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 Post subject: clothesline racer
PostPosted: Fri Sep 25, 2009 9:42 pm 
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I've started working on my clothesline racer and I decided to try and make a high powered h bridge to drive my motor in forward and reverse. However, stupid me didn't do enough reading before my trip to the shack and now i have 4 npn high power resistors. So to make it work, I need to use a voltage doubler so I have a higher voltage on the base than the collector, right?

Will something simple like this be enough for the low current needed to just trigger the transistors?

http://www.coolcircuit.com/circuit/voltage/

I'll be using a 7.2v battery pack to power the motor, so it'll only be about 15 volts.

Then should I just use small pnp transistors to apply the 2xVoltage to the bigger transistors?

Thanks,
Jon

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 Post subject: Re: clothesline racer
PostPosted: Sat Sep 26, 2009 2:47 pm 
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You're close, but still kinda far.

First, that voltage doubler requires some sort of time-varying signal to double. A DC input won't really do anything. The easy way is (if you can manage it) just to have the inputs connected to two pins on a microcontroller and drive them alternating high and low. That'll give you 2x the uC run voltage, minus two diode drops; 8.6V for a 5V Arduino. You'll need to put a resistor in to limit the output current from the micro, or you run the risk of killing your micro. Limit it to 20 or 25mA, using the equation i = C dV/dt. You'll need to choose C such that it's large enough to provide a sufficient i at all times, knowing that it's going to droop slowly between recharge cycles...

You know what? Never mind all that. Let's just say that the exercise I was embarking upon to start explaining how to do that involves enough math and complexity that you are categorically better off finding PNP transistors for your H-bridge. We need to be in the same room, with a whiteboard, for me to really explain it clearly.

If getting different transistors is really, truly, totally not an option, you COULD try using a regulator to provide a DC voltage for the high side of your H-bridge. A TO-220 package device with a good heat sink on it can handle around one amp continuous; some, even more. Do that, and your battery voltage provides the higher V you need without any mucking around with charge pump voltage doublers and such-like.

I forget- were you in the first class I taught? There's an IC in the goodie bag I handed out that can help you- the 7660. It's a voltage doubler that only needs a couple of capacitors- which also should be in that bag.

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 Post subject: Re: clothesline racer
PostPosted: Sat Sep 26, 2009 3:22 pm 
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Thanks, I went and got some pnp transistors, but because my npn ones handle 115w, I had to get 4 pnp ones @ 65w each so 2 will be in parallel.

This is going to be an interesting bastardization of a h-bridge... if it works... and if it does this thing will be speedy.

and if not i have a couple of servos to fall back to, which will work nice, but slow.

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 Post subject: Re: clothesline racer
PostPosted: Sun Sep 27, 2009 1:49 pm 
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Please explain what you mean my attach the transistors in parallel, I am unfamiliar with this.


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 Post subject: Re: clothesline racer
PostPosted: Sun Sep 27, 2009 11:01 pm 
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double the power handling/heat dissipation

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 Post subject: Re: clothesline racer
PostPosted: Tue Sep 29, 2009 1:56 pm 
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Note that paralleling transistors can be a good idea or a bad idea. With bipolar technology devices (BJT, IGBT), there's a positive tempco effect which causes devices which are conducting more current to become less conductive, thus reducing their power dissipation and cutting the amount they conduct. The load naturally balances. With field-effect devices, the opposite is true- if one device starts to conduct a little more, it heats up more, which causes it to conduct more, etc., until you end up with one device handling way more current than it should, and that one burns out.

Note, also, that the driver IC supplied with the Adafruit MotorShield is a BJT based device, so you can stack them to increase current handling.

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 Post subject: Re: clothesline racer
PostPosted: Tue Sep 29, 2009 1:59 pm 
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Note to self: minimal testing required, possible contender for spectacular failure.

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