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Caps on h-bridge IC circuit — Parallax Forums

Caps on h-bridge IC circuit

rwgast_logicdesignrwgast_logicdesign Posts: 1,464
edited 2012-11-23 09:05 in General Discussion
So if im using an l293 h bridge and I want to make a nice clean circuit using 2200uf caps would I place these across the+18v to the l293 or on the acual signal lines between the chip and the motor? Im thinking sticking caps like that on the signal will skew or just elminate the PWM signal correct? Im reading on how to make sure sudden current changes from a motor effect electronics and im thinking if the caps werent of the signal line the sudden starting and stiping of current would effect the l293 negatively, but maybe it built to handel that as long as you have diodes or your l293 variant has built in diode protection

Comments

  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2012-11-22 21:06
    Also is it Ok to use metal film to filer motor noise? I was under the impression metal film wouldnt help decoupling at all but its all I have on hand at .1uf with a high enough voltage. I have .22uf x7rs on hand but there rated 16v, which means id have to drop my voltage from 18 to 16, im not even sure .22 uf is a better idea than a .1uf film.
  • Mark_TMark_T Posts: 1,981
    edited 2012-11-23 02:49
    rwgast_logicdesign wrote: »
    So if im using an l293 h bridge and I want to make a nice clean circuit using 2200uf caps would I place these across the+18v to the l293 or on the acual signal lines between the chip and the motor? Im thinking sticking caps like that on the signal will skew or just elminate the PWM signal correct? Im reading on how to make sure sudden current changes from a motor effect electronics and im thinking if the caps werent of the signal line the sudden starting and stiping of current would effect the l293 negatively, but maybe it built to handel that as long as you have diodes or your l293 variant has built in diode protection

    Large decoupling goes on the supply, always.

    Smaller capacitors can go across the motor windings to reduce inductive noise (RFI), but not nearly as large,
    since such capacitance has to be driven by the bridge and will increase both the peak and average currents it has to handle and slow down the transitions (which need to be much less than the PWM period).
  • Mark_TMark_T Posts: 1,981
    edited 2012-11-23 02:50
    rwgast_logicdesign wrote: »
    Also is it Ok to use metal film to filer motor noise? I was under the impression metal film wouldnt help decoupling at all but its all I have on hand at .1uf with a high enough voltage. I have .22uf x7rs on hand but there rated 16v, which means id have to drop my voltage from 18 to 16, im not even sure .22 uf is a better idea than a .1uf film.

    Don'y scrimp on the cap voltage rating, a motor can generate big inductive spikes, 50V caps would be more like it. High self-resonant frequency is good I suspect.
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2012-11-23 09:05
    ok got it thanks. the thing im still worried about is using a 60v .1uf metalized polyester cap across the windings. Ive repedately been told film caps can not be used for decoupling becuase they dont exhibit the same properties as a ceramic. Im not sure if reducing the motors noise is acually decoupling though, i wouldnt even be attempting this if i hadnt seen erico using big mylar films on that picture above.
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