The case against synthetic voltage

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Background: ProModeler servos are rated at 7.4V (nominal voltage for a 2S LiPo pack under load), However, it's widely known that fresh off a charge, the output of a 2S LiPo may approach 8.4V. Thus, it's not illogical to wonder; is it OK to use 8.4V as derived synthetically via a voltage regulator, a BEC on an ESC, or via a power expander/box? In fact, we're often asked variations - framed in various ways - all asking the same basic question;

Q. Is it OK to set the output-voltage of my XXXX-device to 8.4V with ProModeler servos?

A. The short answer is yes, but we recommend using a battery instead of a BEC.

Long Answer: Many hobby shops sell BEC equipped ESCs (battery eliminator circuits as part of an electronic speed control), power expander devices (often used with multiple servos per control surface on large model airplanes), as well as standalone BECs. In fact, most ESC manufacturers include the BEC feature for free because it's extremely cheap to implement (and this fact alone should tell you all you really need to know). As background, BEC means Battery Eliminator Circuit, and the heart of the idea is to use FETs (Field Effect Transistors) to derive a lower voltage from a higher voltage source.

A well known example involves outputting 4.8-8.4V (user selectable) from a 3S (11.1V) LiPo pack, then using the BEC to power the receiver/servo (instead of a separate receiver pack). It's where the battery eliminator part of the name comes from. Thing is, BECs (and built-in BEC on an ESC) are purely a cost savings measure for manufacturers. And the problem with cheap solutions is that while a $10 voltmeter may show 8.4V, if you use a tool like a $500 oscilloscope, you'll see a different truth. Basically, that 8.4V isn't clean. Not clean like when it comes from a battery. In fact, it's downright dirty and fuzzy . . . like this.

- When viewed on an oscilloscope, the voltage supplied by a BEC is fuzzy - yuk!

And this holds true for power expanders, and standalone BECs (even the best ones that are made in America) and as for why it's fuzzy, it's because of how FETs work. Basically they're fast switches. And they function pretty much the same as making an overhead hallway light look dimmer by flipping the on-off switch really fast with your hand. Anyway, without getting down into the weeds with too much technical stuff, just remember this; the BEC came into existence for manufacturers to save money or add features cheaply. They have absolutely 'nothing' to do with high performance.

Bottom line? The output from a hobby-grade BEC is – at best – suitable for a $200 toy truck, perhaps driving lights, a winch, or some cheap foamie model airplane . . . but it sure as Hell isn't something we'd recommend for powering good electronics! Since this isn't exactly breaking news, we're of the opinion hobby dealers who continue to traffic in BECs are either ignorant, cavalier in their responsibility toward trusting users, or maybe just in it for the money.

But there's another issue. Performance. This is because when you nail the throttle, the propulsion battery voltage is drawn down, the BEC is fighting to provide stable voltage to the servo - but it can't. And this often happens exactly when you need the most performance from your servo, like when a wheel is wedged in a crevasse, or when torque rolling and you're moving all the flight controls as you vary motor thrust with throttle. Bottom line? That's when your pricey servos aren't seeing anything like 8.4V, and this means the servos aren't developing the power you've paid for. A separate LiPo battery isolates your pricey electronics from all this nonsense.

Anyway, the trick to selecting a LiPo is first, size it for how long you operate your model between charges (an 800-1000mAh 2S is plenty for an hour, or more), and second, to get one with a sufficiently high C-rating to withstand the power demands of a stalled servo (without browning out the receiver). For our part, we offer 60C 2S2200mAh packs (probably good for 3 hours). 60C may be rightly considered overkill but the delta between it and a cheapo 35C pack is minimal. Moreover, we have them built with two receiver plugs instead of one to double the available current to the servo bus. Anyway, they weigh about 3-1/2 oz, which experienced modelers know is negligible for a 10 lb crawler, or a model with a 60” wingspan.

Last thing; since most guys get tricked into buying a cheapo battery charger when they first get into the sport, use that one to charge the electronics-pack while at the same time using the good charger for the propulsion-pack. Frankly, dealing with two packs isn't that big a deal and with clean voltage your expensive electronics won't be subject to glitches, spikes (as a motor unloads), draw down (depressed voltage because of the onrush of current due to opening the throttle), or the ripples and noise from the FETs.

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1 comment(s)
Steve Carruthers December 11, 2016 5:03 PM reply

When attempting to power sensitive electronics, i.e. receivers and servos directly from the propulsion battery we have had several major issues and they all revolve around the motor and controller. Several things are happening all at one time and can cause many different symptoms and makes it very difficult to diagnose the problem. The first thing that we noticed that cause a problem is the voltage spikes when loading and unloading the motor. We have seen voltage change as much as 3 to 8 volts with load and a great big spike when the load suddenly goes away. I do not have to tell you what happens to performance of most electronics with large voltage swings in short periods.

The next problem related to voltage is the continued drop in voltage and increase in current during the period of performance and the loss of power to the servos. The other problems we have encountered is the switching power supplies in BEC’s and the chopped power reaching the servos and receiver. We have had many unexplained gremlins in our systems when getting ripple in the power to our systems. Our solution is far too expensive for modelers and more complicated than most would be willing to proceed with. If there is sufficient space and allowance for an extra ounce, the best solution we have found is to use a separate power supply (battery) for the servos and receiver. That has increased our reliability many fold and reduced the overhead in both time and money.

- Project Manager, United States Naval Research Laboratory

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