How many Amps can the servo connector handle?

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Usually, an askJOHN begins with someone writing me a question. This time I added my 2¢ to an old forum post from back in 2007, instead.

Q. Am I correct to assume that the traditional servo and battery connectors (JR or Futaba) are rated for 3 Amps only?

A. Old thread, let's revive it . . . the basic DuPont connectors we use in the sport (regardless of the plastic bit being custom molded for Futaba, Spektrum, JR, Hitec, etc.) are all the 0.1" pitch (2.54mm) pins style are rated by their manufacturers at 3.5A continuous, and 5A intermittent.

Fortunately, servo use is an intermittent consumption so we're still mostly OK using it. That said, we've grown to love . . . and hate the connector, in equal measures for their universality *and* because of their limitations.

Let me explain.

I'll get the hate-part out of the way, first. Basically, these were fine connectors back in the day (late 1970s when their use to the sport was introduced). This, when analog servos consumed a few hundred milliamps under load. These days their 3.5A continuous rating has become marginal because individual servos can draw over 5A. Heck, some can consume as much as 10A . . . all by themselves!

Point being, since the same connector is used between batteries and receivers, then several servos working at once can combine, even if individually below the safety margin of the connector on the servo, to exceed the connector's capability at the battery.

Fortunately, instances when this happens are momentary in nature so we're still mostly OK using these connectors *but* we need to think tactically. By this, meaning adopt techniques that let us extend their use until the RF manufacturers get their act together and settle on a more capable universal connector. E.g. a common standard we can all live with instead of introducing a mishmash of connectors. When? Dunno. Honestly? I'm tired of hoping and now fear maybe not in my lifetime, sigh.

So how big an issue is this? These specs are pulled from our DS635BLHV servo (635oz-in) and they stand in for many other servos, e.g. the very fine quality MKS HBL599, or great value Savox SB2290SG, which like others in our own lineup, may also draw significantly more current than that for which the connector is rated on a continuous basis.

Simply put, the OP asked an important question and in the more than 15 years since he asked, servos have become increasingly more powerful (read current hungry). This, because you can't make power from thin air. That, and consumers are still demanding more powerful servos for their larger models. So heads up!

Anyway, we also love these connectors because outside of a few manufacturers who try to rope you into buying their servos by insisting on a special plug (here's looking at you, especially, Futaba), they're basically universal. This is even true of Futaba because as we all know, after a few moments with a #11 blade equipped X-Acto where you trim off the key, means you're good to go using them with a competing receiver or extension. So being universal, the DuPont connector makes it so you can mix and match equipment. As modelers, this is good!

So to the question; these connectors are rated at 3.5A continuous and 5A intermittent. Is 5A the maximum? Nope, you can squeeze more current than this through the connector *but* once the current draw goes above 3.5A heat begins to build up. Worse, the longer the load is drawing current above 3.5A, the hotter the connector gets. To the point I've seen these connectors melted and deformed such that they couldn't be separated and had to be cut off!

Solutions? Short of getting Spektrum, Futaba, Hitec, Jeti, Core, FrSky, et al sitting around a table playing rock-paper-scissors until they decide on a new connector rated higher, perhaps 10A continuous? Sadly,  there's nothing but workaround. Band Aids.

3 work arounds

Perhaps the most popular Band Aid is the *box* system - clicking this link opens another askJOHN addressing this technique (opens in a separate tab so you don't lose your place). Anyway, you've probably seen box systems advertised by many players - and they work. You connect servos, batteries, receivers, gyros and presto, the receiver is no longer carrying all the current load.

These are especially handy for jet modelers because their models may use 15-20 servos. Downsides are well known and understood. Like their cost (hundreds of dollars before adding gyros), which is a biggie for many. Another downside is the added complexity (it takes a bunch of parts to make one of these things work). As we all know, added complexity means much more to go wrong (and this bothers me enough to eschew using them when I can). Complexity makes their use something of a deal breaker unless I actually have no choice.

