Servo connectors & extensions

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I occasionally field questions regarding servo connectors and extensions (especially for giant scale models). Let's briefly delve into some mechanical and electrical considerations because basically, we (all brands) play by similar rules.


Mechanical

Pin spacing - even though we all use 0.10" spacing, some manufacturers try to lock users in to their brand. How? By doing things like making the plastic plugs themselves a little bit incompatible. This aggravated me as a modeler so I wasn't going to further the practice once I got involved in the servo business. This is why ProModeler servos have a universal connector.

By the way, 0.10" pin spacing is common as dirt - everybody uses it! Note; sometimes you'll see 0.10" listed as 2.54mm, which is the same thing. And note; this isn't some RC modeling-derived standard. Instead, it's an example of modeling adopting something in far wider use within the world of electronics. For example, if you've ever been inside a computer you've seen headers with 0.10" spacing (and matching connectors on ribbon cables).


Electrical

Current handling - pins are good for 5A and are arranged by color in this order: brown/orange/yellow (or black/red/white) because even if you manage to insert it the wrong way around, nothing bad will happen. Basically, when reversed, signal connects to negative, negative connects to signal and thus, the circuit between negative and positive will never close meaning nothing happens. Note; brown/orange (black/red) correspond to negative (-) and positive (+) to a battery (DC), respectively. The yellow (white) is the signal wire.

Gauge - the rule of thumb for gauge (size) is the smaller the number, the thicker the wire, and thicker wire handles more current. Both 24GA and 26GA leads are fairly common in the servo world. Cheaper servos tend to come with thinner leads (they're cheaper than 22GA). Anyway, most ProModeler servos use 22GA - even our micros where it's a little bit of overkill. Reason is, the thicker wire offers more robust power handling. 

  • 24GA - 3.6A
  • 22GA - 5.8A
  • 20GA - 7.9A

By the way, thicker wire is also a little heavier. We've heard modelers who should know better say this is why they use lighter extensions. But experienced modelers grok these few grams are chicken feed in the grand scheme of things. After all, if the wire isn't capable of handling the loads without greatly reducing voltage in the process, what's the point of having powerful servos? 

Note; the signal-wire thickness is immaterial because it doesn't carry significant current but it's typically the same gauge as the two leads powering the servo.

Voltage - all ProModeler servos are HV (high voltage) capable. Yes, they'll operate down to 5V (4.8V nominal) and we share the specs for those interested in low voltage operations, but even a casual reading of the specs shows performance decreases as voltage drops. Since high voltage servos are all about performance, why not get all the performance you've paid for?


Extensions

Rules of thumb - extensions should never be a lesser gauge than the servo lead. Any time they're more than a foot long (~30cm), we recommend increasing to the next heavier gauge. And while we've heard some modelers claim to use 18AWG wire into a servo connector (possibly by shaving down the insulation and taking special care with crimping the strands of wire), commercially, 20GA (20AWG) is about as thick as you can get.

This is due to the need for manufacturing using industry standard crimped connectors fitted to the universal plastic connectors. This is why we don't offer extensions longer than 4' (~1.2m). So how do you calculate this voltage drop? It just takes a bit of grade school math and one well known formula.

Example 1 - let's say you want to connect a couple of DS470BLHV servos on the elevator of your 104" Extra - one for each half. You want to know the worst case scenario for how much torque you'll really get from the servo using 7.4V (a 2S LiIon pack) to see if it's really going be enough to handle anything you may throw at it.

Visit the specs on our website to see the stall current. It's 3.0A at 7.4V and you determine you're going to need a 4' extension to reach the tail of your model. Since I recommend using no less than 20GA once extensions once you go beyond 1' long, we need to know the resistance of 20GA wire for our calculation. Basically, for 20AWG (American Wire Gauge) wire, resistance is about 16.46Ohms/1,000 feet or 0.01646O/foot.

So how much voltage drop will a DS470BLHV experience at full stall, e.g. the worst case scenario? First, remember the 4' extension is actually 8' of wire because to complete the circuit we're dealing with 4' of (+) conductor plus another 4' of (-) conductor, OK?

Now let's turn to Ohm's Law and figure this out. You'll recall from high school physics E=IR (Voltage = Current x Resistance) so let's plug in some numbers and see what we have . . .

3.0A x 0.01646O/foot x 8feet = 0.395V (let's round it up to 0.4V)

. . . this means we lose 0.4V when using a 4' extension of 20GA wire, which is close enough to 0.1V/foot to call it (when the servo is at full stall). Subtract the 0.4V from 7.4V and you'll be getting 7V at the servo. Going back to the specs and interpolating you can see this means it'll actually be outputting about 420oz-in. Surprised? You shouldn't be and fro this you should extrapolate it's basically impossible to get full rated torque and speed using any extension!

Example 2 - what if instead of 20AWG extensions you followed the advice of the loudest voice at your club field, the guy who says he's been using standard hobby shop grade 24GA wire for years? Same math and for 24GA wire the loss is 26.17O/1000' or 0.02617/foot, you just plug that in and you get . . .

3.0A x 0.02617O/foot x 8feet = 0.628V (I'd round it to 0.7V of loss)

. . . and this works out to 7.4V-0.7V=6.8V at the servo. Looking at the servo spec again you'll see it'll be outputting perhaps 380oz-in. 

In fact, the loudest voice in the club probably has been operating just fine with 24AWG extensions but this is because he's probably not stressing the model enough to notice the difference. However, there are pilots easily capable of noticing. And have you ever noticed a correlation between the loudest voice and lack of flying skills because he mostly runs his mouth But I digress and regardless, 380oz-in probably plenty for a 104" Extra because folks tend to overbuy torque - but - this still serves as an example of how you had better do a little bit of math instead of assuming.

Example 3 - using a 20AWG Y-connector to drive two servos from one channel. This is bad on more than one level. 

First, it doubles the voltage loss. Thus, using the values from example 1, instead of a loss of 0.4V it would be a loss of 0.8V, meaning instead of the servos getting 7.4V they'd be seeing 6.6V (or about 370oz-in). But what if you're using a LiFePO4 pack instead of a LiPo? Now nominal voltage of 6.6V instead of 7.4V meaning the servos see 6.0V so torque is down to 355oz-in. Houston, we have a problem!

Second, there's another, possibly bigger problem because the practical limit for 20GA wire is 7.9A. Thus, while you may think you're only stalling at 3A, the real world is worse because there's resistance in the connectors and the resistance is calculated at 60 degrees. Do the math for 120 degrees inside a model on a sunny day and things get worse. 

Bottom line, our advice is don't use Y-connectors with big hairy servos because it's bad juju (but do the math first instead of crying later).

Practical advice - if you're ever going to make a long run to the back of a model (or way out in a wing panel), a lot of highly experienced modelers will snip the wire lead off about an inch beyond the servo case. Then they'll solder 20AWG wire directly - no connector. Why? it's because the connector has a bit of resistance we've ignored in our examples. Plus, the expert is eliminating about a foot of 22GA wire. Do a neat job with heat shrink tubing and it'll look professional - and - the installation will carry more current with less voltage drop. Win-win!

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