How should my stepdown regulator work?

[Zoobie]

I have just bought a 9254 for my tail and I know I have to step the voltage down to 4.8 volts. When I measure without the regulator the output is 5.75v, when I put the regulator on it drops to 5.45? I am using a Western Robotics stepdown regulator and a Gaui BEC. Am I good to run the servo at this voltage?

[bstock]

You want to be closer to 5v (spec is 4.8v) - but you are OK to fly the 9254 at the 5.45v.

It has been debated many times in the forums as to whether it is OK to run that servo at higher voltages... and most agree it is better to run it to spec... but many pilots are not even bothering to drop the voltage down from 6v and are flying it at the higher voltage.

The consensus is that it will work at the higher voltage fine... but it probably is shortening the life of the servo.

[hommer75]

You might want to try it again with a servo attached after the step down regulator and then measure the voltage over that servo. Other one than that 9254 if you don't trust it at all, but it'll probably be ok at the higher voltage if only for a short duration.

Step down regulators are nothing more than a diode, and depending on the current drawn through that diode the voltage drop over it becomes higher. You might find that with a servo attached the voltage on that servo is lower than without.

A voltage drop of only 0.3 volts is strange though; a 'regular' diode will have a voltage drop of about 0.6 -0.7, this almost looks like a schottkey diode which isn't realy usefull in this situation.

[Zoobie]

So If I understand correctly, the more current the servo draws, the more the diode will reduce the voltage?

[hommer75]

Usually it maxes out a bit at higher current, but yes, correct conclusion. If you look at the graph below which I stole from a Faircild 1N4007 datasheet you can see it yourself. At low currect (which happens if you simply measure it unloaded) the voltage drop is only 0.6, but the higher the currect, the higher the voltage drop.

http://www.slatius.nl/hurricane550/1n400x.png

Of course, your regulator only seems to drop 0.3V, which I find a bit strange. But measure it first with some load attached, you'll probably see a lower voltage.