1. Home
  2. Hobby Electronics Basics
  3. testing H-bridge

testing H-bridge

Hello,

I am testing a stand alone H-bridge IC, the L293D. I do not know if this is a problem or just a ormal reaction of this chip. The chip I am using is this one

formatting link

From Texas Instruments, (page10 for the pin)

I am using a 6v battery. Pin 4, 5 and 7 are grounded Pin 2 is on the 6v battery Pin 1 is my enable pin. I suppose when I inject different voltage I should be getting different voltage out, But when I put in over three volts, i get 6v out (pin 3 and 6) and under input of 3 volt, I get no voltage out. How do i control the voltage out ? Because if I where to do a PWM with a pic chip on the enable port, the L293 will see average voltage coming from from the pic. So at 50% duty cycle, (assuming the pic chip voltage is 5v) the output of the PWM port will be 2.5v. I assumed at 50% Duty cycle and the L293 output is a motor. I assume I would be going at half the speed, but my preliminary test from above shows me at 2.5v nothing happens I need to go up to 3v.

ken

Reply to
lerameur
Loading thread data ...

I am answering myself on this one, Its the Vdd from the motor that is turned on and Off from the enable, Thats is what changes the speed. k

Reply to
lerameur

What is connected to the two supply pins, 8 and 16?

Are you intending to run the motor either direction, by connecting it between 3 and 6?

With pin 7 grounded, you are telling the driver that, when enabled, it is to pull down on pin 6. And by connecting pin

2 high, you are telling the driver that it should pull pin 3 up when enabled. Both 3 and 6 should go open circuit when not enabled by a high on pin 1.

I would select a direction for motor voltage by pulsing high (PWM) one of 2 and 7, while lowering the other, continuously. Enable would also be high, continuously. This causes the effective motor voltage to be approximately the supply times the duty cycle. If you want the motor to coast, lower the enable.

The PIC output may read 2.5 volts with a meter, but that is an average reading of a pulse that is 5 volts half the time, and zero volts the other half the time. It is never steady at 2.5 volts. The 293 does not respond to the average voltage from the PIC, but pulses its outputs in synchronism with it (if it can keep up).

I don't think you should be pulsing the enable pin to PWM the motor. This does not apply an effective voltage of the supply times the duty cycle (supply voltage during the on time, and zero volts during the off time, by shorting the motor), but much less. It pulses between supply voltage (minus losses) when on, and negative supply voltage plus diode drops when off, as the motor inductance reverses the voltage to try to keep the current going through the clamp diodes. At 50% duty cycle, the current runs down to zero before the off time is over, so the average voltage is approximately zero.

Reply to
John Popelish

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.