Nelly Update, Bad news.

[Izord]

So my Nelly 1.2 was working great with a Plasmatomic motor. Fast, good brakes, and good reverse. I was getting about 38 kph with an 8 tooth pinion. Ran it like this for about 2 weeks.

Then I tried two lithium batteries from Atomic mods.It seemed to work great, put it on a speed checker and I was getting 54 kph. Then I put it on the ground and noticed no brakes. And reverse was really weak. And when I tried to go forward after using the reverse, forward was weak too. But when I let it alone for a few seconds forward would recover.

I took the car apart and inspected PCB and Nelly. No visible damage. I checked connections with an ohm meter and all appeared intact. Then I tried voltages with a two-cell (2.4volt) pack. Forward was normal, but reverse had low voltage coming out of the Nelly toward the motor. And Nelly got really hot in reverse,(even with just a 2.4 volt pack).

And also the steering wouldn’t work when I was holding reverse down, but steering would recover on forward.

So it seems like something is shorted out when I try to put my car in reverse or use the brakes. I don’t know if the problem is with the car PCB or with the Nelly. I tried all this even with the motor disconnected and just a meter to check voltages, so I know the problem isn’t with the motor.

This is really frustrating, because I was getting really happy with the car.

Ph2t, do you have any ideas how I can tell what the problem is? Could you tell me what kind of signal I’m supposed to get from the PCB thru the colored wires to the Nelly? If you can tell me that I can cut the signal wires, and check the voltages on them so I can tell if the problem is with the PCB or with the nelly. Still I don’t see any visible damage, so it may be possible to recover something out of my car.

Izord

[betty.k]

my guess is if the fets get hot you may have cooked them. it’s a snowball effect, the warmer they get the more they resist. the more they resist the hotter they get!

[Izord]

Wondering what I should do? Ph2t, any advice?

[codesuidae]

quote:

it’s a snowball effect, the warmer [the FETs] get the more they resist. the more they resist the hotter they get!

MOSFETs have a negative temperature coefficent, meaning that as they warm up, their conductance decreases, reducing the current through them. This is what makes it possible to parallel them for increased power handling without the need for any current balancing circuitry.

BJTs are (usually?) the other way around, heat it up and conductance increases, possibly leading to a thermal runaway.

But yeah, sounds like ya done blowed ’em up. Probably will need to replace one or more of the fets.[/quote]

[betty.k]

hey code, help me understand this ‘thermal runaway’ thing. i’m a bit stumped.

[ph2t]

Izlord, sounds like they’ve blown mate. Ship it back to me and I’ll repair it with some new fets. Or, if you want let me know and I’ll ship you out some replacement mosfets if you wish to do it yourself.

nelly v1.2 can handle a 7.2V setup, I’ve tested that. But a high current 7.4V setup (lipo or 2 sets of li-ions in parallel) can blow it up. It was this feedback that made me design nelly v1.3, it handles the 7.x V setup no sweats.

Your call mate. I fyou want to upgrade to the 1.3v due to your investment in li-ions let me know, we can do a deal.

Although I’ll say this. A li-ion/li-poly mini-z is soooo fast it’s just not funny. As betty will atest, my lipo mr02 needed about 10m to get up to speed. Not because of lack of torque but due to the fact that any faster in acceleration and the bloody thing would spin out. This was with an extra slippery diff and foams.

cheers,

ph2t.

[Author]

Posts