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-   -   MM Pro BEC not holding voltage under load (https://www.rc-monster.com/forum/showthread.php?t=24954)

SpEEdyBL 12.15.2009 03:04 AM
MM Pro BEC not holding voltage under load
 
Upon running my Mamba Max Pro in my 8ight, i noticed that the LED's from both the esc and the transponder would flicker when turning the servo with noticeable performance issues. In fact, adding a slight resistance to the servo caused the transponder LED to turn off completely. My Mamba Monster does not have this problem, and neither does another's mm pro running the same servo (JR 9100s). This can't be solved with a "glich buster" capacitor, because this is a sustained current issue.

Is it just my unit that is bad or is this a common issue? If so, is this/has this been addressed by Castle? I would really like to continue being able to use my MM Pro in my 8th scale. It runs cool and is small and light.

May or may not matter:
Set bec at 6.0 volts
Used Zippy Flightmax 4000 mAh 40C 4s pack w/ Tekin 1900

bdebde 12.15.2009 03:29 AM
Probably just BEC gone bad. A guy at the track last week had exact same prob with a Tekin ESC; to the point that the steering would not even work if used too much (quick back and forth). Send it in to Castle, they will fix ya up.

brushlessboy16 12.15.2009 09:00 AM
are your endpoints set correctly. might be stalling the servo pulling alot of current

BrianG 12.15.2009 10:55 AM
I've tested the BEC on a couple of MMPros and the voltage seems to cut out somewhere around 2-2.5A load:

5.94v unloaded
5.76v @ 0.48A
5.72v @ 0.64A
5.70v @ 0.95A
5.63v @ 1.87A
4.16v @ 2.77A

FastXR 12.15.2009 12:15 PM
Any suggestions on what to do about this? I am concerned about this as I'm planning on running a Ve8 on 6s with the MMP and will be using a relatively high draw 1/8 servo, spektrum rx and transponder all running off the MMP bec. I suppose I could run an external bec...

BrianG 12.15.2009 12:18 PM
Yeah, you can add a 1,000-2,200uF capacitor rated for 10v or more on any unused receiver port. This will help provide extra current for those bursts.

Or, use an external, beefier BEC.

FastXR 12.15.2009 12:37 PM
Quote:
Originally Posted by BrianG (Post 339167)
Yeah, you can add a 1,000-2,200uF capacitor rated for 10v or more on any unused receiver port. This will help provide extra current for those bursts.

Or, use an external, beefier BEC.
Thanks Brian I will try the capacitor route first as its a much easier/lighter/cheaper solution.

J57ltr 12.15.2009 12:41 PM
Patrick actually mentioned a problem with some JR servos here:

http://www.rc-monster.com/forum/showthread.php?t=24293

And here is what he said:

Quote:
Originally Posted by Pdelcast (Post 333075)
WARNING: Long boring technical explanation follows: :sleep:

We've seen the same thing on the bench since this problem was reported. Apparently, some of the JR servos draw peaks (short -term) of several (5-6) amps for a couple milliseconds when changing direction. (We have speculated that the drive transistors in the servo are shooting-through during direction changes, but haven't yet verified that.) It's just barely long enough of a surge to trip the BEC over-current protection in the MMPro, and it shuts off the BEC for about 50ms. Adding a big capacitor to a servo connector (like a stutter-stopper) supplies enough current during those big surges to keep the MMPro from tripping the over-current on the BEC.

We are bench testing a hardware change that will add a very slight delay to the over-current shutdown on the MMPro to compensate for the JR servos.

Note that we have only seen this behavior on two specific JR servos, and adding some capacitance to the BEC line seems to be a good fix for the issue.

Patrick

Jeff

BrianG 12.15.2009 12:48 PM
Yeah, that particular servo has issues, but any high torque/speed servo can pull more than 2.5A depending on setup, tire size, etc.

whitrzac 12.15.2009 01:27 PM
that's why I sold my MMp....

FastXR 12.15.2009 01:43 PM
I typically use Ace servos which are generally considered power hogs, so I would guess it probably pulls some decent amps. I'll try the cap first and go from there.

I have an MMP in my Xray T2 and so far with a Hitec 6965HB (think thats whats in there), its performing flawlessly, althought that servo and car doesnt put anywhere near the load on the bec that the ace in an 1/8 scale would.

J57ltr 12.15.2009 02:37 PM
Well Patrick did say that the BEC in the MMP is just as capable as the one in the MMM.

Jeff

SpEEdyBL 12.15.2009 04:50 PM
Sure a capacitor might help with the performance issues, but when the servo is turning around a corner that takes half a second or more to complete, the transponder will remain off the entire time, and a capacitor will have run out long before. I didn't mention the fact that 1/3 of my laps were skipped during the MM Pro's maiden race in 1/8 scale. (It seems to be fine in stock touring with a less powerful servo)

I am not going to run an external bec, or a loaner transponder, which sort of defeats the purpose of running the MM Pro over th Mamba Monster, which has been trouble free.

@brushlessboy,

The end points were set correctly with 5% extra wiggle room. The lights flickered just by turning the wheels in the air, there was no binding, and a slight touch of the hand to add constant resistance to the servo would turn the transponder led completely off.

Pdelcast 12.15.2009 05:06 PM
Quote:
Originally Posted by SpEEdyBL (Post 339210)
Sure a capacitor might help with the performance issues, but when the servo is turning around a corner that takes half a second or more to complete, the transponder will remain off the entire time, and a capacitor will have run out long before. I didn't mention the fact that 1/3 of my laps were skipped during the MM Pro's maiden race in 1/8 scale. (It seems to be fine in stock touring with a less powerful servo)

I am not going to run an external bec, or a loaner transponder, which sort of defeats the purpose of running the MM Pro over th Mamba Monster, which has been trouble free.

@brushlessboy,

The end points were set correctly with 5% extra wiggle room. The lights flickered just by turning the wheels in the air, there was no binding, and a slight touch of the hand to add constant resistance to the servo would turn the transponder led completely off.
The surges pulled by the servo are very short -- less than a few milliseconds. The capacitor will supply the current for the very short high amp surges that are tripping the over-current.

Patrick

Unsullied_Spy 12.15.2009 05:47 PM
Quote:
Originally Posted by BrianG (Post 339167)
Yeah, you can add a 1,000-2,200uF capacitor rated for 10v or more on any unused receiver port. This will help provide extra current for those bursts.

Or, use an external, beefier BEC.
Do you really need even that much? The brevity of the bursts and the bursts only being a few amps you should be able to get by with something smaller like a 220, right?


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