testing H-bridge
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Posted by lerameur on November 11, 2006, 1:49 pm
Please log in for more thread options 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 http://www.datasheetcatalog.com/datasheets_pdf/L/2/9/3/L293D.shtml >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 | |||||||||||||||||||
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Posted by lerameur on November 11, 2006, 5:12 pm
Please log in for more thread options lerameur wrote: > 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 > http://www.datasheetcatalog.com/datasheets_pdf/L/2/9/3/L293D.shtml > >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 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 | |||||||||||||||||||
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Posted by John Popelish on November 11, 2006, 5:45 pm
Please log in for more thread options
lerameur wrote: > 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 > http://www.datasheetcatalog.com/datasheets_pdf/L/2/9/3/L293D.shtml >>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 What is connected to the two supply pins, 8 and 16? > 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. 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. > How do i control the voltage out ?
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. > 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. 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 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. 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. | |||||||||||||||||||
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