I've got FETs in my SE!

Mostly dealing with electronics and pcbs and modifying them, all the things that make electrical-engineers tick and the rest of us cringe in fear.
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DriveWRX
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Post by DriveWRX » Sat Sep 27, 2003 6:17 pm

:eek: Dave, is there nothing you can't do?
Paint, body, chassis, electronics, batteries.... You are amazing!

Thanks for the info! :-)

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Post by cj » Sat Sep 27, 2003 6:48 pm

This may have been asked before but, what FETs are you using?

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Post by crazydave » Sat Sep 27, 2003 11:39 pm

cj wrote:This may have been asked before but, what FETs are you using?
Part number is IRLML2502. You can get them at digikey.com.
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Post by micro_Amps » Thu Oct 16, 2003 10:36 am

I've fet modded these things, and the result is dissapointing to say the least. The problem is that the design of the control signals to the H-bridge (yes there is a full H on the pcb, 2 of them actually) only raises the gate voltage of the transistor (or fet) to a pulsed 900mV, barely enough to turn the fet on at all. The designers have tried to overcome this "half turn on" by stacking the negative transistor in the forward circuit, which helps, but comes no where near the potential speed that this car should be running at. To fully turn on a tranny/fet at least 2 volts, preferably 5 is needed. The IRLML2502 fets start turning on at 700mV, so at only 900mV they are only just turned on, and still in a high resistance state, thats why they dont conduct like they do in a BCG or Gen1 ZZ that has 3.3 volts available to turn on the transplanted fets.
I am still working on a couple of ideas (transistor combinations) to get a decent voltage across the motor from this H-bridge, but it looks like this 900mV issue is going to be the biggest hurdle to overcome now.
BTW, I tried running my ZZ SE on 3.6 volts (simulating a 3 cell) and there was no increase in speed, for the same reasons as above.
I dont know why they have done it, but they are using a 3 volt DC-DC converter to power up the control chip, which keeps the output driving signals at exactly the same values regardless of the battery voltage.(Oh well, thats probably why they have done it) You could run this car on 12 volts and it wouldnt go any faster.
I will keep trying, I want to see performance like that of a fet modded BCG coming from this car.
:) uA
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Post by crazydave » Thu Oct 16, 2003 4:27 pm

Thanks for the update micro_Amps. Can't wait to hear what you come up with.

I agree, compared to a FETed bit, this is very disappointing, but hey, what else would expect from a 25 dollar fully proportional RC.

Keep us updated. I've got my fingers crossed that you come up with a solution.
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Post by ph2t » Thu Oct 16, 2003 8:24 pm

Hey micro, I see your problems man. What a bugger. I hope to get a ZZ SE soon so I can start a bit of an investigation into this as well.

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Post by hogjowlz » Thu Oct 16, 2003 8:34 pm

ya it really comes down to motor combos and gears at the moment. its acceptable but nowhere like i can get a bit moving. but hey the steering kicks any bits ass 300%
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Post by micro_Amps » Thu Oct 16, 2003 11:00 pm

Depends what you want to do with them HogJ, scream around the place or practice your reverse parking.
;) uA
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Post by ph2t » Thu Oct 16, 2003 11:24 pm

micro_Amps wrote:I've fet modded these things, and the result is dissapointing to say the least. The problem is that the design of the control signals to the H-bridge (yes there is a full H on the pcb, 2 of them actually)


This makes sense that one h-bridge would be for the motor driving the rear wheels and the other H-bridge would be for the motor(I assume??) that drives the steering assembly.

Mabee the makup of these h-bridges differ in the sense that one is position orientated (steering H) and the other is speed orientated (drive H).

It is an actual motor yeah? driving the steering assembly? I have no idea 'cause I ain't gots one (yet) but I'm assuming this is the case. It prolly ain't the same type of motor as the drive one, possibly it's a stepper motor given it's role in positional placement.

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Post by micro_Amps » Thu Oct 16, 2003 11:47 pm

No its not a stepper ph2t, its a normal motor (green cap so a 1.6 I think) it has a feedback pot connected to the steering mechanism to confirm position. The front end is pretty much just a 'servo', not a stepper.
:) uA
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Post by crazydave » Thu Oct 16, 2003 11:58 pm

The steering is driven by a motor. It has a green endbell, at first I thought it was just a plain ol' 1.6, but then I realized it is hardwired. I was considering wiring a pink clone motor to see what happens...

...but now that you mention the second Hibridge being for steering (and that makes sense to me) I'm wondering if I've gone and FETed my steering.

