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Old Feb 26, 2012, 09:51 AM
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Mini-HowTo
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:
  • RC Info Share is a website I created to share regulator and reducer product info as well as actual test report data.
  • This RC Groups Voltage Regulator thread is an excellent resource for more info, highly recommended.
Finally, after using a DIY voltage reducer all last season in my Xplorer 3.8, this winter I designed a voltage reducer circuit board that I have been selling "under the radar." Selling these boards is not all about the money, it's just a way to help our RC soaring community. And honestly the low drop-out linear regulators are an excellent option now and are quite safe for your plane. For more info on the SoaringDude Voltage Reducer you can download the user guide here.

Any questions let me know, and Happy Regulating/Reducing. And to your servos: Live Long and Prosper

Chris B.
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Last edited by SoaringDude; Feb 27, 2012 at 03:47 PM. Reason: Added note on recommended current rating for regulators.
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Old Mar 26, 2012, 08:40 PM
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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.

Chris B.
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Old Jul 28, 2012, 01:23 AM
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PM sent re Voltage Reducer
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Old Aug 11, 2012, 11:38 PM
It should fly at least once
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SoaringDude, have you got any of my PM's,email or posts?
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Old Aug 13, 2012, 06:21 PM
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Quote:
Originally Posted by clive45 View Post
SoaringDude, have you got any of my PM's,email or posts?
Yes, I've been very busy this last week, sorry for delay. Already sent you a PM reply.
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Old Mar 20, 2014, 08:50 PM
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Thanks Chris, this is EXACTLY what I was looking for. I thought that a switching power supply was an unneeded and expensive solution to dropping voltage 0.6 volts or so. Have you ever experimented with the 5 and 6 volt 1A IC voltage regulators? One per servo of course.
Scott
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Old Mar 20, 2014, 10:02 PM
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Hey Scott, when I saw your RCG handle I did a double-take, and...I guess I owe you some kind of royalty since you started your RCG account before me .

Anyway no I have not personally used any of the one-reg-per-servo products (like from MKS) but I have seen lots of folks say they work great. The way I see it it's a classic engineering tradeoff between overall electrical reliability of your plane and protection against a single failure mode of any individual non-HV servo i.e. if they failed by drawing a ton of current that could bring your whole plane down.

With as many as 6 regulators in your plane there is an increased probability that any one of those regs will fail due to either flaky wiring or connectors (more likely) or an input-side reg electrical failure that could bring your whole plane down (less likely but still possible).

After considering the pros and cons I decided to go with a single high-current Voltron regulator in my planes and <knock on wood> so far it's worked out well.

This thread is 2 years old. As of today it looks like almost all servo vendors have a nice choice of HV servos now. For new planes now there's little reason to mess with regulators or voltage reducers...finally!

Chis B.
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Old Mar 21, 2014, 09:01 AM
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Why not just put a 10A diode in series with the positive feed? It has a 0.7v drop across it, resulting in 5.9v out. If one was worried about an initial spike, a 6.0v zener diode in series with a small resistor could bleed off the spike, but be inactive under normal conditions (<= 5.9v).
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Old Mar 21, 2014, 09:51 AM
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Hi Randy,

BTW glad you hooked up with JT on your wing repair.

Yep a 10A diode is a good "first order" solution and is more or less exactly what my little voltage reducer board does. Actually, the v reducer gives you the option to jumper in another diode to get more drop and it uses 2 pairs in parallel for load balancing and redundancy. Check the user guide out on that link to see the Vin-Vout performance.

Then you can compare that to the Voltron in the compilation performance graphs on this page. The Voltron is quite flat at even high pulse currents such as what you'd see in an extreme zoom.

Chris
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Old Mar 21, 2014, 09:56 AM
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P.s.

It was learned the hard way in the field by yours truly and others that non-HV servos from MKS are not happy at "6.0v or less," they need less than that. MKS' own single servo regs put out 5.0v. So a regulator does a better job keeping the "just off the charger" LiFe voltage below the danger zone.
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Old Mar 21, 2014, 12:10 PM
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How bout a voltage step up inverter used with a single cell lipo like used in Shred RCs smart lipo??? How reliable robust would a set up like this be??
I'm considering one of these for a handlaunch I'm building

Here's link for anybody who's interested

http://shread-rc.com/?page_id=359
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Old Mar 21, 2014, 12:29 PM
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Originally Posted by Airman74 View Post
How bout a voltage step up inverter...
Any voltage step-up would require switching technology. It would probably work fine but I always try to minimize radio interference sources so linear regs are my preferred route.

