BEC Question

[step child]

My son has a Wraith and we put a solar servo in it and it getting pretty warm. From what I have read about the BECs it is not getting enough voltage so the BEC should take care of that. With my stock electronics in my Hohcho would there be any benefit to using a BEC?

Thanks J.

[Olle P]

1. Steering servos in crawlers do get warm, because they work a lot.

2. I don't know the recommended voltage for your servo, but the "problem" a BEC is (originally) intended to solve is that the battery voltage is to high to feed the Rx (and servo) directly.

3. In crawlers another problem is that the servo will eat lots of current, and the BEC built into most ESCs can't handle that much current without causing the BEC part or entire ESC to overheat or cause other problems.

The solution for 3. is to feed the servo (and possibly the Rx) separately, bypassing the ESC's BEC.
If you have a "high voltage" servo and a 2S battery you can feed the servo directly off the battery. Otherwise you need a separate BEC that can handle ~5A peak and ~3A continously.

[adz]

i recently realised that most rigs needs bec's especially with high torque servos like the alturn hi torque. it aslso gets the most out of the servo, no point running it 75%

[step child]

This is the servo thats is in the Wraith,

D772
Speed @4.8v	0.22
Speed @ 6.0v	0.2
Speed @ 7.4v	0.17
Torque (oz./in) 4.8v	305.8
Torque (oz./in)6.0v	368.35
Torque (oz./in)7.4v	417
Torque (kg/cm) 4.8v	22
Torque (kg/cm) 6.0v	26.5
Torque (kg/cm) 7.4v	30
Length	41.3
Width	21.2
Height	40
Weight (g)	64
Gear Material	Metal
Bearing Type	Dual Ball Bearings
Digital / Analog	Digital
Motor Type	coreless
apply for	Helicopter/plane/boat/car

And it looks like the stock esc only puts out 5v.

AE-2 ESC (Electronic Speed Control) with Forward and Reverse and Drag Brake (Part # AX24259)

• Input Voltate: 6 Cell Nickel or 2s Lipo
• • Lipo Cutoff Set Up From Factory
• Dimensions: 1.7 x 1.24" (43.2 x 34.5mm)
• Weight: 1.6oz (45g)
• Motor: 19T or greater
• On-Resistance FET: 0.0018ohms
• Rated/Peak Current: 106A Peak
• Braking Current: 106A
• BEC Voltage/A: DC 5V/2A
• PWM Frequency: 6Khz

I read on the internet that if the volts going into the sevro are on the low side it'll heat up, thats way I was thinking with the CC BEC I could increase the volts ( 2s 7.4v lipo ) and have the sevro run cooler and get the max torque out of it.

After looking at all this I don't think the Honcho would really need one.

Thanks J.

[Colgout]

With 30kgcm of torque and capable of running at 7.4v, you will better to have a external BEC to maximise the servo.

[JokersWild]

Originally Posted by Olle P

1. Steering servos in crawlers do get warm, because they work a lot.

2. I don't know the recommended voltage for your servo, but the "problem" a BEC is (originally) intended to solve is that the battery voltage is to high to feed the Rx (and servo) directly.

3. In crawlers another problem is that the servo will eat lots of current, and the BEC built into most ESCs can't handle that much current without causing the BEC part or entire ESC to overheat or cause other problems.

The solution for 3. is to feed the servo (and possibly the Rx) separately, bypassing the ESC's BEC.
If you have a "high voltage" servo and a 2S battery you can feed the servo directly off the battery. Otherwise you need a separate BEC that can handle ~5A peak and ~3A continously.

#2 is somewhat wrong. Is correct if you are referring to internal BEC. External BECs were designed to send a separate voltage to the receiver directly powering it and the servo by bypassing the lead from the ESC. Many for quite some time have changed that over to re wiring the BEC to run directly to the servo and run the receiver off the lead from the ESC connecting only the "signal" wire from receiver to the servo.

Quite a bit of Rx do not like to have over 6 volts directly to them so the best way to use an external BEC for what we are going for here on crawlers is to do it the way stated above, either by hard wiring the servo wires to the BEC or to build a jumper harness to go between the 3 (Rx, servo, bec). This way you can control exactly what amount of voltage your servo sees.

