Custom H-Bridge (tha MOSFET mod)
Posted: 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.
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.
