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Thread: Weak BEC on MMM

  1. #1
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    Weak BEC on MMM

    I got a Hitec 7955TG servo for my E-Maxx Brushless which I installed yesterday.

    To my surprise, the receiver lost connection all the time afterwards and the servo did weird things. I tried the Servo on my Slash with the VXL-3s ESC and it worked as it should.

    So, the BEC of the Mamba Monster doesn't provide enough power for that high-torque digital Servo and I just ordered a Hercules BEC to be able to run my new servo. I just don't understand that the most powerfull ESC has such a weak BEC, it seems logical theat the more powerfull the ESC is, the more powerfull will the other electronics be!

  2. #2
    RC Racer
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    I have 2x Hitec 6985 coreless digitals on my e-maxx. Combined, they're about 300 oz/in of torque on 5v. I've never had a problem with the MMM's BEC. I think it's supposed to be good for 5a, continuous I think too.

    I'm surprised your having problems with it.

  3. #3
    RC Turnbuckle Jr.
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    Yeah, very odd for the MMM BEC to behave like that- not unheard off but very rare- it's usually the MMPro which gives people issues.

    Have you tried adding a glitch-buster capacitor to your rx to help keep the current boosted when the servo is working hard, simple fix but it works for many people.
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  4. #4
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    The mamba monster bec doesn"t like the 7955. My buddy was having problems with his. When he called castle as soon as they heard he was running that servo they told him to use a bec with his mamba monster. I instaled a bec as soon as I put the 7955 in my truck. I have it set to 6v.
    When I was running that servo with my mamba max pro on my slash 4x4 I had to use a castle bec with that. The mmp can be adjusted to 6v aswell, if I remember correct.

  5. #5
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    i had issues as well when i added my savox 1256tg servo. i think my issues were because of the flysky receiver i am running, but the novak glitch buster capacitor completely fixed the problem...

  6. #6
    RC Racer
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    I just tried applying a constant current load to my v3 MMM. The esc was not plugged in to the RX. I don't trust servo connectors for the sustained currents I wanted to run, so I pulled the positive and negative pins out of the housing and connected them to my CBA with strong aligator clips.

    The BEC puts out a perfect 5.50 volts (no load).

    Under a constant 3 A load, it dropped down to 5.1 volts before stabilizing.

    Under a constant 3.5 A load, the voltage dropped too far, and the esc rebooted itself.

    Now the sustained load in my test is not a very feasible real world senario. Servo's don't draw continuous amounts of current like that. So getting 3 amps sustained with the bec holding 5+ volts is pretty good. Any load > 3A is going to be brief enough that there shouldn't be a problem. Digital high torque servo's though can draw significantly more than 3A, especially if stalled. Really high current spikes could cause either the ESC, or the servo itself to reboot. I can see a capacitor adding enough of a buffer to stop the voltage sags that cause the reboots.

  7. #7
    RC Turnbuckle Jr.
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    Do you live in a science lab by any chance?...

    I think the spec sheet for the MMM says 3amp BEC rather than 5amp, so it seems to be fine for most setups, just these real high-end servos it doesnt like. I wouldnt mind seeing the same test done on the MMPro for comparison, that esc must struggle with ~2.5amps it seems..
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  8. #8
    RC Turnbuckle Jr. danielhr77's Avatar
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    IMO, the MMM BEC is not that good... For example, I have a MMM on my Summit (and two more in other trucks), and all the lights flicker with fully charged batteries... That doesn't happen with the Velineon ESC for example or other cheap ESCs... For what is cost, I would expect a much better BEC...
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  9. #9
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    Call my cynical, but I'd say Castle do it on purpose. Funnily enough, they sell two BEC's, http://www.castlecreations.com/products/ccbec.html
    They obviously want people to buy their BEC's so they supply a rather weak one in the TOTL (top of the line) ESC's.

    On the other hand, Traxxas do not sell any BEC, so they are kindof forced to supply a decent one in the Velineon esc's.

    Oh, and the cheap EZrun esc's have a terrible BEC - not all cheap ESC's come with a good BEC.
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  10. #10
    RC Turnbuckle Jr.
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    Orrrrrrr, castle might just sell separate BECs for the guys running HV plane/heli/boat escs which are opto-iscolated ( no built-in BEC ), this is standard practice in the world of HV r/c aircraft and boats.

    It would hurt castle's sales alot if the majority of customers complained about their escs not being able to run a decent servo, plus there is the minor issue of there being dozens upon dozens of alternative brands of BEC people could buy besides castles 2 units...
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  11. #11
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    I first wanted to by a CC BEC, but it's currently sold out everywhere (perhaps we know why...), the CC BEC PRO seemed a little overkill for a single servo.

    Somewhere I read about the Hercules, so I ordered that one, it should fit between the MMM and the motor/gearbox assembly. To get power to it, I'll try to just shave the isolation off a small part of the power cables leading to the ESC and solder it to those big wires. I'm not a professionnal when it comes to soldering...

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