A 2nd popular Band Aid is splitting the load amongst multiple receivers (explained as well in the above linked article). As it happen, I happen to opt for this route, personally. For example, I have a 40% model which relies on two receivers with one carrying the load for the right side of the aircraft, and the other carrying the load (servos) for the left side (I split rudder and throttle/choke between them).

Basically, the second alternative solution means reducing the current load by half by splitting it up amongst two receivers. This technique especially works for me because in the event of a receiver going TU (tango uniform - teats up), I can fly and land an aircraft successfully with one aileron and one elevator. perfect? Nope, but at this point all I care about is saving my model.

So what I like about this versus a box, which if equipped with a second receiver automatically falls over to the second receiver (so there are two receivers anyway) is this way I eliminate the box. Meaning reduce the number of components in the failure chain. After all, the odds of both receivers going TU on the same flight are astronomical. However, the odds of one component within a box system going TU (and thus bringing the whole aircraft down), are considerably worse (remember, inside the box are many times the number of components ahead of all the goodness of two receivers and batteries).

Anyway, I'm not telling you what to do, or not do, I am sharing alternatives form which you may pick. And if you have another way, share it with us! This, because we all (individually) place our bets and spend accordingly. Anyway, this method (splitting the load between two receivers) is my favored Band Aid with larger models . . . but we're all big boys here so do what let's you sleep best at night. Ultimately, bear this in mind; what we're trying to solve for is called LM (load management). All we're really doing with all this monkey motion is trying to move current around without damaging anything.

Finally, there's a 3rd Band Aid, one that works for the vast majority of modelers (and nearly everything I own above 60" until I get to 33% and larger models). The technique is detailed within this free white paper: Why's my pack got two JR-connectors? and once again, clicking the link opens a new tab so you don't lose your place. Granted, it *is* self serving (in a way) for me to mention this because we offer packs packs made with two leads *but* you can also buy packs with two leads from anybody. Heck, you can even roll your own. How? Simple, by slitting the heat shrink tubing covering an existing pack, soldering on another 20AWG lead (red to red, and black to black), and then heat shrinking new tubing over the pack . . . and Bob's your uncle! Saying this isn't rocket science.

Reason this 3rd method works is receivers use a power bus. This just means you can connect the battery anywhere, not just where it says BAT on the receiver. So when you use a battery with two connectors, since each allows 3.5A of current flow without heat, then having two leads coming from your pack means they combine to flow 7A without head build up. This is almost to a certainty enough for nearly 100% of modelers in our experience (who am I? ProModeler servos). Anyway, it's a nice Band Aid as workarounds go because cost is negligible.

Plus there's the advantage of using two switches (one on each battery lead to the receiver). What's attractive about this lays in the mathematics once again (I'm an engineer, a numbers guy). Point being, we're down to odds once again because the odds of both switches failing on the same flight are astronomical. Means you can - almost to a certainty - safely land a model with switch failure. And for me, this fact alone is why I resort to a pair of convention hobby grade $10 slide switches instead of pricey and complex switches that fail in the on-position (true only if what makes the magic work isn't what fails). So for me the known quantity of the old school slide switch floats my boat so I use them. But once again, you're a big boy, do as you please switch-wise.


I'll close with this, there's another free white paper on the ProModeler site, which you may find interesting: Thoughts on servo connectors & extensions. Once again, this paper many prove useful to you because within it we discusses wire gauge and current flow, plus how to calculate voltage loss, also. Knowledge is power.

Last thing. I advise you to read up on this product, the Glitch Buster. Forgive the hokey name, this is what they're called within the sport (and they're widely available in hobby shops and our price is competitive). You should install one at the load end whenever extensions exceed 30" regardless of whose servo you're using. Basically, I'm saying their use is dictated by physics instead of servo brand names.

For example, the popular and well made Savox 2290SG will exceed 10A - yes, just the one servo - which is a lot of current at full song. And we have servos going over 10A, also. So do others. Heads up because it's a whole new world out there current-wise these days.

STOP THE PRESSES! There's a follow up to this. It's a bit of a birdwalk but what the heck!

