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I am making rf based metal detecting robo and I had used 8051 for controlling the operation. The rf is fine and programming is also okay I think as I am getting desired outputs at 8051 output pins and L293 output pins.

The problems arise when I connect motors to the L293d's output pins.

1) Even if I get 8V at both sides of L293D, the motors didn't work when I connect them. (The motors are fine.)

2) When I connect one motor to L293D, then the motor works ... and on other side the output is 8V too; but as soon as I connect the second motor too... the first motor also stops.

3) If I keep running one motor with L293D, after 10-15 seconds the robo stops.

I am getting this problem first time in my experience. I have no guide / partner, so I am sharing it with you people. Hope, someone here will help me please.

(And can this be problem that I have used only one 9V battery on the entire circuit of 8051-L293D and H12D (RF IC)? But also the output is 8V on L293D, then why the motors are not working?)

Please help as I had to make my robo work asap.

Regards,

Er S

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  • \$\begingroup\$ Can you measure these voltages: - voltage across single motor when it is working - voltage across either motor when both of them are hooked up Moreover, some motor specs might be useful. What I suspect is that the single battery can't power both motors at the same time. \$\endgroup\$ Apr 19, 2014 at 14:11
  • \$\begingroup\$ i have added the crkt diagram. Yes, the voltages across both motor terminals are 8V approx, but as soon as the terminals are connected to motors, the voltages go from 8V-0v , 8V-0V to 8-8 , 8-8 . That means no operation \$\endgroup\$
    – ersanjit
    Apr 19, 2014 at 16:27

2 Answers 2

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Apart from checking that your 9V battery is "man" enough for the job you are advised to consider how really poor the L293D is in your application: -

L293D H bridges don't realistically do what people expect them to do. That's not a problem of the chip - read the spec and you can see that at about 1A load the output voltage to be expected is a serious amount less than the power supply voltage. Here's a picture from another answer about the L293 and L293D on a 6 volt supply: -

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If the supply voltage is 9V, with a 1 amp load expect the PNP (top) transistor to "lose" up to 1.8V and the NPN (bottom) transistor to lose up to 1.8V. This means, at 1A, the motor will only have 5.4V across its terminals and the L293D will be dissipating a power loss of 3.6 watts and shutting down.

Time after time questions come up about the L293D, the SN754410 (identical in all but name) and the L298 (it has a heatsink but is still inferior in every other respect).

Don't use them. Instead use a DRV8800. It uses FETs with an on-resistance of about 0.5 ohms meaning the top transistor will "lose" 0.5V at 1A and 0.4V the bottom transistor. That's less than a volt lost compared to the L293D which loses over 2.6V (typically).

Alternatively, use the DRV8837: -

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Worst case on-resistance for BOTH top and bottom FETs is 0.33 ohm - in other words, from a 9V supply, with a 1A load, the motor will receive 8.7 volts and the chip will dissipate 300mW. Much more effective.

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  • \$\begingroup\$ thanks for the reply... i have added the crkt diagram. Yes, the voltages across both motor terminals are 8V approx, but as soon as the terminals are connected to motors, the voltages go from 8V-0v , 8V-0V to 8-8 , 8-8 . That means no operation \$\endgroup\$
    – ersanjit
    Apr 19, 2014 at 16:30
  • \$\begingroup\$ motors are 300rpm dc \$\endgroup\$
    – ersanjit
    Apr 19, 2014 at 16:31
  • \$\begingroup\$ I dont really have that much experience with building robots, but if you're not ultra limited by space, and you have a microcontroller already, wouldn't it usually be better to just make your own H bridge with mosfets? At this level of current, keeping the resistance down is pretty important, and with discrete mosfets you can get a way lower resistance, while still not being too expensive (probably). \$\endgroup\$
    – BeB00
    Jun 13, 2016 at 22:28
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The D version can drive a current of 600mA with a max drop of 1.8V. This may be ok for a 6 or 7V motor with a winding resistance of > 10 Ohm for heavy loads.

Motor current exceeding this limit may cause thermal shutdown. An external 25 deg/watt heatsink as shown in spec, may be required.

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  • \$\begingroup\$ well if that's the case I believe the problem is in the battery too... Can a 9V battery source 600mA? \$\endgroup\$ Apr 19, 2014 at 14:33

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