bitPimps.lixlink.com

Good work to all of you guys
And I agree with Sg219 - my head hurts, but you're making me drool!

ph2t wrote:I bet code that in the ZZMT case, stacking mosfets would have an even greater difference between single and double stacks vs. the same setup in a BCG or ZZ.

I am expecting the opposite. If in a single cell car there is insufficent gate drive to fully switch the FET, then the value of Rgs will be large and the effect of paralleling several will be more pronounced.

However, in a MT, with its much higher voltage drive, the FET will easily be fully switched and will have a low value for Rgs. The effect of paralleling them will therefore be reduced.

If Rgs in a 1.2v car is 90mOhms, then the current through the high and low side devices, with a 1.2 ohm motor in the middle would be 870mA. If three FETs are stacked the Rgs would be around 30mOhms, resulting in a current of 950mA, nearly a 10% increase. That would take a 5 MPH car to around 5.3 MPH.

If Rgs in a 4.5v car is 30mOhms (the minimum for a 2502), then with a 1.2 ohm moter you'd get 934mA. Triple them up again and you'd get 950mA, about a 2% increase. From 5 MPH to 5.05 MPH.

ph2t wrote:The Vgs drive is 3V in a zz/bcg. This is a borderline mosfet drive at best.

Well, its low, but it is easily over the 2v level required to get a sufficent gate charge into a 2502.

ph2t wrote:Stacking mosfets increases the input gate capacitance between the Gate pin and Source (known as Ciss on most datasheets). This decreases the already borderline drive's ability to charge the mosfets.

The total gate charge required will increase the switching time but I don't think thats a concern here, particularly on non-propo cars. Even on an SE with a 100kHz PWM the switching time of the FET is at least an order of magnitude or two shorter than one duty cycle (10mS) The inputs we're connecting to the gate were sourcing 10-15mA (at least on the ZZSE) to saturate the original transistor, so I don't think they'll have a problem with the FET gate charge.

But I'm no EE, so I wouldn't be surprised if I'm missing something critical. All I'm saying is that I don't see anything in theory that would lead me to believe that there would be a big increase performance from stacking FETs with an Rds that is already low relative to the motor, presuming the FETs are effectively driven in the first place. But I also don't know all the theory, so I'll be doing some measurements to see why my theory doesn't seem to match up with what ya'll are reporting.

crazydave wrote: i don't feel top speed is relevant to me, since I rarely use it, I'm more interested in the low-mid power band.

Its the same thing really, a 10% increase in motor current gets you about at 6% increase in stall torque and peak RPM (depending on motor effiency).

the best I can advise mate is to do the tests and let the results speak for themselves.....

cheers,

ph2t.

Well, I got the stacked MT from Dave, and he wasn't kidding about the wheelies!!! With an orange motor and red gears, and my sticky lego tires, it pulls a wheelie (not a big one, but it's still a wheelie) FROM A DEAD STOP! Anyway, I love this thing, and thanks Dave!

Ya know, I was a just sit’n here reading this post and thinkin to my self that you-all are just amazin, I sho wish I could do dat to my truck too, but I ain’t got none of them there itty bitty FETs. I just got these here big ones.

Ph2t them ther's some mighty fine batteries, sho wish mine would get here so as I woulden’t have to run around with a big one taped to the top.

Holy crap, Ratman!

You shoved a Xmod PCB in there didn't ya? You upgraded and stacked the FETs too, huh? You're a completely sick and twisted individual, aren't ya?

You're a bad bad person for making me want to kill my Xmod.

you guys are too much

(jealousy)

It's and Iwaver PCB, I just used an Xmod crystal since I had to solder it on.

just give me the low down and how the hell you did the steering!!!!!!


ph2t.

ph2t wrote:just give me the low down and how the hell you did the steering!!!!!!
ph2t.

The hard part is to not damage the pot while you’re sanding it down and installing the gear and shaft. Also everything has to be lined up perfectly there is no margin for error. The pot has to be held steady as a rock, and the gears must mesh perfectly, absolutely no binding allowed. Other than that it’s simple. It was one of your photo’s that gave me the idea.

You also need to remove the centering spring.


p.s. What size/brand LiPo did you use? One or Two? I went with two E-TEC ET80’s, since I couldn’t find anyone with the 90’s, and anything else I found looked too big.

Check it out, you can order an Iwaver PCB by itself for $22.50.
http://www.toyeast.com/v2/proddetail2.asp?pdtid=AIW4V15

Apparently that's the most expensive part of the car.

Looks like it already had a steering pot attached.

LOL, the PCB has been out for about a month or two now. I thought everyone knew.

Anyways, ncie rides all.

I just did my ZZMT FET Mod (GAWD! I cut the crap out of my thumb!)

Wow. I double stacked the n and p channel FETs and torque is quite amazing now. I'm still not getting popping wheelies from dead stop on orange/orange, but it definately has a lot more torque. It rolls in corners now! I can't wait for my lipos to come in. Then it will own all!

Next step is to take apart another ZZMT I have and make it RWD, Stacked FETs and remove some windings from an orange motor if I can pull it off.

-JNY

pimpsmurf wrote:I'm still not getting popping wheelies from dead stop on orange/orange, but it definately has a lot more torque.

If you want to pop wheelies its as simple as adding a 3rd cell bro. A 3rd cell w/ orange is fuckin crazy fast, hell from a dead stop you can ride the bumper on red and if I remember right even green gearing. There is a tutorial here, but trust me, if I can do it, anyone can.

Finks wrote:

pimpsmurf wrote:I'm still not getting popping wheelies from dead stop on orange/orange, but it definately has a lot more torque.

If you want to pop wheelies its as simple as adding a 3rd cell bro. A 3rd cell w/ orange is fuckin crazy fast, hell from a dead stop you can ride the bumper on red and if I remember right even green gearing. There is a tutorial here, but trust me, if I can do it, anyone can.

I think he was making reference to Viperout's claim. I did viperout's car, and his orange motor was just a tad hotter than either one of mine, and it did indeed pull wheelies from a dead stop. My FET modded monsters don't pull wheelies from a dead stop, but they do have a tendency not to keep their front wheels on the ground. They're torquey boogers.

Tri-cell with FETs would be insane with out some kind of propo control. That's why I'm done modding my monsters until I get around to ordering an Iwaver PCB. I'm also thinking about a chassis stretch to keep the front end down.