no shunt controller?

[lynjed]

hi, newbie here, been reading this great site for a long time but now first post for help.
I have a self made trials bike - low speed torque is more important than milage - I get 45 mins at the moment, which is ample, as I use it in the back yard.

I have a 1000 watt brush motor on 48v - this is mounted to a five speed gearbox, in turn mounted to the rear wheel via a big cog.
the system ran great until my 1000 watt controller, packed in on the throttle connections. So I got a new unit - supposedly heavy duty at 1500 watt to replace it.
the old unit only had 5 mosfets, barely any solder on the circuits for amps, yet still was very good.

Imagine my disappointment when the new supposedly bigger controller is much lower powered through the system - about 50% lower. I decided that it must be the way its programmed and decided to open it up and modify the shunt - dead easy!.
So I opened it up, the components are much heavier duty, lots of wire in the solders tracks to carry amps etc - but no shunt to modify?

Does anybody recognise how it works and how to modify it? Is it using the 2 large resisters as a shunt? Im an electrical idiot so plain English for any replies, many thanks.
jed

 

[amberwolf]

Best guess is they are using the voltage across the FETs while "on" to determine current thru the motor. That's how 4QD does it with thier 2QD controller--there's a little info on htat at the 4QD site somewhere.

If that's how they're doing it then if you trace out the circuitry on the board, there should be an extra connection from at least one of the FETs, not on a gate line, thru resistors and possibly other components back to the controller chip.

Optionally, they could be using a very low-ohms resistor between the FET source and ground, those are usually power resistors and kinda large, so they could be the big ones you see there. However, it is just as likely that htose resistors are part of a low-voltage power supply to convert your battery power to 5V or 12V for some of the electronics in there. You'd have to trace it out to find out.

 

[lynjed]

hi
thanks for the reply - can you tell me what the very big fet is - next to the normal ones. there is one at each end - you can just make it out at the end in the photo.
cheers
j

 

[amberwolf]

Don't know, wihtout a schematic. They may be using them as freewheeling diodes; it depends on how they made the "bridge" for controlling the motor. There are half bridges and full bridges, for instance: half bridge can only run hte motor one direction, but full bridge can reverse it.

If the controller has a reverse function, or any kind of "active" braking or anti-theft that locks the wheel, then it is a full bridge design, so probably each "side' of the PCB is one half of the bridge.

IF there's no reverse or any active braking (not including regen) then it is probably a half-bridge, and all the small FETs are probably paralleled, and the two large ones are probbaly paralleled with each other and are the "top" of the bridge sending power to the motor. They may even be P-FETs instead of N-FETs in that case (you cna find that out by looking up their part numbers).

 

[lynjed]

hi,
the controller has no functions other than power in, power out and a throttle connection. I don't have any schematics for it.
should have been a simple mod job....................