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I am using an H-Bridge chip, given inputs from a PIC16. My problem is that when a motor is NOT attached, I can measure the voltage across the wires that are supposed to be connected to the motor and it is 5V (which is what I want). However, when I connect the motor, it drops to 0.5V, and the motor barely turns.

What could be the problem?

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  • \$\begingroup\$ Your motor seems underpowered. Does the output current capability of your supply match the rating of the motor? \$\endgroup\$ – Count Zero Mar 3 '12 at 20:15
  • \$\begingroup\$ I'm running the circuit off 3 AAs. When I connect the 3 AAs directly to the motor, it runs fine. So I think the current is fine. \$\endgroup\$ – Amandeep Grewal Mar 3 '12 at 20:21
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    \$\begingroup\$ The your H bridge is either not hooked up right or isn't rated for the current. \$\endgroup\$ – Olin Lathrop Mar 3 '12 at 21:54
  • \$\begingroup\$ Can you post a circuit diagram of how you have the chip hooked up? \$\endgroup\$ – Optimal Cynic Mar 4 '12 at 15:55
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    \$\begingroup\$ Which H-bridge IC are you using? \$\endgroup\$ – Anindo Ghosh Dec 3 '12 at 4:00
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This is basically Ohm's law at work. You have a circuit going from your batteries, through a transistor, the motor's coil, another transistor and back to your batteries. To know exactly what's going on measure the voltage drops across the components.

I can think of a few of things that could be going on:

  1. You aren't driving the H bridge properly and the transistors in the bridge haven't switched properly. So they look like a higher resistance in the circuit and measuring with no load (close to zero current) will show almost no voltage drop on them but once you hook up a load (higher current) you'll drop a lot of voltage on them.
  2. You are overloading your power source (batteries) so your supply voltage is dropping under load.
  3. Something the PIC is doing, what SW is it running? Is it PWMing the motor?

Measure the voltage drop across the components (battery, bridge) and post the schematic and parts that you're using for a more detailed answer.

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I think you have used the same power supply for both the logic supply (vcc1) as well as the motor power supply (vcc2). You should power up the pins using separate power supplies as per the rating in datasheet.

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