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Compensate for resistance?

Discussion in 'General Electronics Discussion' started by qweets, Sep 26, 2010.

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  1. qweets

    qweets

    48
    0
    Sep 26, 2010
    hello : )

    i have a 10v motor and a 10 volt regulator to power it. When i connect them together directly the motor turns very fast and works great. i connected up a h-bridge(SN754410) to add reverse , and it works as expected. However, since connecting the motor to the h-bridge, the motor turns slightly less as quickly as it did before , as if the H - bridge has a little resistance about it, slowing the motor down a little. ( not much, just a little).

    i wonder how i could over come this ? Maybe a 12 volt regulator with a resistor instead of the 10v regulator?

    thanks for any advice .

    qweets : )
     
    Last edited: Sep 26, 2010
  2. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    Run the motor from the bridge in both directions - adjusting the output of the bridge for the motor speed you need. Any kind of solid-state switch is going to have some inherent voltage drop. You just need to anticipate that drop - and compensate for it.
     
  3. qweets

    qweets

    48
    0
    Sep 26, 2010
    hello, i am a bit new to this.how do you mean adjust the output of the bridge?

    do you mean use a 12v supply and trim it down until I reach the desired motor speed?

    thanks : )
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    Is your H bridge made using mosfets or bipolar transistors?

    If the former then what is the Rds(on) of these devices, if the latter then what is the Vce(sat).

    Alternatively, what are the devices?

    Are you supplying enough gate voltage/base current to them?

    There may be multiple solutions to the problem, and raising the input voltage may be one of them (in fact, it may be one of the easier ones)
     
  5. qweets

    qweets

    48
    0
    Sep 26, 2010
    hi
    it is a h -bridge IC SN754410 , i am giving it full pwm and full voltage but the output is a little weak, i think i'll buy an 12v regulator to replace the 10v regulator to power the 10v motor and trim it down a little to reach the speed i need. just wondered if anyone else had experienced with this type of thing.

    : )
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    If your PWM signal never reaches 100% duty cycle, then the full continuous voltage will never appear to the motor.

    Since you're using PWM to control the speed, the motor's inductance will combine with the PWM signal and the H bridge to form what is effectively very similar to a switchmode power supply. So if you adjust your maximum speed to be the same as when the motor is powered from 10V, the motor will be operating almost exactly as if you were powering it from 10V.

    If you look at the datasheet for the SN754410 you'll see it uses bipolar transistors as the switching elements and that at 0.5A they have a saturation voltage of between 1.1 and 1.4 volts. Using 2 of the half bridges results in 2.2 to 2.8 volts dropped across the two half bridges. This varies with current, and since the datasheet I found does not have a graph of this voltage wrt current, you'll have to estimate or measure it.

    Aside from building your own bridge, the easiest way is to increase the voltage. Then trim your PWM range things to get the same maximum speed.
     
  7. qweets

    qweets

    48
    0
    Sep 26, 2010
    thanks steve : )

    i'll try a 12v supply and see how the motor spins.

    Although I was using pwm in my tests , i just thought, i do not really need pwm ...I need full speed all the time. So maybe i can just fire 5v dc into the pwm input instead (i wonder is that the equivalent of 100 duty?)...I am rather new to all this as you may be able to tell . lol :p

    thanks for your help.
    : )
     
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