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Pin numbers being equal to BCE or GDS will depend on the parts selected, you'll have to determine that when you select your parts.

Browse digikey.com,
search for p-channel mosfet,
look for in stock parts,
FETs-Single,
filter on logic level and through hole

This one looks good:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=869-1055-ND
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Pin numbers being equal to BCE or GDS will depend on the parts selected, you'll have to determine that when you select your parts.

Browse digikey.com,
search for p-channel mosfet,
look for in stock parts,
FETs-Single,
filter on logic level and through hole

This one looks good:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=869-1055-ND



I got it working with the NPN/PNP and you werent kidding the TIP120 got super hot.

So with the MOSFET it will run cooler and then would I connect it the same way? Or would it just be 1 MOSFET? I guess I have some reading up to do.
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Replace the PNP with a P-channel MOSFET - will run cooler as the calculations show.
Coolness will depend on the Rds of the selected part, the lower the cooler.
Just make sure it is logic level MOSFET, otherwise it needs 10V to turn on full, and you only have 7V.

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Where do the Emitters connect to?  The one the NPN and PNP/MOSFET? Just so its clear.
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Thanks helps. The Emitter on the NPN isn't connected? or does it goto ??
« Last Edit: March 14, 2011, 11:50:09 pm by MobileWill » Logged

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Goes to ground. You gotta learn some basic schematic reading dude!
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Goes to ground. You gotta learn some basic schematic reading dude!

Well isn't ground usually 2 lines, one shorter than the other?

Thanks for all the help, my robot is so close to being completely wired.
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Both are recognized as ground.

Eagle (arduino schematics) uses a line with GND under it.
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Both are recognized as ground.

Eagle (arduino schematics) uses a line with GND under it.

Thanks, now I know. So the narrower arrow is the VCC?

One more question, on my orginal design with the just the TIP120, the only reason that didn't work was because the TX line was causing the Motor controller to turn on, would a diode stop that? Is that a better solution as then the current isn't going through a device? I think the above picture I don't have that problem unless the MEGA is connected to the PC via USB, then I saw that the LED on the motor controller was dim. After the mega is on and the motor controller is on, if set the pin low, the led is still dimly lite unless I remove the TX wire.(Also the step-up is outputting some power 3v~)  Is there a better way to isolate the TX? opt-isolator (I think it is called) Or should I not worry about that?
« Last Edit: March 15, 2011, 12:22:29 am by MobileWill » Logged

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I just realized that I am dealing with the 3.7V, since its before the step-up that I am switching, does that matter?
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The P-MOSFET Drain or the PNP-Collector will drive the Vin of your boost/stepup regulator.
Drive the circuit from whatever pin you want from the Mega - when it goes high, current will supplied to your reguator.

I don't know what you're talking about with Tx and an LED.
The current has to go thru something if you want to control it.
Maybe switching relay on/off is what you need. Are you going to be turning this on/off/on/off quickly under software control? Or just once in a while?

Anyway, that's tomorrows question, I'm off to bed. 1:30AM my time.
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The P-MOSFET Drain or the PNP-Collector will drive the Vin of your boost/stepup regulator.
Drive the circuit from whatever pin you want from the Mega - when it goes high, current will supplied to your reguator.

I don't know what you're talking about with Tx and an LED.
The current has to go thru something if you want to control it.
Maybe switching relay on/off is what you need. Are you going to be turning this on/off/on/off quickly under software control? Or just once in a while?

Anyway, that's tomorrows question, I'm off to bed. 1:30AM my time.

Sounds good.

There is a connection from the mega TX to the RX on the motor controller and it is causing problems. If the turn off the motor controller the power led on the motor controller is dimly lit and the step-up is outputting 3v vs the 7v it doesn't normally.  

It seems that when I cut the power to the motor controller it seems to get power from any connection.  

The only reason to turn off/on the motor controller is

1) when the robot is off to keep it in the off state, like for storage/transport, but batteries are connected still.
2) if the batteries are low, turn it off to charge faster or to be able to charge from solar (future option) (idea is if batteries are really low, turn off as much as possible. I pretty much can turn every thing off via software. I have a regulator for the sensors (instead of the onboard one) that can be shut off)
3) if I need to reset the motor controller / battery if it is tripped from overload.


I noticed that with the current setup the TIP120(NPN) gets much hotter than the TIP42(PNP). Wouldn't the PNP be carrying the load? The NPN is just turning the PNP on right?

I just noticed that the NPN get hot just being idle.

So maybe a relay would be better?


In testing I noticed that if I use just the TIP120 then the step-up outputs 4v. So that isn't right. So I guess this leaves using a relay.

Which one though? I have only worked with automotive relays.

Thanks so much. This robot has been a great learning experience and its growing. I can't wait till I can play with all the sensors and such.
The orginal problem was that 1 battery wasn't enough and the battery would go into protection and shut off.

« Last Edit: March 15, 2011, 12:58:33 am by MobileWill » Logged

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Can the TIP120 be used to switch the positive?  Trying to read up on transistors.
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Quote
the package has 1 being the base 2 being the collector and 3 being the emitter.
All transistors are different you have to look at the data sheet for the transistor you are going to use.
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