And the BEC you would be looking for would be a castle creations 10 amp external BEC.

So to answer the OP question. Yes an external BEC is the way to go..... there are different benefits to it, but will not fix the servo getting warm or hot. That was answered above on why that is happening

[Postclanker]

I have a solar servo, definitely run a BEC ...actually run one in every rig I own.
It's a cheap part to protect your investment...I know the Solar is dirt cheap but as soon as you have to replace a couple, you could have saved yourself the hassle and down time had you got a BEC from the beginning.

To me it's just common sense...you know, like a Homer forehead slap "DOH" !

[Olle P]

Originally Posted by JokersWild

#2 is somewhat wrong. Is correct if you are referring to internal BEC. External BECs were designed to send a separate voltage to the receiver directly powering it and the servo by bypassing the lead from the ESC.

That doesn't defy my statement above.
The original reason for BECs (internal as well as external) is to reduce the voltage to one that can be handled by the Rx. Period.

The reason for external BECs was to be used when the speed controller didn't have one.

Originally Posted by JokersWild

And the BEC you would be looking for would be a castle creations 10 amp external BEC...

... because it can regulate the output voltage to levels above 6V.
But if you're using a 2S battery you won't need an external BEC but can feed the servo straight off the battery.

[JokersWild]

Originally Posted by Olle P

That doesn't defy my statement above.
The original reason for BECs (internal as well as external) is to reduce the voltage to one that can be handled by the Rx. Period.

The reason for external BECs was to be used when the speed controller didn't have one.

... because it can regulate the output voltage to levels above 6V.
But if you're using a 2S battery you won't need an external BEC but can feed the servo straight off the battery.

Fully charged 2s Lipo is at 8.4 volts.

Figured I should have posted that in the past not all esc s had bec s...... But then again most prolly never seen and used a manual speed controller so it would confuse most.

[step child]

Originally Posted by JokersWild

Fully charged 2s Lipo is at 8.4 volts.

Figured I should have posted that in g past not all esc s had bec s...... But then again most prolly never seen and used a manual speed controller so it would confuse most.

I still have the eraser and brush in my rc bag from the old speed controls I was thinking about just hooking the sevro to the battery, but if a fully charged battery puts out 8.4v could that be bad?

Thanks guys learning a lot....

[Trubble]

Originally Posted by step child

but if a fully charged battery puts out 8.4v could that be bad?

Depends, is getting to buy a new servo with bearings and metal gears considered "bad"?

Before I got my BEC, I was having problems with power outages. Ran an unregulated 2S into my receiver, which fixed them right up. Both servo and receiver were fine at 8.4 volts temporarily, but I didn't have it like that for long, maybe two packs (an hour runtime). Put in a BEC set for 7.0 volts (ran through the receiver). Servo temps were constantly in excess of 210°, and it burned out not long afterwards. Personally, I blame the plastic bushings bagging out and the servo getting the shakes when trying to center for the burnout. Don't think the extra voltage on a 6 volt servo was a particularly wise move, either. Sure did have some extra zip when it was on 8.4, though! (Seven volts was barely discernible from six, imo)

Now the comp crawler has a 3051 servo with the BEC direct to the servo and set at 7.4 volts, (ESC powers the receiver). And the trail truck runs the BEC through the receiver, set at 6.0 volts, with the stock servo. All seems well with both trucks.

I just like the idea of getting as much load off the ESC as possible. Even stock servos can really heat up your internal BEC circuit. And you're not just frying that part, but you're also cooking any nearby ESC components, too.

Suppose, you could just run things as they are, and see who wins the race. Will your bushings bag out and servo fry first? Or will the ESC's internal BEC give up the ghost and blow smoke first? And if the ESC goes first, will it still be operable? (other than it no longer having an internal BEC)
Now you almost can't run a BEC because you curiosity's piqued, isn't it? Y/W.

