Inconsistency in my L293D
May 23, 2009
Sorry, I have no schematic prepared to make this easier.
L293D by ST Electronics (datasheet) (quad half-H-bridge), only one power source: 5V from battery.
V1 = V2
Enable pins for 1+2 and for 3+4 are high.
Input pins for all half-bridges are pulled to GND via 22kOhm resistors
Motors connected to outputs: M1 to outputs 1+2, M2 to outputs 3+4.
All on the breadboard, I have an extra wire in the V+ bus. I use it to make the inputs high. Start experimenting.
start as above
inputs: 1 high, 2 low = M1 forwards
inputs: 1 low, 2 high = M1 backwards
leave one input (1 or 2) high, motor keeps on running, remove V+ from Enable pin "1+2", it is now floating. Motor stops
Cool = as I expected.
start as above
inputs: 3 high, 4 low = M2 forwards
inputs: 3 low, 4 high = M2 backwards
leave one input (3 or 4) high, motor keeps on running, remove V+ from Enable pin "3+4", it is now floating. Motor keeps on running!
What the Freak?
situation as produced by B)
I pull Enable "3+4" down to GND via 22kOhm resistor. Motor Stops. Finally!
situation as produced by A)
I pull Enable "1+2" down to GND via 22kOhm resistor. Makes no difference. Motor was already stopped.
Make enable high (with pull down resistor still in place). Motor starts running again. Makes sense.
The half bridges 1+2 need no pull down resistor on their shared enable pin. Half bridges 3+4 do. It does not hurt to pull them both down.
Remove the pull down resistors on the enable pins. Make them high.
Make both motors run whichever way: for example input pins 1 and 3 high, 2 and 4 pulled low.
Remove V+ from Enable 3+4 (floating): M2 does not stop without pull down resistor on the Enable pin.
Makes sense now that we have learned the above.
Make it high again.
Remove V+ from Enable pin 1+2 (floating): both motors stop!
Fudge around with some more pull down resistors on enable pins: makes no bleeping difference!
I gotta get me some sleep.
I cannot make M2 run (enable pin high) while M1 is stopped (enable pin low). But I can do that the other way around.
Suggestions? Cheap suppliers for replacement chips? Last time I spent 7 Euro.