I didn't notice a difference, because I was focusing on the speed, and shortly after took the jumper off my steering pot, which made it super sensitive.

I like the proportional steering, but proportional throttle doesn't help me drive much, because of my driving style. I'm thinking I wouldn't mind ditching the propotonal throttle, to get power.

micro-Amps, you think it would be possible to put a mosfet at the output to the motor, and set it up to switch the juice straight from the battery when current to the motor start to flow?
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Post by ph2t » Fri Oct 17, 2003 12:03 am

micro_Amps wrote:No its not a stepper ph2t, its a normal motor (green cap so a 1.6 I think) it has a feedback pot connected to the steering mechanism to confirm position. The front end is pretty much just a 'servo', not a stepper.
:) uA


Cool, thanks for the info dude :). Is it interchangeable with other motors?
Also, this stacked bjt intrigues me, I take it's the npn one of the pair? Was that 0.9V measured with a DMM or a CRO? Also dude was it an open circuit measurement? Sorry, so many questions man...Is the drivetrain h-bridge the same layout as a BitG apart from this stacked transistor?
:lol:

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Post by ph2t » Fri Oct 17, 2003 12:19 am

crazydave wrote:The steering is driven by a motor. It has a green endbell, at first I thought it was just a plain ol' 1.6, but then I realized it is hardwired. I was considering wiring a pink clone motor to see what happens...


Have you unsoldered it and put it in a normal bit for speed comparisons?
That would be an interesting exercise.. :???:

...but now that you mention the second Hibridge being for steering (and that makes sense to me) I'm wondering if I've gone and FETed my steering.

I didn't notice a difference, because I was focusing on the speed, and shortly after took the jumper off my steering pot, which made it super sensitive.

I like the proportional steering, but proportional throttle doesn't help me drive much, because of my driving style. I'm thinking I wouldn't mind ditching the propotonal throttle, to get power.


Yeah, but if it could be done it would be absolutely mad! :D

micro-Amps, you think it would be possible to put a mosfet at the output to the motor, and set it up to switch the juice straight from the battery when current to the motor start to flow?


So Dave, are you saying you should connect the gate pin of the mosfet to the terminal where the motor is connected on the pcb? this would imply that you are not using the output pin of the chip, yeah? interesting...

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Post by micro_Amps » Fri Oct 17, 2003 12:23 am

Ok,
Dave the output of the chip pretty much already goes straight to the fet (via a current limiter resistor) anyway, so the same problem exists. There is not a high enough voltage to turn the bloody thing on. Also the output to the drive H-bridge is constantly (and variably) pulsed to allow for deviation in drive motor speed control. Its not on/off, this is a completely different beast this time. I might try and investigate this further, but I'm not really confident of a successful result, good idea though.

If you put in a 4.2 as a steering motor you may get a quicker end to end (L to R) reaction but Im not really sure this is desirable as it contradicts the smoothe operation of the proportional control.

Ph2t,
I 'm sure the steering motor can be changed if you try hard enough.
The stacked bjt, is the npn in the forwad cct, thats why forward is faster than reverse in a stock ZZ SE. So further stacking is an option to increase H-bride voltage. Although the transistors used are a new kid on the block, I havent seen these before.
Signal voltages were taken in circuit pre and post fet transplant with DMM,output from chip seems regulated to max out at 900mV pulsed to govern the duty cycle on the driving transistors. Just too damn low.
Both drive and steering H-bridges are as per bcg setup, 4 transistors in both. Standard, simple, most importantly, cheap.
:) uA
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Post by crazydave » Fri Oct 17, 2003 9:17 am

ph2t wrote:
crazydave wrote:The steering is driven by a motor. It has a green endbell, at first I thought it was just a plain ol' 1.6, but then I realized it is hardwired. I was considering wiring a pink clone motor to see what happens...


Have you unsoldered it and put it in a normal bit for speed comparisons?
That would be an interesting exercise.. :???:


The wires run directly into the endbell, I could probably modify it to work in a bit, but it would be very difficult, and probably would not go back in the SE very easy.

ph2t wrote:So Dave, are you saying you should connect the gate pin of the mosfet to the terminal where the motor is connected on the pcb? this would imply that you are not using the output pin of the chip, yeah? interesting...
Yeah that's what I was talking about. I wanted the FET to essentially switch on a bypass around the PCB. i know this has been done with bit PCBs for multicell dragsters, but I wasn't sure if the SE's pulsed current would be strong enough to turn it on, and micro_Amps seems to agree, but still it's an option I'm willing to look into.
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