That said, when I was doing pulse load testing on the Castle CC BEC switcher I also scanned it for RF interference up to 6GHz and measured nothing within 1/2" of the case. So it is possible for switchers to be designed so they don't contribute interference that could increase the noise floor of your plane's receiver.

If you try that combo I would do a careful range test (set TX to low power, then rotate your plane at distance in all possible attitudes while your TX is held by someone at normal operating height) before flying it.

Chris
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Old Mar 21, 2014, 02:39 PM
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Quote:
Originally Posted by SoaringDude View Post
Any voltage step-up would require switching technology. It would probably work fine but I always try to minimize radio interference sources so linear regs are my preferred route.

That said, when I was doing pulse load testing on the Castle CC BEC switcher I also scanned it for RF interference up to 6GHz and measured nothing within 1/2" of the case. So it is possible for switchers to be designed so they don't contribute interference that could increase the noise floor of your plane's receiver.

If you try that combo I would do a careful range test (set TX to low power, then rotate your plane at distance in all possible attitudes while your TX is held by someone at normal operating height) before flying it.

Chris
Havnt seen anybody reporting probs with these guys in the HL threads,, but of course I'll do range check.
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Old Mar 23, 2014, 03:24 PM
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I have built and tested an extremely simple solution to allow use of a 6.6v LiFe battery with equipment rated for 6v. I used this circuit at yesterday's ALES contest for all flights with no issues. Following Kontronik's recommendation for their 40A KOBY controller, I used the battery/diode pack in parallel with the controller's BEC output. This provides a redundant power source for the receiver. The battery/diode setup could be used as an independent power source for the receiver if you prefer. This circuit could be used to parallel two or more LiFe batteries safely, as diodes prevent reverse current flow.

Steve Henke asked for the part number used in the circuit. For everyone's benefit, here is the circuit diagram with the part I used. Metro Electronics (Sacramento, 19th & J) had a 10A 400V diode in stock, so that's what is shown in this schematic. They did not have a 10A 100V diode in stock, but it would serve equally well. The diode was installed in a plug converter harness so the battery can be charged through its standard connector, as no current can flow through the diode in the reverse direction. The schematic shows an optional charging jack (or on/off switch) that could be installed through the fuselage, but balancing connections are not shown.

The circuit uses a silicon diode that has an inherent voltage drop of 0.7v. Germanium diodes have a voltage drop of only 0.3v and therefore are not suitable for this circuit. Schottky diodes are also unsuitable, as their voltage drop is 0.15v - 0.45v.The diode's 4000W capacity so greatly exceeds requirements that the circuit is essentially 100% reliable. The photo shows the large diode in its shrink wrap protection and a few 1A diodes (1N4001) for comparison.

The 0.7v drop results in a typical output of 5.9v across the receiver with a LiFe battery. Depending on your charger, the voltage could be slightly higher for a brief time until the battery reaches its nominal voltage. This happens very quickly and has not caused any difficulties for me. The circuit was first tested in an EasyStar as the sole voltage source for the receiver without problems. After that, it was installed in a Graphite 3.45m in parallel with the BEC output from the speed controller, powering a Spektrum AR9020 receiver. Over ten flights have been made with the Graphite, including five at the ALES contest.

The first graph from the Altis v4 altimeter shows the BEC-only performance as a baseline. The typical voltage range is 5.1 to 5.5 volts with a brief reading of 5.0v. The second graph shows the battery/diode circuit in parallel with the BEC; its voltage range is 5.2 to 5.6 volts with a brief reading of 5.1v and several readings of 5.7v. In my opinion, the performance of both setups is quite similar and acceptable. In the parallel installation, the LiFe battery serves primarily as a standby battery. In the future, I will provide a similar graph with the battery/diode as the sole source for receiver voltage.

The circuit was tested with two Spektrum receivers and an assortment of servos: Hitec HS-55, Hyperion Atlas Digital and old Volz Micro-Maxx. No problems were encountered. The diode costs only $2.00 from Metro; similar diodes are available at Mouser and other sources. I hope it can benefit other pilots who wish to use LiFe cells.
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Old Mar 23, 2014, 03:37 PM
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Quote:
How bout a voltage step up inverter used with a single cell lipo like used in Shred RCs smart lipo??? How reliable robust would a set up like this be??
I'm considering one of these for a handlaunch I'm building
I've used similar inverters in LED circuits made for a display case. They have been around for a while and are quite robust at this point. I like the smart LiPo shown in the link - it's a nice solution at very low weight. Please let us know how it works for you!
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