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(Corrected version as last text was somehow corrupted)

I have made a circuit with L298N in order to control direction of 2 DC motors. When I power the circuit I noticed that voltage between 2 output pins is lower than expected.

When measuring voltage between 2 output pins I read around +- 5-6V instead of Vcc 9V.

Then I measured difference of each output pin to GND and when input was high, there was voltage as expected near Vcc (9V). But when input was low, there was somehow 4V on it.

I cannot figure out how does this voltage on pin occur. The end result is that voltage difference between output pins is too low to turn the motor.

I also tried to disconnect Enable pin to eliminate some possibilities and it worked as expected - voltage on output was 0V no matter the input state.

What am I doing wrong and is there a way to eliminate voltage on output pin when input is low.

I'm controlling input pins from shift register 74HC595.

Circuit schematics: L298N control 2 DC motors via 74HC595

(Voltages were measured without a load)

UPDATE: I also tried to hook up the motor and then voltage between pins dropped almost to 0V: I then change Vcc to 12V and output on low input is still 3.96V. When motor is connected, voltage between output pins and GND is always Vcc and except when both inputs are low - then it droppes back to 3.96V.

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  • \$\begingroup\$ Have you tried putting some pulldown resistors on the outputs? \$\endgroup\$ – MCG Feb 20 '17 at 13:38
  • \$\begingroup\$ This should give you the answer: electronics.stackexchange.com/questions/108686/…. Bottom line is that this chip isn't very good. \$\endgroup\$ – Andy aka Feb 20 '17 at 18:57
  • \$\begingroup\$ You probably want to point out that current causes voltage drop. Well motor that I want to drive is 3W 12V, so thats around 250-300mA. I have tried to put pulldown resistors, which caused voltage to drop a bit, but also voltage on high dropped and the end motor still didn't turn. \$\endgroup\$ – Delphi4U Feb 21 '17 at 7:45
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I figure it out. It looks like current sensing pins have to be connected to the ground and not left unconnected. After looking the schematics again, I figure out that those pins actualy connects H bridge to the ground.

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