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RC Car Hack (beginner questions)

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DirtyLude

Well-Known Member
After my first attempt at making a robot drivetrain with 6v non-gear motors failed (it hardly had enough torque to move at all), rather than buying gear motors which are exceedingly expensive, I just went out and bought an RC car from Radio Shack for $30.00 CDN, about $20.00 USD. The Turbo Clamper (same as the Silverlit turbo gripper). It has two drive motors that it uses the change direction, like a tracked car.

I took the thing apart, but had to put it back together again; my son decided it was now his most favorite toy.

I'm going to toss the circuit board, but I had some questions about the design.



The power transistors on the right, all run the motors. Whats the purpose of running so many smaller transistors rather than one larger one for each motor. Cost? Or is there a benefit to this?

The front headlights actually use generated current from the motor. I'm assuming they run on battery power when the car is on, and the motors generate electricity for them when turned and the power is off?

I first tried driving my 6v motors with a TC4424 chip, but they heated up way too much with just 3 seconds of running. I switched to just using simple TIP120's to run it, and these seemed to work, but I only get forward direction. I didn't think about this much, yet, but is there a way to use the TC4424 with the TIP120 to run bigger motors, or should I just go for a more capable h-bridge controller?
 

tansis

New Member
Seek out the website of "Pericom Technolgy Inc" they make R/C model chips.
APPLICATON NOTE PT8A996S/201S DEMO BOARD is worth a look and has good application schematic showing the cofiguration of the four transistors required to build a Bridge Driver circuit for motor bi-directional motor control.
 

nettron1000

New Member
The power transistors on the right, all run the motors. Whats the purpose of running so many smaller transistors rather than one larger one for each motor. Cost? Or is there a benefit to this?
Whoa! thats alot of of transistors and alot of used board realestate.

Ive heard of using paralleled TO-92 transistors to increase the transistor H-bridge power handling capability but thats the first ive seen. Yes the bottom line is cost. Another advantage i can see is redundancy. If one transistor burns out the circuit can still function, assuming of cource that the remaining transistors can handle the extra load.

But the advantages of a power H-bridge IC,tho a bit more expensive, out-weigh these advantages



The front headlights actually use generated current from the motor. I'm assuming they run on battery power when the car is on, and the motors generate electricity for them when turned and the power is off?
Lost me there.




I first tried driving my 6v motors with a TC4424 chip, but they heated up way too much with just 3 seconds of running. I switched to just using simple TIP120's to run it, and these seemed to work, but I only get forward direction. I didn't think about this much, yet, but is there a way to use the TC4424 with the TIP120 to run bigger motors, or should I just go for a more capable h-bridge controller?
According to the specs if the resistance of the load is less than 50 ohms you cannot use the TC4424 series, but you can use it to drive MOSFET power transistors which can then drive the higher loads.

Heres a link that explains it pretty well:


http://www.robotroom.com/HBridge.html



An even better alternative is to use an L298 or LMD18200 Power H-bridge driver IC's, which are the ones i use most often. I got them free as samples from National Semi awhile ago, but i think they charge a minimum of $8 on all sample orders now, new policy...phewy !
 

DirtyLude

Well-Known Member
Thanks for the replies.

After doing more research on H-Bridges I finally have a better idea of what I would need to do to drive seperate MOSFETs. I think I'm just ordering some lmd18200's to make it easy on myself right now.
 

bmcculla

New Member
Hi

Actually with Bipolar transistors having one transistor drive the gate of the next (called a Darlington) allows the second transistor to output more current. This is because the output current is proportional the the current flowing into the gate. The first transistor amplifies the small current at its gate causing a bigger current into the second gate giving an even larger current at that output.

Brent
 
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