L293D 5V in - 3V out

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1

Hi, I have a problem with L293D. On Vcc2 is 5V, but on JP1, JP2 is only 3V.

L293D-low_voltage.jpg

2

If you look at the datasheet,

you will see your output will be between VCC2 - 1.4 and VCC2 - 1.8. Your 3V out is pretty close to the low side, and, therefore expected.

3

You will probably also loose

You will probably also loose 0.7V on the input diode.

4

Good catch, Geir. I didn’t

Good catch, Geir. I didn’t notice the input diode on VCC2. 

That will drop about 0.7V, plus the drop of the h-bridge itself easily explains why he is only getting 3V to the motors. 

Try a higher voltage for VCC2. Maybe a 7.2V NiMH battery. 

5

And with SN754410 it will be

And with SN754410 it will be the same? I know about the 0.7V on diode and 5V - 0.7V = 4.3V , but on out was only 3V, something what I did not expect :smiley: .

6

The datasheets list

L293/L293D

Screenshot-wwwticom-lit-ds-symlink-l293dpdf-GoogleChrome.png

 

SN754410

Screenshot-wwwticom-lit-ds-symlink-sn754410pdf-GoogleChrome.png

 

I am not positive, but, it would seem to me that with HIGH being less that VCC2, and, LOW being more than 0V. Your motors will never see full VCC2. I would guess that you will only see the swing from VOH to VOL. So, your reading was only telling half the story.

 

7

Check the datasheet for the

Check the datasheet for the SN754410. I think you will find it is about the same. They are both transistor based h-bridge circuits. 

The P-N junction of a diode or transistor is about 0.7V. Since the circuit drops through two transistors to run the motor, that’s about 1.4V, in theory. The actual drop can be higher.