codesuidae
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quote:
it’s a snowball effect, the warmer [the FETs] get the more they resist. the more they resist the hotter they get!
MOSFETs have a negative temperature coefficent, meaning that as they warm up, their conductance decreases, reducing the current through them. This is what makes it possible to parallel them for increased power handling without the need for any current balancing circuitry.
BJTs are (usually?) the other way around, heat it up and conductance increases, possibly leading to a thermal runaway.
But yeah, sounds like ya done blowed ’em up. Probably will need to replace one or more of the fets.[/quote]
I didn’t notice a huge increase in the few I’ve done either, however, when racing with or pulling against unmodifed trucks, they always win. So I know its working, I just don’t think you really see the difference without a tri-cell or lipoly.
You might want to try double stacking the FETs as well, ph2t and others have reported pretty good results. But again I think you’ll need the increased battery voltage before you start seeing really snappy performance.
I generally run the fastest motor/gear combo (orange I think), its got so much power that the only reason I have to go to a slower setup is if I just want to drive slower.
For anyone else who would like to FET their own MT using the guide linked above. I didn’t really design it to be as simple as possible, it is just what appealed to me while I was diagraming the board. I’m sure you can do without bridging resistors (the board has a 5v boost converter, so the gate drive for the FETs is more than enough), and its not necessary to do the jumpering exactly as I’ve shown it. It is my intention to redo the page to simplify the method, but I’m currently busy on other projects. Also, I apologize for the poor formatting you get when accessing it through the codesuidae.net domain. I haven’t quite got the hang of the content management software I’m using.
I like that it comes with a wench. I mean, ya gotta have someone to bring you beer and stuff after you get to a campsite.
Hey, does that winch on the front work?
Great photos as well, its nice to see good photography of peoples efforts.
Edited by – codesuidae on 18 January 2005 06:33:31
Yes, the code is in C, compiled on the computer and transfered to flash memory on the chip.
Its a little difficult to get a good curve going with such low resolution input. I choose the default values to put most of the steps below the 50% mark precisely so that most of the control would be at low speed. The last two steps take you to 75 and 100%
There are several possibilities for improving control. One would be to ramp the power level from step to step, so instead of a sudden jump from 50 to 75, you’d get a quarter or half second or so ramp. This would smooth out the bumps a bit, but I don’t know how noticable it would be. Might make it a little easier to avoid swapping ends at the high end.
I’d also like to improve the PWM frequency, currently it uses a 100Hz pulse frequency (thats what causes the buzzing you hear when the SE moves). The next software version of this mod is going to bump that frequency up to something much higher, which should quiet it down considerably.
Another option is to give it control of the reverse circuit too, then remap all but one of the reverse speeds to forward speeds, resulting in higher resolution propo (maybe 8 or 9 steps) for the forward circuit, which, IMO, is much more useful than reverse.
Quote:.. if ya got it to non even steps u could run expo..Explain? And whats that about a PC link?
eh, that didn’t work, lets try it again:
Hmm, well, I’m not sure I understand that.
The chip uses the following steps:
10%, 19%, 30%, 45%, 75%, 100%
I haven’t run into any SE’s that use more than 6 forward speeds.
Quote:codesuidae.. how is the chip steps setup becasue i can see a expo setting here setup via a D9 link to a pcThey are US$10 for the kit, US$17 installed. I’m offering a FET mod and DC mod installed for $30.
hmm, I don’t charge shipping in the US, I can just put ’em in a letter and send it. I dunno how much it would cost to ship ’em down there. Lets say I’ll pay shipping up to a buck, anything else is extra.
I can do custom power curves too. Some people have expressed an interest in bumping the power up to about 80% max, to make it a bit snappier without having the full dual cell power, which is a little difficult to control on less-than-ideal surfaces. Basicly you have 6 steps for speed, and I can set the chip up to run the motor at whatever power level you want at each step.
The microcontroller is an Atmel ATTiny12, you can get the datasheet from the Atmel website or a google search will pull it up.
The efficency of power usage is unchanged, I’m just providing more power to the motor. The battery does run down faster if you use the high power capability, but that is to be expected.
D’oh! Case sensitive URL, my mistake. I’ve corrected it.
Most manufacturers publish detailed charging info in their datasheets. If you are interested in more detailed info a web search will turn up several of these, along with some projects for building your own chargers using the various battery charging chips available from Dallas, Maxim and others.
I’ve considered setting up a custom charging station using one of those chargers so I can top-up these tiny RC batteries after I’m done running the car for the day. I’m thinking that I could use the quick-charger that comes with the car while playing and let it charge up normally after I’m done.
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