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Thread: Help with diodes

  1. #1
    Flashaholic cityevader's Avatar
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    Default Help with diodes

    Hello, I'm planning to use diodes to drop down some voltage. It's in a tiny RC airplane. I got some super duper teeny tiny servos which I realized too late are made for a single lipo cell around 4V. The planes speed controller is for a 2 cell lipo pack for the motor, and a regulated 5V output for the electronics...so a diode or two should drop .6 to 1.2V off the 5V if my memory serves me correctly.

    Here's where my limited knowledge stalls...I got a 5v Radio Shack diode (I wasn't sure if I should have gotten the 12v diode?) and alligator clipped it into the positive lead to a tiny motor and connected to 8.4v source to see which way it was biased. I ran fast in one direction and slow in another...but shouldn't it have not run at all in one direction? I seem to remember if a diode gets too much reverse voltage it would open up and allow flow? Is this what is happening with the higher voltage source?
    The voltage drop across the diode was 5v, and across the motor 3V while motor spinning slower. So I assume it was wired backwards? Then I realized that I was using a full 8.4v instead of 5v, so now wondering what would happen if I kept it wired the same, would it not work at all if not over-voltaged?

    So long story short, given the 5v source with a 3-4V servo, am I correct in assuming two "proper" diodes in series in the proper polarity would drop the voltage down to proper range? Secondly, would the "proper" diode be a 5v or 12v?

    Sorry if it sounds confusing, but to those smart folks here, I'm the dumb azz.
    Edited... here's the servo in question: http://www.hobbyking.com/hobbyking/s...c_0_075kg.html
    Last edited by cityevader; 05-07-2012 at 09:27 PM.

  2. #2
    Flashaholic ss2nv's Avatar
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    Default Re: Help with diodes

    Wouldn't resistors be better suited for doing this?
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  3. #3
    Flashaholic cityevader's Avatar
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    Default Re: Help with diodes

    Perhaps, but then i'd have to research how to figure out the math. From memory, a diode will drop about .6v, no math.

    Last night I unsoldered the 5v zener and soldered in a regular diode (12v). Confirmed it works, but haven't done any measurements yet as to what the servo is getting. For what it's worth, I didn't do any testing with the original setup "properly" (as in under actual operating voltage).

  4. #4

    Default Re: Help with diodes

    Hi Mate,
    You are right, a couple of diodes in series will drop the voltage by about .65V each (1.3V total)
    You want to make sure that the diode that you use can handle the current that the motor will draw CONSTANLY, not as PEAK current.
    The diodes that you were using are called Zener diodes (5V and 12V reverse voltages) and are meant to be used in reverse polarization, and are usually low current (you did not say which type).

    If it is a beefy servo, try a 1N5403 or 1N5404 these diodes can take up to 3A. for a smaller servo, try an 1N4001 or 1N4004 these last ones can take 1A.

    Good luck and happy landings!

  5. #5

    Default Re: Help with diodes

    What he said. I have RC planes as my main hobby. The diode solution works. One normal silicium diode should drop around 0.5V-0.8V, that should be enough.
    But you have to install the diode in the main lead going to the servo (positive/negative/pulse), not in the cable going from the servo electronics to the motor.
    Install it in the positive cable of the main lead.

    Cheers,

    Julez
    This message was morsed into the open end of a fiber-optic cable with a flashlight.

  6. #6

    Default Re: Help with diodes

    the 5 volt zener should have dun it.. all bar to hi a current (amps).. but as mensioned by ammlione.

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