upgraded FETs
 

o wow, its been a while since i posted... anyways, I replaced the Xmods' stock FETs with 4562's. I had some left over from stacking mini-z FETs.

At the moment, the FETs lie 2x1 cuz of the heatsink in the way (and of course, i'm too lazy to cut the heat sink). I don't notice too much of a difference cuz i'm using a stock motor but I do notice some torque increase. Before the modification, I couldn't tire spin, but after, i can spin the tires whenever i want when i bury the throttle.

I'm hoping to get a faster motor so I can feel more of this upgrade.

Sorry, I can't post pics cuz my digi-cam is horrible and can't make out the words on the FET. But however, I'll try to take pics of the stock FETs cuz they have this white goo visible on it.

Stock FETs are 2SA1792. Four of them are used, two on the top of the board, and two on the bottom. Not sure if this is how the Mini-Z does it or not, I only saw stacks of two in the pics.

I posted on Mini-ZRacer about an idea to use much larger FETs instead of stacking the small ones (though I didn't say it was for an XMod ;) ) and I still think this is the way to do it. It avoids the heatsink problem (Heat sinks would be attached directly to TO220 packaged FETs) and allows you to pass as much as 70A through them. Of course at this point, thicker wires will be needed, and traces on the boards might need to be beefed up as well. But the reliability of such a setup should be far better, and more cost effective. Such a setup would cost about $10 for the FETs (two each of N channel and P channel would be required), and a couple of bucks more for the board and wires, and would be the equivalent of having more than 10 stacked FETs.

I just need to order my parts and test this setupto be certain it works.

Do you think that by having fets on the top and bottom of the board, it already has a "stack" built in?
Now the othere thing I'd be wondering- do you put the new added ones on the top or the bottom? Or both?
One extra on each?

That's what I'm thinking. I need to examine the circuit board more closely to make a determination, but my initial look led me to believe that we are getting four FETs to run the motor, rather than just two. If that is indeed the case, then it really wouldn't matter which way you stack (up or down), as they are all connected the same anyway.

The FETs on the bottom run the motor, and the FETs on the top run the steering servo. There's no point changing the servo FETs unless you want super high speed steering; which is kinda a waste of FETs....

What we really need to know is what the specs are on the stock FETs. I have been unable to find them anywhere so far. If I knew who made them, it would help, but they only have part and lot numbers printed on them with no maunfacturer's logo. A1792 is what they say, I'm assuming they are 2SA1792 (Toshiba). They could actually be UPA1792 (NEC).

Well, the only way I can figure it out would be to see what it can take, and compare it to what the known values are.... ummm I don't want to do that. I can wait.

In theory, the manufacturer should have a data sheet out for the stock FETs. International Rectifier is really good about this, putting all of their data sheets online. Once we figure out who makes these things, then we can probably get this data.

Anyway, I ordered four of the big TO220 style FETs today from Digikey, and also a set of 10 IRF7389 FETs in case the other method doesn't work out.

what's the diff of the P-channel and the N-channel power rating(Amp)..

which one is this one i should go by....i belive the p-channel is a negative amp rating and the n-channel is positive...

Newbie...
 

i just bought an rsx x-mod friday and im hooked...i already bought the handling upgrade and the stage 2 engine upgrade...could some one clue me in on all the abbreviations... fet, nml, and so on?? thanks

Re: Newbie...
 

FET - Field effect transistor, used in the...
ESC - Electronic Speed Controller
NML - No Motor Limit, an individual that hand winds and assembles motors.

So what does all of this mean?

read about FETs ESCs and NML at http://www.mini-zracer.com

do a search there and you'll find tons of topics that discuss this in extreme detail... in fact, so much detail, that you'll be able to teach a class....

how do you get the old fets out 2 put new ones in i am new so just curious.

You'll need to unsolder the old ones first.... then replace it with the new ones....

Well, experiment phase 1 is a success. I replaced the stock FETs with IRF7389s, and everything works! :D

http://users.adelphia.net/~neurokinetik/7389.jpg

Now it is time to remove them and begin phase 2...

Well, phase 2 is built, but not working at this point. Time to troubleshoot. The FETs I have chosen may not have the proper characteristics for this circuit.

Here's some pics showing what I have attempted.

The first pic shows the add-on board with the new FETs:

http://users.adelphia.net/~neurokinetik/FET_board.jpg

The second shot shows a closeup of the soldering of a CAT5 wire to connect the ESC to the add-on board.

http://users.adelphia.net/~neurokinetik/ESC_closeup.jpg

The last shot shows the entire electronics of the XMod with the add-on board attached.

http://users.adelphia.net/~neurokine...te_circuit.jpg

great to hear about stage 1!
I can see where stage 2 would be a problem.... that's a lot o' wires.
How do the mini z turbos do it, with only 4 wires?
They must be a lot more complicated than they look.