Someone responded with . . . Castle Creations ESCs have BECs mostly rated at 5 amps, with some, such as the Talon series, rated much higher. And as far as I can tell all deliver that current, which powers the receiver and all of the model's servos, through the connector you describe above.

A. Do you measure this with a voltmeter or an oscilloscope? Remember, voltmeters are averaging devices. And most respectfully, they (Castle Creations, and others) in fact do offer BECs at various ratings *however*, if you read the fine print (AKA the asterisk within the specifications) you'll learn BEC output is temperature dependent *and* dependent on input voltage.

For example, Castle's very well regarded standalone CC BEC PRO 2.0 (which in full disclosure, we resell), when connected to 6S pack is outputting between 8-9V (to the receiver). At that point, it's offers up between 10-11A of current - BUT - their own footnote discloses this in the fine print, as follows . . .

Ratings are determined with a 5mph airflow at 25° C (77° F)

What's this mean? Simple, it means voltage regulator circuits are measured in the lab with both a cooling breeze and at a standard temperature. Thing is, especially on a hot summer day, assuming it's outputting rated current when mounted within a model would be a questionable decision because then you're presuming there's actually sufficient (any) airflow (who measures?).

Just saying it's wildly optimistic to assume it's meeting the advertised specs, so heads up. Note; import products make no mention of how they gather data for establishing specifications (but I'd bet you a milkshake theirs are also measured under optimum laboratory conditions instead of in the real world). After all, you gotta put lipstick on that pig in order to sell it!

So riddle me this, how many modelers have a fan ensuring cooling of the BEC circuit? These are common in the RC truck world but they're present because the modelers are concerned with cooling the FETs of the ESC, not the BEC circuit. And while you may see fans in high end ESCs for aircraft . . . these NEVER come with a BEC circuit!

Moreover, when the BEC-circuit is built into an ESC, considering an ESC is a device that gets *much* warmer than standard conditions, and considering when it's mounted flat to the airframe (as we all do), can anybody reasonably assume the BEC circuit is actually near the cooling breeze, or is maybe mounted against the model (meaning within stagnant air)? After all, cooling fins exist for the benefit of prolonging the life of the FETs and a BEC is basically an afterthought. And as if it couldn't get worse, the whole thing (standalone BEC) is typically buried beneath heat shrink tubing. So as a practical matter, what do you think the real-world airflow is across the BEC part of the device? I would suspect it's 0mph, e.g. MUCH less than how it's rated.

Add to it . . . on a 95° day (and I note you're in South Florida, so these kinds of days are much more common, than not), I suspect the device is easily seeing 130°F, or more, before the system is even turned to begin generating power. Once it's working, then the internal temperature climbs even more, thus making things worse (FWIW, we've measured in Central Florida so in pretty much the same conditions you experience). So I wonder this . . . do you want to take the over or the under on what real-world current output is?

Finally, take note; high quality standalone BECs like this one (Castle part number 010-0154-00) actually have two leads instead of one (just like our batteries). Why? It's because there is no magic way of outputting 10A of current through a single Dupont connector on one lead (rated at 3.5A continuous) without it overheating. After all, the laws of physics aren't being suspended just for them - agreed?

Anyway, here are a few more white papers, which those curious to learn more may appreciate:

Battery or dedicated pack?
The case against synthetic voltage

. . . and note, with a battery pack, the limiting factor is also the number of connectors (just like with the BEC). But additional leads are even more important because a battery (any pack, any brand) can easily output 30A, or more! For example, eyeball this photo of one of our packs and note the C-rating.

. . . here's how you figure out how much current it'll give you (and this calculation works for all batteries regardless of brand). You take the capacity in A and multiply by the C-rating, so for this pack, max current flow is 2.5 * 15 = 37.5A.

So a battery (ours, or anybody else's) easily beats any BEC you can buy. Moreover, if you make time to review the 2nd linked article above, you'll learn this juice is perfectly smooth, no 1s and 0s because it's clean analog output instead of noisy digital output.

