Custom H-Bridge (tha MOSFET mod)

ph2t · Post by **ph2t** » Fri Jun 13, 2003 7:36 pm

Well like every other like minded techo out there I had to have a shot at this. Originally I got Micro_amps to make this mod for me since he had the FETs ordered from overseas. The change is amazing with this mod.

Here's some video of the FET Mod in action.

The three cars pictured below have the following:

Silver Compact Char-G S2000, stock TOMY 2.2 motor.
Blue Compact Char-G Skyline GT-R, Perfection 3.0 motor.
Yellow MicroSizer Mistu Lancer,Perfection 3.8 motor. (FET Mod)

You can see the video of the differences in take off here, in DivX OR MPEG format.

The reason why the FET mod is so great is one of resistance. The original transistors in the PCB drop the voltage before it hits the motor, so you are only getting a smaller fractional voltage than the 1.2V from the battery. The FETs have a very low resistance (known as the value Rds in the FET datasheets) that helps push more current and voltage to the motor.

First things first, a schematic. Everywhere I have looked I haven't found a proper @%#^& circuit diagram of this mod. So I did a little study and constructed one.

N-Channel MOSFET H-Bridge

The FETs are connected differently than the normal Transistor H-Bridge that's stock in these cars. This is due to the fact that 4 N-channel FETs are used instead of 2 N-Chan and 2 P-Chan FETs. The German daulcell FET mod uses 2 P-Chan FETs as well I think.

MAKE IT EVEN BETTER!

I wanted to find a FET that had an even lower Rds value than the FET made popular by this mod. The IRLML2502 has a typical value of 0.045 Ohms. I found this FET NTMS7N03R2 with a typical value of 0.023 Ohms. Half the Rds of the normal FET used!!! Cool! (In a very sad freakazoid sorta way...).

So enough of this stuff, lets see some pics.

The package these NTMS7N03R2 is different to the ones normally used. So different in fact I'm gonna have to make a new PCB for 'em.

The FET is that little black package, I put a steering knuckle next to it to give you pimps an idea of its size. On the right is the prototype track layout for the PCB.

Cause I don't have the tools I'm gonna have to draw the track work onto the virgin copper PCB using a special reistive pen.

Now I let it dry for a few minutes then I put the PCB in some etching solution (basically an acid). After about 7 minutes all the uncovered copper is eaten away and I'm left with this:

Now to solder those FETs on.

Putting the FET board on the car

Now that the FET PCB is ready I need to connect it to the car. The car I've chosen to mod is an Lxx booster, the auto modellista one. First I've ripped the old transistors out and circled the area where I need to connect the FETs to in yellow.

After connecting the steering wires to the transistor pads circled above I then connected the FET PCB to the power terminals on the car PCB and then I removed the wires to the motor from the car and attached them to the FET PCB.

So the power wires go to the FET board, the steering wires (from the transistor pads, not the actual fine steering wires!) go to the FET PCB, BUT the motor is wired straight to the FET PCB.

That's it. I guess one day I will do a PROPER tutorial based on the original IRLML2502 FET but I like this one better. The only biatch is making the custom PCB, but I guess I could get the parts to do this fabrication using the PC instead of hand drawing it.....

I will post video of my testing 'cause I want to see how my own version of ths mod performs against the original German one. Just quickly I put the Lxx car on a cushion on my couch which is on an angle of about 50-60 degress. In un-boosted mode the car stood still and didn't roll back. In boosted mode the @#$%^ acutally climbed the bloody cushion! More tests to follow.

End rant....

ph2t.

kwikbb · Post by **kwikbb** » Fri Jun 13, 2003 7:52 pm

holy shit thats just sick... oh mother of pearl, i think i'm aroused lol

Post by **CaboWabo** » Fri Jun 13, 2003 8:02 pm

I think I'm scared! lol
I wanna do it, but I'm pretty sure I'm gonna be meltin my PCB.

sm2osley · Post by **sm2osley** » Fri Jun 13, 2003 8:31 pm

Can you do mine

sessiz · Post by **sessiz** » Fri Jun 13, 2003 8:44 pm

Damn.
That is what I need for my truck.

crazydave · Post by **crazydave** » Fri Jun 13, 2003 8:44 pm

If this mod works even on the LXX board then I'm going to have to try it. I think I can handle the traditional one, but this one, with the extra PCB kinda confuses me.

If anyone wants to try it ph2t's way you can get this PCB etching kit from Radio Shack.

mpbiv · Post by **mpbiv** » Fri Jun 13, 2003 9:41 pm

I am also pretty sure that RS carries dry transfer type decals for laying out PC board curcuits for etching. BTW the etching kit crazydave was talking about would include all you need to do a few boards, I think it was like ~$14 or less last time I checked.

mpbiv · Post by **mpbiv** » Fri Jun 13, 2003 9:49 pm

Oh and great tutorial PH2t, I am now very tempted to try your fet mod

ph2t · Post by **ph2t** » Fri Jun 13, 2003 11:57 pm

I did this mod because I believe it is actually easier to do than the traditional FET mod. Give me a few days, I'm gonna do some testing that should tell me if I can do this mod WITHOUT having to remove the old transistors. Dave, have you seen the directions given on how to do this mod? Bloody hard. I had to re-create the schematic because it didn't exist anywhere until now, afaik.

So Cab's don't worry man, there might be a way to do this mod without some serious component frying...

If people are interested I can send you a PCB with FETS all ready to go and include instructions for installation. Will that help everyone? I'm more than happy to make the pcb 'cause I've already invested in the equipment to do so...

I believe soldering a few wires to the car's PCB is a hell of a lot easier than the traditional german fet mod way.

Keep in mind the FETs I use are less than HALF the resistance of the other IRLML2502 FETs.

And the torque is killa!

ph2t.

sessiz · Post by **sessiz** » Sat Jun 14, 2003 12:00 am

I'ld be interested. Keep us aprised, and good work ph2t.

ph2t · Post by **ph2t** » Sat Jun 14, 2003 12:07 am

Will do homie...

cj · Post by cj » Sat Jun 14, 2003 1:26 am

mpbiv wrote:I am also pretty sure that RS carries dry transfer type decals for laying out PC board curcuits for etching.

They don't have those in any stores near me any anymore.

Post by **CaboWabo** » Sat Jun 14, 2003 10:16 am

I'm interested as well ph2t

crazydave · Post by **crazydave** » Sat Jun 14, 2003 3:53 pm

If the price is right, I may be interested too. I' m pretty sure I could pull it off myself, but I'm always willing to take the easy way out. Save me the trouble of tracking down the mosfets too.

ph2t · Post by **ph2t** » Sat Jun 14, 2003 10:55 pm

I'm gonna do some tests today and I was thinkin of the best way to get some accurate results.

I was gonna use three cars, one virgin, one German FET, one Ph2t fET.
All will use a 2.2 motor ('cause that's wat I gots zee most of). All will use the stock orange gears and softs for rear tyres. I will charge all cars fresh of my custom charger and then test from a stopped start on a variable incline, eg: a piece of laminated chip board.

I'll see how I go and let you fellas know.

ahhh, the mind is sharp after a bit of

ph2t.