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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crazydave
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Post by crazydave »

Well I finally got my P-channel FETs in. Since I had already done the N-Channels on the bottom, I didn't even have to remove the PCB to add the P-channel ones to the top. Took me about 5 minutes. :grin:

Well again, slight improvement, but we're still not there. I stacked the the FETs, and that improved it just slighty still, but alas, no cookies.

Still though, until we figure uA's technique, if you have an SE, and FETs laying around this is worth the effort, as there is an improvement over stock.

I put a little video clip together, it's kinda messed up, for one I used the same controller for both cars to assure they started simultaneously, but since the steering trim is set by the radio, one car wanted to wander off. Then instead of hitting the button, then saying go, my wife said go as she was hitting the button. So the modified car got about a 6 inch head start before the vid started, but I'm showing it to you anyways, because I think the fact that it got a 6 inch jump in the time it took my wife to press the button, says something. The jump off the line is drastic, then it slowly accelerates past the stock car. The stock car is veering off to the right, so that makes it seem a tad slower.

The stock car is on the left, the modified on the right, both running the stock 2.2, and red gears.

---> [url=members.cox.net/strollerpusher/sefet.wmv]Video Clip (*.wmv)[/url] <---

I have a few experiments I'm going to try, but I have to wait until I get my other SE working right again. I like to have a backup car. ;)
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Post by ph2t »

Great work there Dave. Did you stack the nchan fets or the pchan fets?

Hopefully one day soon I'll figure this out, even if it kills me!

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Post by VooDooMafia »

Keep it up guys, I have faith that you will figure it out :)
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Post by crazydave »

Well, I stacked the P-channels yesterday. I believe I had stacked the N-Channels before, but as I mentioned I didn't bother removing the PCB this time, so I'm never sure unless I see it.

I gotta tell ya, there a slot cut into the plastic piece that holds the PCB down, that the P-channel FETs poke out of, and it looks kinda cool with the 2 FETs poking out the top of it. Makes it look like the super modified beast, that it's not. :lol:

Got a few ??s for ya ph2t, I was looking at the specs for the P-channels, and noticed they have a slightly lower operating voltage. Does that mean they will turn on at a lower voltage? What were you saying, 3V? Any chance flipped P-channels in the N-channels' position will help?
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Post by ph2t »

I'll check the datasheets later today and let you know. Sitting in my jocks at the moment in front of the PC getting ready for work.....Talk about fucken sad!

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Post by CaboWabo »

ohh, davey I'm anxious as all hell...
keep going bro - you're getting there!
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Post by mad_haxor »

here is something i think will work to raise gate voltage.
lift the gate pins on the fets and connect to the output of a 741 op amp.
then connect the rest of the 741 as follows: vcc to dc/dc converter,
gnd to gnd,inv input thru a 100k resistor to gnd and thru a 270k resistor to output of 741, non inv input to where the gates were connected.
thats it. you should have close to 3 volts on the gates of the fets at w.o.t
foward, but still have full proportion speed.
hope this helps.
if you try it post a reply and let us know how it works for you.

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Post by hogjowlz »

new dude steppin up with some mad ee knowledge!! welcome to the boards pimp.
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Post by mad_haxor »

Thanks, i will finish my a.a.s. in e.t. degree this month, glad to use some of it.

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Post by hogjowlz »

haha well feel free to spread some of it here.
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Post by VooDooMafia »

mad_haxor wrote:Thanks, i will finish my a.a.s. in e.t. degree this month, glad to use some of it.


Yea thats usually the first thing people do with there degree is find away to make there toys that much better :)

Welcome to the Hood :-)
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Post by ph2t »

wow, some fresh blood! cool.

as to the mod I dunno. I've looked into the opamp setup but I think the DC gain would need to be really high and that cold cause a problem. Also, you have a 3V power rail to play with and I think the 741 needs like min +/- 9V ??? Anyways I still appreciate the suggestions. You got 0.9V output from the controlling chip into the fet/bjt. This was measured open circuit, no load. Still gonna give this a go though, ou just never now....

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Post by mad_haxor »

Yeah, the 741 likes a lot more voltage than 3 volts, but I think it might work with 3v.it would be too easy to try not to. as for gain it would be: 1+ (270/100)=3.7 so .9v x a gain of 3.7 = 3.33v.
i have got a 741 to work off of a 5v dc supply from a 7805 with no trouble.
I have never tried 3v though. worth a try,anyway.

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Post by CaboWabo »

hogjowlz wrote:haha well feel free to spread some of it here.
that aint no shit
welcome aboard mad_haxor
I'm already entriged by your posts, be looking at more as you make em.
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Post by mad_haxor »

Thanks, not sure it will work. it was the first solution that popped into my head. less than 5 minutes thought was put into it. there are several other ways that will work, but they would require more calculations and experimenting. I dont have one to try it on or any fets yet, and my kid wont let me butcher his, lol. the 741 seems to work on multisim, (3v supply,.9v pulsed dc in 3v pulsed dc out),but that doesnt mean it will work in real life.

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