Smokin' Mamba BEC?

[microgoat]

This is the second time this has happened to me, so it can't be a coincidence. I've got to be doing something wrong, but I can't figure out what.

My setup is:

Mamba-25
Outrunner, one Axi, one Park 400
Six cells in one rig, eight in the other
Same radio gear in both rigs, down to the transmitter.
Disabled BEC on the Mambas (removed red wire from the harness of a 3-inch extension (to keep programmability thru Castle Link), clipped switch from the circuit board and insulated the wires from one another).
Electronics on both rigs fed thru a tap on the 5th cell from each battery pack.

Here's the weird part. Both my Mambas have melted the connection between the servo extension and the Mamba ESC. The 8-cell rig actually smoked at this point, and I went back to running it on 6 cells. Obviously there's some voltage getting through, but where, and why?

On the 6-cell rig I eliminated the extension and everything's kosher now. Would eliminating the center wire from the extension at the ESC end help on the 8-cell rig (It's currently only clipped at the receiver end)? I can't imagine there's enough juice there to jump the gap, especially with the switch removed.

People run extra cells in Mambas all the time without melting them, so what am I doing wrong?

I could call Castle, but it's after hours, and you guys are pretty smart. Help a brushless brother out!

 

[slobin3d]

did it hurt the radioi gear at all?

 

[microgoat]

Aside from charring the harness, it didn't even hurt the Mamba. It runs just fine on six cells.

When I plug in 8 cells, there's no ringtone, just smoke from the receiver connection. Plug in six, it rings and everything's fine.

 

[slobin3d]

plug a voltmeter between the servo extention wires w/ the extra cells and check the voltage a receiver can handle 7.2 volts w/ no prob usualt but 8.4-9.6 would cause smoke maybe you your tap off the 5th cell is shorting

 

[microgoat]

Now that's something to think about. The 8 cells are split up into two packs, 6 and two. The BEC comes off the fifth cell of the 6-pack, and the two packs are joined by a series harness. Theoretically, that shouldn't affect the BEC tap, but it's worth looking into.

 

[JasonInAugusta]

Goat, I think I know what's wrong.

I'll draw up a diagram to bounce the idea off your noggin'.

Be back in a bit.

 

[microgoat]

Receiver voltage is unaffected by the extra cells, so that's not it. And I double-checked my series harness, so that's not it either.

 

[slobin3d]

give us the answer jason were waiting lol

 

[JasonInAugusta]

Take a look. If the amp draw from the batt to the Mamba increases (that's I2), that causes total circuit current to increase (It).

Then you've got the two tied to one another through the servo leads...which creates a pretty unique circuit. :lol: It effectively bridges the two current paths (I1 and I2) together through the RX and ESC...even though you've yanked the red lead.


Try a standard RX pack (which would eliminate I1 and the bridge between I1 and I2) and see if the problem persists.

$5 says this fixes it.

 

[slobin3d]

the thing that confuses me is even with that idea (which makes sense) when the esc is idle it should work fine but

"microgoat Aside from charring the harness, it didn't even hurt the Mamba. It runs just fine on six cells.

When I plug in 8 cells, there's no ringtone, just smoke from the receiver connection. Plug in six, it rings and everything's fine."

 

[JasonInAugusta]

8 cells increases applied voltage...which increases current.

I =E/R

He's bridging the two current paths. When he does the puny servo wires/plug can't hang...they puss out and melt.

 

[slobin3d]

goat i have a 5 cell flat pack you can have if you want it 1200mah

 

[microgoat]

I'm not quite sure I follow, but then I'm not an EE. I would think the signal would be more or less independent of the voltage load of the system. The operative word there being "more or less."

Wouldn't a receiver pack produce the same type of differential voltage at that point as the 5-cell tap? If the Mamba is trying to feed BEC volts to the receiver that the receiver is already getting (and I think that's what's happening) then you'd still have the same problem, but with a different source.

And here's some more freaky-deekiness. It did the same thing on the six-cell rig, only not as bad. The problem seems to have gone away when I eliminated the extension and plugged directly into the receiver. Or if the problem is still there, at least the receiver is undamaged, let's put it that way. Maybe the receiver can just handle more heat than the servo extension.

Testing the 8-cell rig with the main pack dumped, and the two cells not charged, everything worked fine. Only after a full charge did the smoking begin.

The first meltdown happened when I turned on the Mamba's switch instead of the switch on the 5th-cell tap. After that, I removed the switch and use a jumper when programming.

Unfortunately, my only receiver pack is permanently mounted inside the tow truck's toolbox. But since I have to go buy a servo lead anyway, I can borrow one from the LHS and see if that fixes it. One of my perks :lol:

Yes, that's why everything you buy has extra staples in the package

 

[JasonInAugusta]

The RX pack would eliminate the bridge between I2 and I1.

Look at your pic. The negative lead is what's melted. That lead is bridging the two current paths.

Remember, negative is referred to as common...meaning it's a common point in the circuit.

 

[slobin3d]

lets see if i understand this right, since they share a common ground it isnt somuch that the bec or battery is forcing to much power into the circut as it is letting the power flow through unresticted hence overheating the link??

 

[JasonInAugusta]

You win the prize!

No idea what it may be. :lol:

Maybe a night with Dirk in his pink Tu-tu. :lol:

 

[slobin3d]

at least im keeping up with your train of thought by having the rx pack you lose that open connection

 

[JasonInAugusta]

There's no such thing as an open connection.

It's either connected (short) or open...not both.

 

[slobin3d]

ive heard about that tu-tu:???:

 

[slobin3d]

There's no such thing as an open connection.

It's either connected (short) or open...not both.

now you just getting techincal