Well, it depends. The "twin turbo" setups probably use 8 wires as well. Most of their turbos are only for the forward direction. I could concentrate on that as well, if I wanted.

What's important is that I have confirmed for myself that the main drive circuit of the XMods ESC is virtually identical to that of the Mini-Z. That opens up alot of possibilities.

BTW, the wiring isn't that bad at all, it took me about an hour and a half to build that entire setup from start to finish.

Anyway, if I get this working, it won't really be a stage 2, it'll function more like a stage 20...

Is there no room to stack the IRF FETS?

nope, they would stick thru the bottom of the chassis. Good for airflow, but bad for anything hitting it.... Unless you flipped over the board, or re-oriented it in some way.

Flipping the board might be a possibility. You could also use a decent heatsink on top of them if you did this. I've got the perfect one from cooling cache RAM chips on overclocked Slot A Athlons. I have six of them from that project, and one of them will exactly fit the two FETs side-by-side.

The mini-Z turbos send power directly from the batteries to the motor. They connect to the stock FETs and are simply a set of 4 more powerful ones. This way they are easy to install and bypass stock electronics entirely. I'v been thinking of building a simple one with RadioShack MOSFETs. This way I don't need to solder or mess with the PCB, just solder wires to the batteries and use those black connectors for the motor and motor output.

You'll still need to connect to the board... How else are you going to get the signals to the MOSFETs to turn them on? Also, check the Specs on the MOSFETs that RS sells. They are IRF510, which are only capable of handling 4A continuous. The IRF7389 handles almost double that, and the MOSFETs I am using handle 10x what the IRF7389 handle.

Anyway, I've been doing some testing, and it appears that my circuit is fine, but one of the two types of MOSFETs I chose is not suitable, and I think it is the N channel. The circuit is not working because the drains on the N channel FETs are not staying at ground when they are supposed to, while the P channels go to 5V. What you end up with is 5V on both the positive and negative sides of the motor, and no voltage difference means no rotation.

What I have learned about this motor drive circuit:

IC5 controls the reverse voltage, and IC6 controls the forward. When you want to go forward, the gate on N channel FET in IC5 goes to 3.5V (not enough to turn the transistor on), and the gate of the P channel FET in IC6 goes to ground, which turns the transistor on. The output (drain) of IC6 goes to 5V, and spins the motor forwards.

The issue with my circuit is that I need to change my N channel FETs to logic-level ones, so that they are not turning on when they are not supposed to. Once this is done, the circuit should work just like the stock one, but with more than 10 times the current capacity.

Here's a couple of pics of the revised circuit. It's down to seven wires now, as I noticed that pin 1 on both IC5 and IC6 are grounded.

http://users.adelphia.net/~neurokinetik/FET_board2.jpg

http://users.adelphia.net/~neurokine...e_circuit2.jpg

I am very new to xmods and mini-z tech stuff. I read for quit a while last night about stacking the FETs, but you have a seperate board and is that what the MOSFET your are talking about is? Also where are you going to install the extra board

The little 8 pin chips that the speed controller uses are the MOSFETs. Each chip contains a pair of MOSFETs, one N channel, and one P channel. Most people parallel a bunch of these chips by soldering them on top of one another to maximize their current capability. Instead of doing that, I am using much higher current devices that replace the chips with two MOSFETs each. These devices give the same or better capability as stacking up 10 of the small chips, which would be both unwieldy and not really possible where the board is located, even if you were to flip it over (the driveshaft ends up in the way). The small board will probably be attached to the roof of the car, like the light module is.

Obviously, with this setup, you won't be able to run the light module, but if you are really going for maximum performance, you aren't running the lights to begin with. ;)

With the seperate board and FETs would you be able to run a 5 or 6 cell set besides the lack of room

With this setup, I can run up to 55V. :eek: Well, at least the motor drive circuit will be able to handle it. The rest of the board OTOH... :lol:

Actually, if I replace the steering servo FETs with IRF7389s, I could run as much as 30V with little risk of frying things.

How much will this FET upgrade cost me and where can i purchase them?

What is the stock FET rating? The reason I ask is that I am running 8.4V on a stage 2 yellow motor (next to highest speed motor - lots of current draw) and the controller seems to be taking it fine.

At this voltage, I can't drive it through turns as fast as it can go - it simply does flips. 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. And I can't see anyone wanting to go much faster than 20-25MPH with one of these.

Of course, there are the hard-core speed freaks out there I suppose, but you reach a limit where the power simply is not useable abymore without increasing size or weight to keep it on the ground.

I guess I need to take some current measurements to get a baseline. Remember that as you increase voltage, the current goes up too.

Good luck on the project though - I am interested in the final product.

S

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

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.

...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.

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!

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.

wow :eek: 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

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.

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... :p

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.

Here's some pics of the revised circuit.

http://users.adelphia.net/~neurokine..._revision2.jpg

http://users.adelphia.net/~neurokine..._revision2.jpg

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?