[Johnnysplits]

I just bought a D772. In my bench testing, it did get hot, but I was working the shit out of it. It does pull the numbers that it claims at 7.4v and at stall it drew 2.71A so it's a winner so far. The best benefit of using a separate bec is what Trubble stated...taking the load off the esc. The esc has enough to handle just running the motor. Most esc's don't put out enough amps over and above what servo's demand and that's when you have problems.

[AutoCrawler]

Originally Posted by Johnnysplits

I just bought a D772. In my bench testing, it did get hot, but I was working the shit out of it. It does pull the numbers that it claims at 7.4v and at stall it drew 2.71A so it's a winner so far. The best benefit of using a separate bec is what Trubble stated...taking the load off the esc. The esc has enough to handle just running the motor. Most esc's don't put out enough amps over and above what servo's demand and that's when you have problems.

I'm interested in what you used to measure the stall current. A multi-meter or do you have a servo tester of some kind?

Originally Posted by step child

AE-2 ESC (Electronic Speed Control) with Forward and Reverse and Drag Brake (Part # AX24259)

BEC Voltage/A: DC 5V/2A

I read on the internet that if the volts going into the sevro are on the low side it'll heat up, thats way I was thinking with the CC BEC I could increase the volts ( 2s 7.4v lipo ) and have the sevro run cooler and get the max torque out of it.

After looking at all this I don't think the Honcho would really need one.

Thanks J.

Stall current is the the number you need to know to evaluate whether the BEC you are using is adequate for the servo you want to use. It is the nominal maximum current draw of the servo motor. It was missing from the specs you quoted so you can't determine if it needs an external BEC from what you quoted.

If the stall current of the D772 servo is 2.72A at 7.4v the its probably around 2A at 5v which means that the AE-2 ESC BEC is marginal at best since it is only rated at 2A.

If you bind up the steering a lot you are probably pushing the servo into or close to stall which will over tax the BEC. It will be working at or above its quoted limit which may cause it to work poorly or fail. It will overheat and may not be able to deliver the stable voltage the RX needs to function correctly.

Ideally you want the BEC to have some capacity margin above the max current your servo can draw. A 5A BEC would be adequate here. The CC BEC is nominally 10A so more than you need but it also has the advantage of having programmable voltage level and spare capacity if you want to run some additional servos. You can get a cheapo 5A switching BEC for less than $10 but the CC BEC is only about $20 so that is why everyone likes it.

The servo should run cooler if you run it at higher voltage because it will need to draw less current to do the same work. However if you still bind up the steering a lot when driving forcing it into stall it will probably get hotter faster since the stall current is higher.

Also note that if you run your ESC at a higher voltage it is likely that the internal BEC will perform worse and be able to deliver less current before it overheats because it has to drop more voltage to maintain 5v. This is another reason an external BEC becomes necessary.

[Johnnysplits]

Originally Posted by AutoCrawler

I'm interested in what you used to measure the stall current. A multi-meter or do you have a servo tester of some kind?

I use a Turnigy 130A power analyzer inline between the power source and servo. I have a test rig made up of a piece of 1"x4" pine, an aluminum servo bracket, a 4/40 pushrod, and a digital luggage scale. Power comes from a 2s lipo through a Castle 10A bec so you can easily change up the output voltage. I use the rtr radio equipment to control the servo. I do operational and holding tests.

[step child]

[QUOTE=Trubble;4353079]Depends, is getting to buy a new servo with bearings and metal gears considered "bad"?

Hey now, if I read it on the internet ( on another forum ) its going to work fine .

This is why I ask so many questions before I screw up something. I really like all the information I have gotten from this, you guys ROCK .

I won't be a cheep ass, I'll buy the BEC and the programmer for it. Probably have some more question about it .

Thanks J.

[Olle P]

Originally Posted by JokersWild

Fully charged 2s Lipo is at 8.4 volts...

Your point being?

The servo rating of "7.4V" means that it's designed to be fed directly from a 7.4V (nominal voltage) battery, for example when used in a nitro truck.
Many servo manufacturers even use speed and torque values measured that way when they present the servo capabilities. (Meaning that if the servo is presented to provide 250 oz-in @ 7.4V it's actually only 250 when fed straight off a fully charged 2S LiPo (above 8.0V).)

I know this by doing the tests myself...