upgraded FETs

[rmyc]

yeah... the stock fets can handle up to 10v @ 2.9ah

[neurokinetik]

Originally posted by FESCSteve
I wonder what the gain on adding the higher current capacity fets will be? I know that the on resistance will be lower, but you will probably negate any benefit with the long wires to the FET board.

Not true, as those wires are only carrying the signal to turn the FETs on. From that board, they go right to the motor, and I can also use thicker wires to make that connection as well, further cutting losses. Cutting power losses is only a beneficial side effect of what I am trying to achieve. I want to be able to run any motor I choose. I want to achieve that goal wihout having to build big towers of FETs that won't easily fit into the XMod chassis.

Originally posted by rmyc
yeah... the stock fets can handle up to 10v @ 2.9ah

...and you have found this information where? I'm assuming you mean 10V @ 2.9A, because AH is a rating of battery capacity, not current capacity. 10V @ 2.9A is 29W. I highly doubt that the mystery manufacturer has come out with chips that can handle 10x the current without adding better forms of heat dissipation.

[raven5x]

Wow, I love the use of the board!! So what type of results should you see when you replace the FET's?? I can get them wholesale(from a friend) but I don't know if theyre worth changing. I'm assuming that the better the FET combination> the more the wattage put into the motor> thus faster RPM in the motor and shorter lifetimes for the circut??

I'm not at all new to MOSFET chips becuase you can take them out of old car amps, but what ya'll do w/ ur XMODS is ultimately crazy!

[neurokinetik]

It works!!!

I just got my new N channel FETs in, soldered them in place of the other ones I tried, and it works! No delay issues, just smooth power. Bench testing the setup with a Stage 1 motor, I could hold full throttle for a couple of minutes, and the heatsinks on the transistors are still cold to the touch, while the motor is warm.

So here are the transistors you need:

N channel 2x IRL3705N $2.60 each in single quantities
P channel 2x IRF4905N $3.21 each in single quantitiies

Total cost of the parts excluding the small circuit board (about $3) $11.60

Here are the specs...

IRL3705N:
VDS: 55V
ID (Cont): 77A
RDS(on): 0.01 ohms
Max power dissipation: 130W

IRF4905N:
VDS: -55V
ID (Cont): -64A
RDS(on): 0.02 ohms
MAx power dissipation: 150W

Compare these to the numbers on the IRF7389:
VDS: 30V
ID (cont): 7.3A (N channel) -5.3A (P channel)
RDS(on): 0.029 ohms (N channel) 0.058 (P channel)
Mx power dissipation: 2.5W

What does all this mean? Well it means that for the price of a 3x3 stack of IRF7389s, you can get the performance of a 10x10 stack*. Bring on the hottest motor you want, I think you'll find that the batteries will now become the limiting factor in how much current you can draw.

Oh yeah, I got all my FETs at www.digikey.com


*Max power handling and current capability is 10x that of the IRF7389, but the RDS(on) value is only half to a third of that of the IRF7389. This means that you don't get as much of the benefit of the decreased resistance as you would get with a set of stacked IRF7389s. However, we are talking about really small amounts of resistance here, small enough that you should be worrying about the wires connecting to your motor more than the RDS(on) value.

[platinum_player]

wow that is amazing, i wish i knew how to do this upgrade because i got an mini-z 4th Generation Modified Motor and i would love to put it in my xmod, do you know anyone who can do this for a small price, i would love to have this in my car so i could run just about any motor that i want thnx

[neurokinetik]

I'll post some new pics and findings later on tonight, but last night, I revised the FET board, and rewired the whole thing again. Now the MOSFETs lay flat on the board, and the board itself is only half the size that it was before. I improved the routing of the wiring, making it easier to install the FET board where I want it, and also switched to stranded CAT5 for the wires, which are more flexible and easier to work with. It fits well enough that I can actually get the C5R body on with it in place, though it doesn't quite sit as low as it should. Stock XMod bodies should fit no problem, I just need to transfer over my body mount and wheels from the RSX and shorten the wheelbase to see if it fits.

[TypeZer0]

why wouldn't the lights work? i thought the lights would be wired separate from teh FET circuits.

[neurokinetik]

Originally posted by TypeZer0
why wouldn't the lights work? i thought the lights would be wired separate from teh FET circuits.

The lights wouldn't work in my case because I removed that (unnecessary) part of the circuitry...

Actually, the problem is that you run out of space in the roof of the car with two boards up there, not to mention you'd have to put something between them to prevent shorts.

[neurokinetik]

Here's some pics of the revised circuit.