There's a reason batteries are the gold standard against which everything else is measured.

Anyway, the whole analog versus digital with power sources for models is almost identical to HiFi guys arguing about analog versus digital amps. Serious audiophiles buy analog amps and newbies buy digital amps because a) they're cheaper, and b) because they don't know the better yet. This is because to their inexperienced ears both seemingly output pretty good sound.

However, the old dogs (experienced audiophiles) will never buy a digital amp. Digital amps are exclusively the domain of rookies versus pros. And as a practical matter, and I mean no offense, we could argue the exact same thing holds with current sources for our onboard avionics system when comparing BEC circuits (digital) to battery packs (analog).

Look, I'm not trying to sell you anything, I am just trying to clear up issues with current that may cost you a model, or worse.

The fellow subsequently responded with . . .

No, I'm talking about the connection between the ESC and receiver, which feeds BEC power to the receiver and servos, while also connecting the throttle control signal from the receiver to the ESC. The few reports of fires that I could find were related to a Castle Creations model called "Mamba" and appear to have been caused by the motor control circuitry. I found photos of burned up ESC circuit boards but that would have nothing to do with the connectors the OP is talking about.

A. I think we're speaking at cross purposes. The connector is rated at 3.5A continuous. This is fact that stands alone.

Point being, whether someone uses a 10A BEC or a battery capable of 30A, if there is only ONE connector (whether going to the receiver through the BAT port or as part of the throttle lead from the ESC), then it doesn't matter what the capability of the source is, the connector is the limiting factor *if* consumption exceeds 3.5A continuous.

So you explain you've been using the single connector without issue, and in your experience many others have as well. Well, as it happens, I agree with you, 100%. Many modelers are just fine with the one connector.

However, that *you've* not experience a problem is just indicative that you're not using equipment that exceeds the ability of the single connector to handle it. Note; this is nothing to do with *you* as a pilot, or anything, it's just another fact . . . else you'd have seen melted connectors because, once again, continuous current above 3.5A will melt the connector.

So my entire purpose of sharing this information is that we're at a point in time where individual servos are drawing more than 3.5A. That, and to advise folks about this fact because not being wary of using a single lead could cost them a model. Meanwhile, we've gotten sidetracked with the issue of BECs vs. batteries - not my intent at all.

Instead, it's that using two leads (whether from the battery or the BEC) is really advisable for most modelers. Look, I'm not arguing with you, I am just explaining the physics of current flow and offering advice regarding managing it. Again, this isn't opinion, these are just facts!

To your point you've not had a problem. Agreed, everybody does not need to concern themselves with exceeding 3.5A. There are tons of models flying around served perfectly well by the single lead connector. You are 100% correct in this observation, and I agree with you. Modelers whose total current requirements are lower than this don't need to even care about this - as you yourself espouse. But if anybody is ever using a model where the combined servo load means consumption is more than 3.5A continuous, then they're putting it at risk. And not just the model but they're also shouldering the burden of responsibility for hurting someone.

Next, to your comment of blown ESC only being Castle Creation Mamba, respectfully, you're mistaken. Recently, a fellow mentioned his experience on this very forum with a Castle Creations Talon 90.

Sadly, when the ESC failed, because he was depending on a BEC to power his avionics, it crashed. Cost him a nice Extreme Flight model, too. Point of fact being, had he been using a separate battery, he would have merely experience a dead stick landing instead of being a spectator to a crash because the failed speed control took his avionics power, also. But as we both know, this is a side issue to that of using a single connector.

And for what it's worth, his replacement model now uses a dedicated avionics battery pack, instead of the BEC circuit (his own words and thus, proving he learned from the experience). So my observations on depending on a BEC are merely meant to help folks watch out for the gotchas in this sport. Bad enough we do stupid thumb things and crash, but increasing our crash risks simply due to not learning from the experience of others stinks.

Finally, another fact is we all learn (or not) in different ways. Some folks learn from the experience and advice of others. Some only learn when it happens to them. And saddest of all, some never learn. We're in this for fun, right. So let's learn from each other!

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