|Feb 26, 2012, 10:51 AM|
Voltage Regulators and LiFe Battery Packs
The growing popularity of LiFePO4 (aka LiFe) battery packs has raised the issue of servo reliability when subjected to voltages that exceed servo manufacturers ratings. While some servos are now coming out in "HV" (high voltage) versions, many of the popular ones are still rated at 6.6v or less. For example MKS servos are rated at 6.0v maximum. Since LiFe pack voltage can exceed 6.6v during the early part of their discharge curve (RCG ref), this puts unhealthy electrical stress on the servo motors (per a discussion with the MKS USA rep on this topic) which can lead to in-flight failures.
While there are many RC pilots who use LiFe batteries and offer testimonials that "my servos work fine without a regulator," the fact is that the servo manufacturers say the servo motor lifetimes can/will be reduced if you run them at the higher LiFe voltages (it turns out that most servo electronics--ICs, discretes, capacitors--are up to the task but not the motor itself).
Know your Voltage and Current Requirements
There are several easy and reliable solutions to this problem: add a voltage regulator or voltage reducer between your battery pack and your receiver. Before you can do that it helps to know the voltage and current requirements in a typical 6-servo plane. Most non-HV servos will work fine with between 5.0v and 6.0v but just to be safe you should check specs on your particular servos. In terms of current consumed, the typical 6-servo plane will draw 300-500mA during normal operation. However during the zoom phase of launches (or other such hi-G maneuvers) the very short term battery pack currents can get as high as 10A! Thankfully that only lasts for very short (few seconds) periods so you don't have to guarantee continuous currents from your regulator at that level. But certainly the higher usable* peak current a regulator can handle the better. For a good real-world compromise I recommend a peak current rating of 3-5A for your regulator or voltage reducer.
* Making the distinction between manufacturer's ratings and the actual performance of a regulator.Voltage Regulators versus Voltage Reducers
A voltage regulator comes in two flavors--linear and switching. Virtually all of them on the RC market use specialized integrated circuits that do all the heavy lifting to produce stable voltage with varying input (battery) voltage. While switching regulators have been very popular in certain segments of RC and have an intrinsically wide input voltage range, they produce noisier voltage under peak loads and at least theoretically have the potential to generate RF noise that can interfere with your receiver. Low drop-out linear regulators, on the other hand, produce clean power and in our LiFe application where the difference between input and output voltages is very small, are every bit as efficient as switchers.
A voltage reducer is significantly less complex electrically than a regulator; it uses power diodes to drop a more or less constant voltage (as opposed to generating a fixed output voltage). Different diode types can be combined for different voltage drops. One very nice attribute: voltage reducers have no hard limits on peak current; rather, power diodes will simply drop slightly more voltage at higher peak currents as opposed to regulators that may significantly reduce the output voltage during peaks.
Which to Use?
I have to admit that going into this project I was not comfortable about using regulators in my expensive planes. After doing the research and performing tests on several units I have completely changed my perspective: now there is no excuse to NOT be using them. Personally I like the linear regulators better than the switchers. I also completely trust a good voltage reducer in my plane. A few references to help you with your selection:
Any questions let me know, and Happy Regulating/Reducing. And to your servos: Live Long and Prosper
|Mar 26, 2012, 09:40 PM|
Pulse load testing data now available
I just announced in the Voltage Regulator thread here that we are now able to do even better testing of voltage regulators. All test reports posted here so far now include pulse test data that gives a much better performance picture of how regulators will fare with LiFe 2S battery packs.
If you have any questions let me know.
|Aug 13, 2012, 07:21 PM|
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