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Topic: Building a car, possible voltage drop? (Read 773 times) previous topic - next topic

Jan 02, 2013, 04:49 am Last Edit: Jan 02, 2013, 04:55 am by ShadowfoxXXX Reason: 1
I'm rather new at this, but I've  purchased an RC car and me and a friend are trying to remote control it via arduino.
THE ARDUINO ISNT CONNECTED IN THIS IMAGE, but it connects through the resistors into the base pins of the transistors.



I destroyed the board that came with the car, so I've recreated something similiar on this breadboard.
It consists of 2 H-bridges made up of NTE2504 transistors. 
The arduino pins connect to the BASE of the transistors via 2 680 OHM resistors per pin
The whole thing is powered by a 6v lithium battery.
The steering uses a regular motor.
When the steering motor is powered, it doesnt seem to have enough power to turn all the way, but when the battery is connected directly to the steering motor, it will turn completely.
Is it possible the transistors arent fully saturated? did I do the math wrong?
I calculated that a 2 500 OHM resistors would allow 20MAH through each pin., so 10MAH per transistor.

I checked the battery, its at 7 volts and plenty of current.

These are high gain transistors (800-1600 HFE) so I assumed that would be plenty.
The motors pull 700-800MAH each, so 10MAHx800= at least 1600 should be plenty
Is there some kind of voltage drop going on? I'm lost.

TL;DR THE CAR WORKS, but the voltage seems to drop through the circuit.

arduinoPi

how much current do your motors use? breadboards have a limit of about 1 amp before a voltage drop occurs.

jackrae

You WILL lose about 1.4 volts through each H-bridge circuit.  Why not replace the transistors with FETs which have extremely low turn-on resistance.

dc42

Apart from voltage loss due to saturation of the transistors, as you are using NPN transistor throughout you must have the upper transistors configured as emitter followers. When you drive the base with 5V from the Arduino, the most you will get on the emitter is about 4.3V. To do any better, you need to use PNP transistors or P-channel mosfets in the upper positions, and level-shift the drive to them.
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