thanks to fechter for the details on the current limit section. i have never had problems with that part of the circuit, so i have not needed to draw a schematic of it yet. i have increased the shunt conductors to raise the I limit on mine, but having it adjustable would be nice, especially if someone else rides my bike. full throttle at low speed now will push 3 kW, and most of it just goes to waste. i use the drainbrain and watch the current, so it is ok for me to have a 50A limit, but not everybody knows about throttle control, and it is a bit difficult to finesse that last few degrees of throttle control needed to keep the current down. i think i will try using a voltage controlled fet instead of a pot and then i can adjust the current remotely without injecting noise into the current comparator section.
if paul has not mentioned the part, the IRFB4310 can be substituted for the IRFB4710 used in the 72v controllers. the 4310 has half the on-resistance of the 4710 and only 10% more gate capacitance, so the driver will handle them no problem. it would be nice if the 4110 would work, as it is the 100v fet with the lowest Rds-on, but the gate charge is pretty high, and i am not sure the ir2101 driver could handle it.
in a 20A controller they just use 6 fets and they are driven directly by the ir2101 driver chips. you could drop in a set of 4310's and add another piece of bus wire to the shunt making one of these a 35A controller. just please don't come crying to me when your motor fries if you are not cautious
in the 35/40A controllers they use pairs of the same fets and the low side pair is driven directly by the 2101 through a pair of 10 ohm smt resistors. the high side pair, which handles the pwm, has a complementary transistor pair added to the output to boost the drive current. you can replace all 12 of these fets with the 4310 and if you beef up the high current paths it could be a 100A controller. i set mine at 50 because there is an advantage to limiting the current to a reasonable level. in addition to being easier on the batteries, very high currents can actually damage the magnets in the motor, resulting in permanent loss of power.
one thing i have noticed in doing the repairs is that the 10 ohm gate resistors tend to get overheated and change value. i replace them with a stacked pair of 20 ohm 0805 thin film resistors to prevent this.
the IRFB4310 is about $2.50 for qty 25 pcs from irtronix. this is a very good price. for those looking for somebody else to repair their controller or to upgrade it to higher current, contact me off list. i am doing it because retirement is just not keeping me busy enough, and because there seems to be a need. it takes me back to my early days fixing tv's back when you actually fixed them and didnt just toss them in the trash.