If your load has a low input resistance, or draws significant current, then the voltage will droop to lower than what you wanted. Can you tell me more about the device you need to drive with the lower voltage?

 

[NewBlackDak]

OK, we have several(23) small hand-held devices that run off a pair of 1.2V F6 cells. They're used so much that it would be better for us to have a longer lasting battery source. I'm unsure of the load they draw, but it's not very big. I can find out tomorrow. The problem we run into is they don't make it through an 8 hour shift much less a longer one. I'm trying to figure out a better yet still portable power source for them. I'm thinking 4 AA, or 9V, or whatever.
Hopefully they'll be able to hold out for the entire shift if not we can give them an extra charged battery pack to swap in on the fly.

The units have built-in protection, and will not run unless the input voltage is 1.1~1.3.
a 9v would fit nice in the battery position, and still have room for the step-down board.

Maybe I'm crazy, and should just order a case of f6's, but we already have a whole bank of charges and batteries to fill them.

 

[TuxDave]

How much room do you have for this voltage divider? Room for a couple resistors, or room to put in an IC chip. The resistive divider may work if it doesn't draw too much power.

I'm also unsure about it's battery life since you will always be dissipating energy through the two resistors. You can give it a shot, but I recommend having at least one of the resistor be variable (a pot). 9V batteries (like.. the typical radio shack version) aren't accurately 9V, I believe when bought, they sit around 9.4V or so. So you may need to do a little tweaking on the resistors.

 

[NewBlackDak]

I think there's room for an IC. I'm unsure a 9v would be the most efficient. Maybe AA's would be the best?

I've been OKed to build a couple prototypes, so any ideas are welcome.
We've got some battery holders that I can swap fairly easily for the built-in ones. If I do that I can build the circuit inside the unit. I'll have about 1.5" wide x 3" long x 1/2" depth for working room.

 

[AnnoyedGrunt]

Can you do something like hook up a bunch of the 1.2V batteries in parallel? Maybe make your own little battery pack using the existing batteries? (In case you can't tell, I'm a mechanical engineer, hence a mechanical solution). If you hook up a bunch of the batteries in parallel, you will keep the output voltage the same, but increase the life. Are the 1.2V batteries rechargable? If so, then a battery pack might work pretty well, but if not then you'd probably need to design something that would allow you to quickly swap batteries.

The problem I see with using a resistor to drop the voltage is that it would burn up extra power while your system is in use, so you would reduce your battery life to some extent which is going in the wrong direction (although perhaps the losses are small enough that it wouldn't matter).

I've used a voltage regulator IC on some projects in the past, but they burn up extra power, too (not sure how it compares to using a resistor). The ones I used had a 5V output, but maybe they have ones with a 1.2V output as well.

Good luck!

-D'oh!

 

[ecopure]

"I'm trying to figure out a better yet still portable power source for them. I'm thinking 4 AA, or 9V, or whatever. "


If you make a battery pack out of 4 aa nimh's wired in parallel their output will be between 1.2 -- 1.3 volts.

Or am I missing something here

 

[KalTorak]

A bunch of AAs in parallel would kinda work; only concern is if they don't have exactly the same voltage - you'd want small resistors in series with each. With a 9V, you'd want to do some sort of a high-efficiency stepdown voltage regulator to get it to 1.2V (or whatever your target is). Something like a buck-topology switching regulator would work.

 

[VTHodge]

You stated that the units use "a couple of 1.2V batteries". Does that mean that the unit operates on a voltage of 2.4? The easiest solution to me would be to use AA batteries or your 1.2V batteries in parallel with a simple smoothing circuit to keep the voltage level (resister, a capacitor if noise is an issue, and a diode to prevent back charging).

 

[NewBlackDak]

Here's the specs on the current batteries H-F6 750 1.2V 750 mah.

They are wired in parallel, so they're still delivering ~1.2V.

 

[TitanDiddly]

Have you thought about using a simple IC, such as a 7805 or a 2940? This one spits out 5v, but they're available in other voltages:

http://www.uoguelph.ca/~antoon/gadgets/reg-rx.htm

National Semiconductor will give away samples, sometimes:
http://www.national.com/

Always a good idea to use caps on the input and output like in the first link, but it's not do-or-die. Have you looked in to dispostable lithium batteries? They can often have up to five times the capacity. NiMH and NiCd cells are almost always 1.2v, and come in high capacities:

http://battlepack.com/loosecells.asp

 

[Geniere]

I don?t really follow the logic of most of the posts to this thread. If you go to a higher voltage battery and use a resistor divider network or an active component (voltage regulator) you incur additional losses and will end up with less hours of operation. Your best recourse is to use a battery combination to provide the proper voltage and no additional components. Select batteries that offer the highest energy per given volume, i.e ma-hours.

 

[AnnoyedGrunt]

OK, here's an idea for my "battery pack"

D-SIZE

You mentioned AA's as well, so here's a couple choices:
AA #1

AND:
AA #2

Maybe you can rig something up with those.

Oh yeah, I don't know what kind of space you have to work with, but I remember that my RC car had a 4 AA battery module for powering the receiver and servos. I don't remember if it was series or parallel (but I guess it would just depend on which way you install the batteries, so you can make it go either way), but maybe a hobby shop or radio shack would have something you could use to make a mock up.

-D'oh!

 

[TitanDiddly]

Originally posted by: AnnoyedGrunt
OK, here's an idea for my "battery pack"

D-SIZE

You mentioned AA's as well, so here's a couple choices:
AA #1

AND:
AA #2

Maybe you can rig something up with those.

Oh yeah, I don't know what kind of space you have to work with, but I remember that my RC car had a 4 AA battery module for powering the receiver and servos. I don't remember if it was series or parallel (but I guess it would just depend on which way you install the batteries, so you can make it go either way), but maybe a hobby shop or radio shack would have something you could use to make a mock up.

-D'oh!

R/C reciever packs are series for 4.8v or 6.0v depending on manufacturer. I take it you're talking about the kind where you put in disposable AAs?

Addendum to my last post in this thread: 2940 chips are special because they're low-dropout, you can feed a voltage into them that is quite close to their rated output before they give up and just pass the given voltage though. 7805 chips are more rough in that region.