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servo questions - voltages

Place for general submarine conversation

Postby Mike D » Fri Feb 21, 2003 11:12 am

Hey SC members.

I using three 1/4 scale servos (futaba) in a large sub.
Im not using a typical batery Im using a 12v deep
cycle marine batery. i have a transformer to bump
down to 6v. for the r/c stuff.
Its one of those car types with differant
voltage outputs. I have its + wired directly to a 3 amp
fuse. then it connects to the receiver. Gound is directly
connected to neg on the batery via a .01uf and 10uf @ 6v
caps. (to take out electrical noise) when i put a meter on
the + get a reading of 6.35v. Going to the reciver.
Is this bad? I have been running this setup for 5 years
with not many problems but need to know...is this bad?
dose voltage have anything to do with torque?
what is the norm. for operating servos (volts and amps)
dose it matter if you are running a little higher?

Also what is the usuall voltage and amps provided in those
gell cells i see people using in r/c stuff?

one more...my servo leads are very very long....6 feet to my rudder one...is the lenth of the leads something to worry about?

As i said befor...i havent experienced many problems over the years but i am now installing a pic microcontroller to make the sub atomomus...the controller has outputs for servos and also has a external supply hookup for just the servos. (i guess this is so you dont need to use the voltages thru the controller. )
so if i use the external supply, can i hook it up to the transformer Im using now with the currents its supplying now?

Or should i do something else? One thing to remember , i would like to use the 12v batery (bumping it down to 6v) for all of the
r/c stuff. I do not wish to add other bateries for the r/c stuff.

any feedback or comments would be great to hear from you guys on this subject.

Michael
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Postby rcade » Fri Feb 21, 2003 5:57 pm

Most 12 volt batteries when completely charged actulaly put out 12-14 volts. If you are stepping down the voltage with a BEC it will put out more than 6 volts if supplied more than 12 volts.

I run a 7.2 volt battery in my helicopter and it does boost the torq and speed a little. I have had no problems in using higher voltages and I have been doing this for 8-10 years. I use Futaba and JR systems. Keep in mind that JR transmitters use the black wire as positive. The receiver uses red as positive. Don't know why.
Rob Cade
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Postby 69-1045870627 » Fri Feb 21, 2003 7:39 pm

Hi Mike:
To answer all the questions takes a little understanding. Typically servos have three wires ground, power, and signal. The power and ground can be separate and most people as a matter of convenience draw power from the radio. A radio and servo have a certain internal resistance and as you increase the voltage you are only increasing the heat the unit will produce unit something gives. While you can increase the power to a servo, you will shorten the life of the servo’s motor. At some point the tiny wires inside the motor will overheat and stop working. Best is to switch to a heavier servo if you really need the torque.

Servo motors are D.C. type (Direct Current) and the have permanent magnets inside. The coil (armature) is also a magnet created by the voltage applied. If you have one strong magnet (the armature) and one weak magnet (the permanent magnets) then you really can not get more torque.

Batteries come in a variety of flavors distinguished by the number of “cells”. Typically, 1.2 volts per cell, and the current varies. What you are looking for is the highest number of “amp hours”. This says how much current you can have for what time period. R/C batteries are marked in MAH or mAh or Milli-Amp-Hours. So a 2400 mAh battery provides 2.4 amps for one hour.

D.C. or direct current unlike A.C., goes through the entire cross section of the wire. If the wire is too small over a long run, then the wire will act as a resistor and you will have a voltage drop at the end of the wire. The wire will also get warm.

If you plan to use a PIC, take a look at how much current it will handle. If it will not handle the necessary power, you may wish to use an “optical isolator” on the PIC’s output. This way the PIC only sees a low current L.E.D.

Usually for applications where you have a 12 volt supply and only need 6 volts, an I. C. device like a 7806 works well. Be sure to use the recommended capacitors so it won’t oscillate. You can get fixed voltage regulators that will handle up to 5 amps each. No, you can’t easily run them in parallel. Sorry.
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