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DC Motor Question

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Hypnobeard:
Hello everyone,

I'm fairly new to the practical side of robot building, and have almost completed construction on my first autonomous bot. I studied Cybernetics at university, so I'm alright at the software stuff, but the electronics can stump me sometimes.

I'm using a PIC16F876 to control a differential drive 'bot using 2 DC motors. The test circuit I've made uses an H-Bridge to control the direction of the motor. The circuit is set up on a prototyping board. I know the control signal from the control circuit is OK. The output of the H-Bridge is providing a voltage drop of +3.3V across the connector for the motor. however, when I actually connect the motor, it nothing happens (the motor doesn't turn). If I measure the voltage across the connection terminal again, I get ~+0.5V. If I simply connect the battery cells directly to the motor, it turns fine, as I'd expect. The motor is rated to operate at 3V.

What am I doing wrong, and why isn't my motor turning when it's connected to the circuit?

It's doing my nut now, and I'd appreciate some help!

Admin:
It appears that your circuit isnt supplying enough current to run the motors, and the voltage is dropping because of this. Check the current draw both at the power source (before the circuit), but also at the motor (after the circuit), too. This will tell you where the power drain is happening at . . .

I am guessing you are PWM'ing MOSFETS? Are you doing 100% PWM (keeping the lines high)?


--- Quote ---It's doing my nut now
--- End quote ---
wha?!  :P

Dont feel bad, my first H-bridge melted and burned my hand  ;D

Hypnobeard:
Hey Admin,

Thanks for the reply. Not PWMing at the moment, the control line is set to high (so would be 100% I suppose) for testing. I'm using a combination of PNP and NPN transistors 2N5551 and 2N5401 with diodes in parallel.

Will test out the current draw at the suggested points.

BTW, do you know of any driver ICs that would be suitable for a 3V motor (in case I can't get this working)?

Militoy:
The transistors you are using are rated for 600mA (max), but are low-signal amplifiers that are only spec’ed for gain characteristics up to 50mA. They are probably coming out of saturation when loaded. If your circuit only needs 0.25A or so, and you want to use a complimentary set of PNP/NPN transistors in a TO-92 package, why not consider the 2N2222/2N2907 combination, that are beta-rated up to 500mA? I suspect though, that your 3V motor may be drawing more current than that, and you will need to select a higher-current set. Be sure to load your motor while measuring current (use a little drag from your hand, if necessary).

Hypnobeard:
Thanks Militoy,

What should I be looking for on the datasheets to enable me to choose the correct current ratings? if, for example, I want to anticipate a current draw of 1A from the motors (unlikely, I know) how to I spec this up when searching for transistors?

Thanks again.

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