Initially, running it in the heavily modified chassis with the C5R body, it seemed slow. The motor was the Mini-Z Racer Z130-A-BB, which I've not been very impressed with (neither fast nor torquey). I was using the 8 tooth pinion. Well, that paricular chassis has a bent driveshaft, and the rigged C5R wheels, so it wasn't performing optimally. Bench testing the same motor connected to a normal chassis and comparing it to the FET modded chassis showed that there was no discernable difference in the peak motor RPM. Acceleration was hard to judge by ear.

So, I did some major parts swapping, basically swapping over the entire rear pod off the good-running RSX with the tweaked stage 2 drag motor (timing advance) a normal set of wheels and tires, and gave it a run. I thought the throttle response was soft when I was running the other setup, but not anymore. Its quite punchy and quick. Hard to keep it straight on full-throttle application now. Best of all, it fits under the stock RSX bodywork. I'd post a pic of that, but it just looks like a normal RSX.

[EMX]

Originally posted by neurokinetik
Here's some pics of the revised circuit.





Initially, running it in the heavily modified chassis with the C5R body, it seemed slow. The motor was the Mini-Z Racer Z130-A-BB, which I've not been very impressed with (neither fast nor torquey). I was using the 8 tooth pinion. Well, that paricular chassis has a bent driveshaft, and the rigged C5R wheels, so it wasn't performing optimally. Bench testing the same motor connected to a normal chassis and comparing it to the FET modded chassis showed that there was no discernable difference in the peak motor RPM. Acceleration was hard to judge by ear.

So, I did some major parts swapping, basically swapping over the entire rear pod off the good-running RSX with the tweaked stage 2 drag motor (timing advance) a normal set of wheels and tires, and gave it a run. I thought the throttle response was soft when I was running the other setup, but not anymore. Its quite punchy and quick. Hard to keep it straight on full-throttle application now. Best of all, it fits under the stock RSX bodywork. I'd post a pic of that, but it just looks like a normal RSX.

So would you say its worth the work to do this?

so basicaly now, I can put any motor in my XMOD and it "should" run now? also what about more power? or is more power needed?

[neurokinetik]

I don't really think there is any more work involved in doing it this way than stacking 4562s or 7389s. This is equivalent to two stacks of 10 each (one for forward, one for reverse) of the 8 pin ICs. That's 160 solder connections to make. This method involves about 35 solder connections.

At this point, it would take a truly sick motor to kill these MOSFETs. I'm going to push the limits on it and wind something like a 10 turn motor to use with them. The current draw of the motor could cause some issues with the radio and steering servo, but I have a solution for that as well. I also plan to upgrade the battery wires and all power connections right up to the motor.

As far as adding more power, it could be done, but it is not required. I want to see what can be accomplished with just 4 cells at the moment, before moving on to bigger power sources.

[TypeZer0]

i really like the progress of your turbo unit.

what if you used 4 smaller batteries or a li-poly to power the ESC and dedicate the 4 aaa's to the motor? or move it up to 4 AA's which may allow a higher constant current drain tahn AAA's.

jus wondering, are you going to provide us wiht a schematic?

[rallger]

**dont read this if u get peeved by noob question***

what ter heck is a fet and why are u stacking tehm?

hey in my other thread about the 180 size motor and upgrades i talked about using a low voltage drop regulator to control voltage and current to the radio assembly. this type of regulator would limit the voltage and curret when the voltage from the batteries are as low as .45 v above the required voltage, i.e. 5.45 v needed to regulate 5v @ 500ma. i have been thinking of setting up a circuit where a group of 6 six cells are made in to 2 custom battery packs and then put in parralel with the motor and radio w/regulator, batteries in the center
(bad ascii art)
# # #
#(radio)---<<5v@500ma--#(regulator)---#<<(battery)>>(fets)
# # #

I think this would allow running of the lights w/o blowing the board. i testes the xmods board to find out the max current draw
and here are the results
car on, no servo motion(including holding the wheels straight) the car draws about 22ma, with servo moing back and forth it draws about 100-130ma, and with the servo holding wheels against an opposing force the board w/servo draws about 130ma with spikes upto 150ma, so 500ma should EASILY run the radio and lights, i choose 5v because it's a standard value for voltage regulators. now the problem is SPACE!!

FYI i hooked up the gate of the p and n channel fets that form the h-bridge to a scope to test for the BW needed for the mosfets and the board(ESC) switches from 3.5 to 5v @356hz, just like wedginator said

WHOOPS, just like neurokinetik said