As is often the case, these askJOHN articles come about because of a problem someone's having. A fairly common question regards using a HV servo like the DS930BLHV with the stock Traxxas X-MAXX radio system (6V). In this brief article I'm going to share a trick for how to run a HV-servo with a separate high voltage power supply so as not to release the
stock radio's magic smoke!
What's the problem?
The basic issue is this; servo output is related to the input voltage, so consulting the specifications page for the DS930 shows it still produces plenty of oomph for the job at 6V but the stock BEC doesn't produce enough current. Of course you can reach into your pocket for a new radio system but if you want to save money by keeping the stock radio system, the solution is to separate the power input to the receiver from that of the servo. We show you how and it's both cheap and easy!
Note; in addition to an X-MAXX, this technique applies to crawlers like a TRX4, buggies, or pretty much 'any' vehicle of any brand, which is relying on a 6V BEC-circuit to operate the steering servo.
What do you need? Either a 2S LiPo or an aftermarket BEC capable of outputting 8.4V, plus a short servo extension (as the source of material), and a bit of heat shrink tubing.
Tool-wise, I find a hobby knife like an X-Acto with #11 blade handy for releasing the tangs holding the pins within the connectors, but note; these are sharp and also very good at releasing blood . . . the point being (pun intended), please be careful, eh?
And by the way, the colors of the lead don't necessarily need to match so from one extension you can make up enough to modify three servos with a single extension. And finally, this is reversible because we're not cutting or altering anything. The point being, we can reassemble everything just like it was when it brand new. Ready to get started?
As usual, the first thing I do when a fellow calls is direct him to the specifications tab for the particular servo. In this case, the DS930BLHV. This, so he can see how this servo still output over 700oz-in at 6V (plenty enough, trust me). With his concerns allayed, next we continued our conversation and delved into current requirements.
Note; at ProModeler we disclose specs like torque, speed, current draw etc. at various voltages and offer up the data for all of our servos. Curious? Just click the specs-tab where you'll see something like this (below). Making use of the information is easy. Just find the 6V column and scan downward noting torque is over 700oz-in and stalled current is 5.5A - these are important pieces of information!
Anyway, a few more questions established he was planning on replacing the radio with a new one. Not because he wasn't satisfied with the features and performance of the stock Traxxas radio system, but because he wanted all the performance advantages of running the servo on high voltage. Reasonable enough, and not surprising me in the least, he was basically waiting until he had the money. This, because he needed to take things a step at a time financially . . . after all, he'd already dropped a bundle buying the rig - plus - the dough for the high performance servo! Honestly, spending a few hundred more for a new radio would stink, agreed? Especially considering the feature set of the stock unit is plenty good enough (and it is).
So then I said, "Why buy a new radio?", which confused him. I knew it would else he wouldn't have called me but I wanted to set the table for another approach. One where we separate the power inputs between radio receiver and servo. One thing bothers me; how many folks don't call and instead, try to suss it out on their own? Or maybe they take their rig to someone supposedly more knowledgeable only to be taken advantage of? Dunno, don't want to speculate further, but it happens. Anyway, let me share what I taught him if you're curious.
So back to the guy on the phone, where it's clear he didn't quite understand the nuances of how the stock radio/ESC/BEC works. Happens a lot, and there's no shame in this because none of us are born knowing anything!
In brief, the BEC-circuit steals juice from the propulsion battery,
which produces too much voltage for the control electronics (or the
servo). The BEC circuit synthetically re-configures the propulsion
battery voltage into something suitable to run the receiver and servo.
Said juice (now at a lower voltage) is back-fed into the receiver
through the throttle lead, which is connecting the ESC to the receiver.
Two problems; first, 6V means rather less performance than the servo is
capable of outputting and second, the stock BEC-circuit doesn't deliver
enough current! Clear? If not, keep reading.
Basically, the servo gets the juice to operate from the receiver because it uses a power bus (meaning you can pick off DC voltage anywhere on the receiver regardless of channel assignment). As for the servo itself, the source of the voltage/current (what I am referring to as juice) doesn't matter - it simply doesn't care where it comes from! And this is important because we're going to use this information in a moment.
Anyway, thinking this bit of data may be useful to others (neatly explaining the 'why' regarding my penning this article), what I told him for how to extract the performance is this; "You've already paid for a high voltage servo, wouldn't you rather not fork over even more money for a new radio? What if I can show you how to use the full power of the servo without spending much more than pocket change?"
To say he was interested would be an understatement because everybody likes saving money, right? This is especially important because remember, directly connecting a higher source voltage to the standard voltage radio system will fry it. So the procedure is; we're going to separate and isolate the high from the low voltage! The best part? We're talking about spending relatively little money, and nowhere near what it costs to buy a new high voltage capable radio - and - it applies to other brands of servos and trucks/cars so this is good useful info even if you allegiance is to another brand of servo!
To recap; I'm going to show you how to use an HV servo with your 6V radio system without spending big money . . . and we'll do it with pictures!
So why bother with this if the servo outputs more than 700oz-in on 6V? Put another way, what's the problem with using the stock radio system? It's because of what the specs indicate regarding current draw is for the servo at 6V. Basically, because the servo stalls around 5.5A while the stock BEC outputs maybe 2.5-3A, it means the servo won't get enough current to turn the wheels at speed. Put another way, everything may seem fine on the workbench but once you're going fast and try steering the rig, it's maybe going to be unresponsive because there's insufficient current to power the servo - yikes!
So you're going to need a source of more current. And preferably at high voltage, also. One method is to install another BEC. One that'll run in parallel with the stock BEC but only used to power the servo. Be very careful with this because if you connect it to your receiver while it's outputting 8.4V you'll fry the radio system (won't hurt the servo but the receiver will be toast).
So one reason we've done this little mod to the servo-wiring connection is to let you connect the servo to a 2S LiPo (thus powering it separately from the radio, which continues getting its power via the stock BEC) while picking off the signal. The signal wire, which we connect to the receiver in the same place the steering servo is plugged in, completes the required circuit to tell the servo where to go. The other lead connects to a higher voltage source (battery or secondary BEC). In essence, we've separated and isolated two voltage levels, a high voltage for operating the servo separate and apart from the 6V output by the stock BEC for operating the receiver.
And the real beauty of using a LiPo is that even a inexpensive 2S unit delivers 30A of current, which is easily enough to power the servo! And at the higher voltage, you get way more than 700oz-in of torque! "This' is why you want to run your servo on higher voltage, capice? Or in the alternative, you may power the servo using an aftermarket stand-alone BEC instead of a 2S LiPo. Then you can go to even higher voltage (up to 8.4V) and the servo outputs more power than with a 2S LIPo.
And FWIW, if you insist on using a BEC (I personally prefer the clean power of a LiPo), I recommend using a quality unit from the likes of Castle Creations versus a cheapo import! The downside? Not much of one but you 'will' have to charge a separate 2S LiPo (the one for the control electronics) but this is no different than the nitro-guys have been doing ever since the Earth cooled and is really no biggie).
Last thing; if you believe torque at 6V is adequate for your needs, then if you opt for an aftermarket BEC, you could set it to 6V instead of 8.4V and plug the servo in directly to the receiver without the little harness-mod. However, be careful not to never ever set the BEC output any higher than 6V or you'll fry the receiver, OK? Me? I'd rather let the receiver run off the stock BEC at 6V and feed the servo 7.4V with a battery. Anyway, go forth and enjoy the full benefits of the high power servo without buying a new radio.
For the curious, here's further reading regarding why I don't love BECs: