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h-bridge problem.

Discussion in 'General Electronics Discussion' started by qweets, Jan 13, 2011.

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  1. qweets

    qweets

    48
    0
    Sep 26, 2010
    hello : )

    i wonder if anyone can help? as i am a bit stuck on this one. i am hoping to pwm a motor with a "a3953" h-bridge. The PWM signal i have (coming from an arduino) is functional, i have an led on it to test and i can vary the led 0-100% so that seems o.k. ( @31kh freq which i need for my motor to keep it running silent as possible)
    here is the pdf of the h-bridge :
    http://www.google.co.uk/url?sa=t&so...pvffO7LcDuWZ6y5oA&sig2=JKVtpRnsQjVZpj8Wd0IeJw

    from what i can gather (i may be wrong) in the PDF the H bridge requires "low" logic pulses at the enable pin and toggling the phase pin will alter the polarity.( forward and reverse)

    but there are other aspects to this h-bridge that are not all that usual ,
    here is a line from the pdf maybe someone can understand it a bit better than me.

    "For applications that PWM the ENABLE input and desire the internal current-limiting circuit to function in the fast decay mode, the ENABLE input signal should be inverted and connected to the MODE input. This prevents the device from being switched into sleep mode when the ENABLE input is low."

    there is a truth table in the pdf that shows if you H logic the enable it will go into sleep mode. o i am unsure how to wire this up as a standard h-bridge.

    i have it all wired up like in this diagram.below.

    [​IMG]

    i can make the h-bridge work , when i connect the enable to ground it allows through my 10v ( 9.78 or about) from my regulator, when i remove the ground it goes to 0v. so i assume that if i can pulse this action, i can vary the voltage. ( i read that ground = logic low? i hopes.)

    i wonder how you guys would go about connecting a pwm signal to this h-bridge? ive tried inverting the pwm from my arduino as it outs logic high pulses.( i beleive) so i used a 2n3904 transistor but with not great results.

    maybe i have the wrong idea and need to convert my PWM into a more suitable signal?or just wire it up a bit different?

    thanks for any help on this : )
     
  2. qweets

    qweets

    48
    0
    Sep 26, 2010
    i have just found that when i use a lower frequency PWM the h bridge is o.k i can vary at the output as normal. But, when i use 32kh the h-bridge snaps on and off almost like a switch , my arduino is not at fault as i can connect an LED directly to it and the 32kh varies the led very well, and the transistor i use to invert the pwm is also o.k as i attach an led after that and it too is o.k,

    when the 32kh finally reaches the h-bridge enable pin , the h-bridge is not happy ( unlike it is with low frequencies) it works instead like a switch.


    this is just like the previous 2 h-bridges ive tested , they do not like the high frequency it seems.

    i thought this h-bridge could handle 70kh. maybe i am reading the pdf wrong?
    anyone know of another h-bridge i can try? i've tested 3 so far : /
     
  3. qweets

    qweets

    48
    0
    Sep 26, 2010
    Oh darn it qweets!!! 70hz is the minimum not a 70kh max.

    if anyone knows of a DIP h-bridge that can handle the 32kh PWM , id be so happy to give it a try !!

    thank you : )
     
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