Need help with voltage divider

[dynodale]

I have the need to step down the voltage of a battery pack to
6.0 volts. I don't won't to limit current just the voltage.

The pack is, 2S 250mah E-Teck Li-Polys.
Can somone tell me what value resistors to use as a voltage divider. I don't won't to fry my receiver and servos.

[Mr.RC-CAM]

A passive voltage divided based on resistors will not work in this application. You need to use a voltage regulator. It's probably easiest to just buy one made for R/C use (or just use the BEC output of your ESC).

RC-CAM

[dynodale]

Thank you

I am going with the 7805 regulator. Radio Shack has them for $1.50 each.

[Mr.RC-CAM]

The LM7805/LM340T-5.0 Vreg is not a great choice in this app. They work reliably down to about 7.0V on their input. But, a two cell LiPo pack can drop below that, while under load, during its useful discharge curve.

When that happens the results are not predictable. Some flavors of 7805 will continue to work, but at a low output voltage, whereas offers will fully turn off! If you use the 7805 then test it down to 6V to ensure that you like what you see.

I suggest an LM2940T-5.0 LDO Vreg. This is an old device that is cheap and easily found from a well stocked electronics outlet. Radio Shack does NOT carry them. There are hundreds of other 5V LDO candidates, so if you do not like this one then choose your favorite.

RC-CAM

[Gary Warner]

Another low-drop-out 5 regulator is the LM293, though it maxes out at 100ma. It's in a TO92 case (small). It can work for micro equipment if the servo loads can remain below 100ma.

Also, in any of these regulators, be sure to use filter caps on the output (one at .01uf, another at 10mf) to prevent oscillations in the output voltage; a common problem with LDO regulators. When these regulator oscillate they can draw a lot of idle current.