L293D Great Current Loss !

Using Arduino Motors, Mechanics, Power and CNC
#1

hi, well when i connect directly i get so damn good performance my blade just flys itself even tought its glued !! but whit l293 driven from arduino i get low speed !

a mosfet didnt reduce speed !

i used my multimeter to check things i got 7.75v from 7.95v which on the datasheet its normal:((

but the current
falled from 250ma(direct) to 170 (using L293D) anyway to reduce this ??? i connected my 8v batterys (6 1.2 v)

  • to l293d left lower corner and the - to power rail connected both sides and arduinos ground to breadboard ground and + to breadboards + i will upload fritzing image if you want !!!

heres code !

int s = 0;

void setup(){
Serial.begin(9600);
pinMode(5,OUTPUT);
}
void loop(){
if(Serial.available() > 0){
analogWrite(5,Serial.parseInt());
}
delay(1);
}

#2

Measurements done with:: “digitalWrite(5,HIGH)” ??

#3

That's the problem with BJT transistor, they can drop a lot of voltage.
Leaves less voltage across the load.
(Vsource - Vtransistor)/Rload = current.
Want higher current? Need lower Vtransistor >> MOSFET

#4

i tried digitalWrite(5,HIGH); too gave me exact same results.

mosfet takes a lot of space :frowning: im already out of space im ordering new chassis but on my current one ( 4 wheels 2 motors one im ordering got 2 wheels) dont have any place for battery ! mine is huge ( lego triangle 6AA batteries holder)

#5

L293D is a darlington driver, so you lose 2 to 4V automatically. Not recommended
for low voltage motors, but the chip is cheap...

Incidentally a good BJT H-bridge that's not darlingtons can be a good performer,
like MOSFETs, but you need the right transistors.

#6

MarkT:
L293D is a darlington driver, so you lose 2 to 4V automatically. Not recommended
for low voltage motors, but the chip is cheap...

Incidentally a good BJT H-bridge that's not darlingtons can be a good performer,
like MOSFETs, but you need the right transistors.

L293D isn't really cheap! but ok i will live whit it, i also tought this amout of voltage drop is so much!

#7

See page 5 - Voh: VCC - 1.8 to VCC -1.4.
So 1.4 to 1.8V of voltage drop across the output.
And that's at just 0.6A - it increase as current goes up to 1.2A max limit.

#8

And you double that as both the high side and low side switches have a voltage drop.

Compare price of L293D with that of two LMD18200T's!

#9

CrossRoads:
http://www.ti.com/lit/ds/symlink/l293d.pdf
See page 5 - Voh: VCC - 1.8 to VCC -1.4.
So 1.4 to 1.8V of voltage drop across the output.
And that's at just 0.6A - it increase as current goes up to 1.2A max limit.

:frowning: that is so bad:((
seeing if L298 got same amout of voltage drop !!!! but i dont really need speed for my robot but still thats so sad ! wasting battery!

#10

That's technology.

Check Sink & Source saturation voltatge at the bottom of page 3.

Low Rds MOSFETs are the way to go for high current.

#11

CrossRoads:
That's technology.
http://www.st.com/web/en/resource/technical/document/datasheet/CD00000240.pdf
Check Sink & Source saturation voltatge at the bottom of page 3.

Low Rds MOSFETs are the way to go for high current.

thanks trying to load it but guess St is forbidden in iran !

.:iran:.

#12