74HC595 chain for dc-motors switching problem

Discussion in 'The Projects Forum' started by Regress, Jan 14, 2011.

  1. Regress

    Thread Starter New Member

    Jan 14, 2011
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    I've run into a problem when chaining the 74HC595 part and using them to switch on and off DC-motors using FETs.

    The motors are in parallel, and the first 8 can be switched on and off perfectly using the first shift register.
    When switching one of the next 8 motors in the chained 595, the motors seem to go on very shortly but not strong at all.
    I switched the shift registers to see if the second was malfunctioning, but they are both working.

    When I bypass the shift registers and send a pulse to any of the FETs, the motors turn on just fine.

    I've placed capcitors from the 5v to the gnd in front of the 595s, a large one and a small one. (20uF and 0.1uF)

    Maybe someone on here could take a look at the circuit to see if there is a problem.

    I Hope to get some help *:)

    I've hadded some pictures of the circuit
    [​IMG]

    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Can you post a schematic?
    That way we can see if there is something wrong at some point.

    Bertus
     
  3. Regress

    Thread Starter New Member

    Jan 14, 2011
    6
    0
    Hi Bertus,

    The motors and Fets are placed based on the following schematic, in parrallel:
    [​IMG]

    The following is my circuit:
    [​IMG]

    Thank you for taking a look, I really appreciate it!
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    The schematic shown me that you are using TIP120's.
    Those are darlington transistors and not fets as you told us.

    The saturation voltage of the TIP120's is much higher as fets.

    [​IMG]

    I also attached the complete datasheet of the TIP120.

    Bertus
     
  5. Regress

    Thread Starter New Member

    Jan 14, 2011
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    Bertus I'm sorry, I forgot to add that I did use fets in the actual circuit.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What type (part number) of fet are you using?
    You will need logic gate fets for your application.

    Bertus
     
  7. Regress

    Thread Starter New Member

    Jan 14, 2011
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    I am using the IRF520N. They seem to be working for the first 8 motors in the circuit.
    For the other 8 there seems to be not enough current.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Is the powersupply stable?

    Better use the IRL520 instead of the IRF520.
    At a gate voltage of 4.5 Volts the IRF520 can handle 2 Amp as the IRL520 can handle about 10 Amp.

    I attached both the datasheets for comparison.

    Bertus
     
  9. Regress

    Thread Starter New Member

    Jan 14, 2011
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    Hi Bertus,

    The power supply for the motors is a converted atx supply.

    As for the shift registers, it seems I have solved the problem by connecting them to a seperate power supply. I was using the supply from the microcontroller before which did light up leds for the second shift register (dimly) but I think it wasn't strong enough to switch the fets.

    There seem to be a few shift registers that are damaged and are malfunctioning. Any reasons this might have happened?
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It could be that spikes on the powersupply damaged some of the 595's.

    Do you use decoupling capacitors at all the IC's ?

    Read this thread why it is important to have them:
    Decoupling or Bypass Capacitors, Why?

    Bertus
     
  11. Regress

    Thread Starter New Member

    Jan 14, 2011
    6
    0
    That's an informative thread, thanks. I also found this for beginners who might be interested: Basic Circuits - Bypass Capacitors

    I have two boards with shift registers, one with capacitors and the other without.
    The capacitors are placed from the 5v to the ground and can be seen in the earlier pictures. Correct me if they're placed wrong :)!

    Oddly enough, the one without capacitors is functioning, while the other is not.
    The one that is not functioning has very thick buses soldered on.
    Makes me wonder if this might be generating some kind of inductance and damaging the IC's

    I will continue to do testing tomorrow, probably resolder and post if I find anything interesting
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Do you have a scope or a Good DMM?

    Some measurements are needed to go any further, since the basic issues seem to have been covered.

    Please add photo of bottom side of boards as well.
     
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