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  1. #11
    Hi Cropwell
    mate as soon as i connect pin 3 lm78 to 16 on l293d the voltage drops from 5V to 2.56 and the motor power drops by about 50%
    unhook pin 3 and gets 5v perfect
    cant see where ive gone wrong i have 8.3 v at pin 1 LM78 and 8 on L29
    motor connections 11, 14
    switch power from pin 16,9
    other switch wires pin 10,15
    Have tries removing switch power at 16 and feeding both + sides from pin 9 but doesnt work
    any help please?
    also Doddy may have a thought

  2. #12
    Hi Cropwell
    mate as soon as i connect pin 3 lm78 to 16 on l293d the voltage drops from 5V to 2.56 and the motor power drops by about 50%
    unhook pin 3 and gets 5v perfect
    cant see where ive gone wrong i have 8.3 v at pin 1 LM78 and 8 on L29
    motor connections 11, 14
    switch power from pin 16,9
    other switch wires pin 10,15
    Have tries removing switch power at 16 and feeding both + sides from pin 9 but doesnt work
    any help please?
    also Doddy may have a thought

  3. #13
    Apologies - I started to reply to this earlier before work but somehow lost my post. This is an abridged version:


    I'd be concentrating in the first instance on establishing the logic supply (the +5V) to the L293 - without this you're going no-where. Add decoupling caps if you've not done so already (ref. the LM7805 datasheet for detail). If you can, measure the current drawn by the L293 - the standard 7805 can source 1A, and will current limit somewhere above that (by dropping the V-out). The L293 datasheet suggests the logic supply should be 60mA, max - above this and you have a problem - either wiring, or a knackered L293. Any metal tab on the 7805 is internally connected to common (ground/0V) - so take care that this doesn't short to something that it shouldn't.


    I wouldn't even worry about the motor wiring etc until you stabilise the 5V logic supply.


    If you find you keep blowing the L293s, I would consider removing the motor and channel-by-channel testing the circuit/switches with a DMM to ground.


    The circuit is trivial (I mean that in the sense that there's little to go wrong) - it should be straightforward to diagnose with the above. If you report back we can take it further.

  4. #14
    Hi Doddy
    thanks for above
    decoupling caps never heard of but googled and think i know where to put them. I dont have any so onwards
    the output of LM708 is between 3.8 and 5v ( just changes sometimes!!!!! )
    regardless the button for cw produces an output around the battery input so i guess you could say that circuit is working
    BUT i can get any power out of anything when i press CCW button even tried the switch on the other side of the l293 any nothing there either
    Hsvr tried 3 different L293 ans 2 lm708 and the same happens
    when i do have a static current of 5v to pin 16 the same results (cw works fine but ccw nothing
    Am i wiring the l293 wrong?
    looked at its data sheet and all seems correct in my brain ( not sure what inverting means
    all i want is to have 2 buttons
    1 to make 6v motor cw push another and ccw.
    Is there a better module ? i only have 23mm max width of any piece so a whole aduno breadboard thing wont fit
    just tried new Lm708 and when pin 16 is NOT connected i get a static 5v. as soon as wired to 16 drops to 3.8 ish seeems strange i have a 1.2v on pin 14 and or 11 like its bleeding power to these pins ( motor connections )Click image for larger version. 

Name:	l293d diag revb.jpg 
Views:	97 
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ID:	28690

  5. #15
    Quote Originally Posted by rjsutton View Post
    Hi Doddy
    thanks for above
    decoupling caps never heard of but googled and think i know where to put them. I dont have any so onwards
    the output of LM708 is between 3.8 and 5v ( just changes sometimes!!!!! )
    regardless the button for cw produces an output around the battery input so i guess you could say that circuit is working
    BUT i can get any power out of anything when i press CCW button even tried the switch on the other side of the l293 any nothing there either
    Hsvr tried 3 different L293 ans 2 lm708 and the same happens
    when i do have a static current of 5v to pin 16 the same results (cw works fine but ccw nothing
    Am i wiring the l293 wrong?
    looked at its data sheet and all seems correct in my brain ( not sure what inverting means
    all i want is to have 2 buttons
    1 to make 6v motor cw push another and ccw.
    Is there a better module ? i only have 23mm max width of any piece so a whole aduno breadboard thing wont fit
    just tried new Lm708 and when pin 16 is NOT connected i get a static 5v. as soon as wired to 16 drops to 3.8 ish seeems strange i have a 1.2v on pin 14 and or 11 like its bleeding power to these pins ( motor connections )Click image for larger version. 

Name:	l293d diag revb.jpg 
Views:	97 
Size:	155.6 KB 
ID:	28690
    The caps should go as close as possible to the input/output pins on the 7805 - you might find that it's possible to mount them physically to to the 7805 pins.

    The behaviour of the caps is the stabilise the operation of the 7805 - which can go into oscillation in the absence of these - that is bad (I'm not suggesting that's your problem - although I could imagine you reading a different voltage on a DMM than 5V if the 7805 goes into oscillation). If I've time tonight I might try testing a 7805 with/without capacitors to observe the behaviour.

    The 7805=5V dropping to 3.8V on connecting to Pin16/L293 is very significant, and I'm thinking that's the thing we need to fix.

    Can you take a photo of the electronic assembly - I just want to eyeball what you've done wrt the schematic, take out any uncertainty on my side. Also, the underside of the board (I'm assuming some form of breadboard/vero board). In particular there I want to make sure that you've isolated pins on the same row on the L293 (1-16, 2-15 etc).

    Re "Inverting" - the only reference on the datasheet is the non-inverting inputs on the L293 device. This simply means that if the input voltage is "high" (near 5V) then the output voltage will be "high" (near VCC2). If the input voltage is "low" (near 0V) then the output voltage will be low (near 0V),

    The L293 is a suitable device for what you're trying to do.

  6. #16
    Hi Doddy,

    A couple of questions.
    1. Can a logic input float or does it need to be strapped to ground with a 10k resistor?
    2. Do the CW and CCW buttons operate independantly or do you have to press one of them for movement and the other at the same time to change direction.

    When operating a module from an arduino, I make it standard practice to have a resistor to ground on the input (I have been caught out big time in the past with floating logic)

    Rob

  7. #17
    Datasheet says TTL compliant - so they can float (but float high). Looking at the equivalent input circuit on the datasheet also corroborates this - and really you'd want the switches to pull low, rather than pull high. But that doesn't explain the behaviour of the PSU.

  8. #18
    Doddy
    im not using any breadboard etc im soldering directly to the L293d pins i dont physically have the space to have a breadboard
    Ive been super careful to keep the heat down and not shorting with the solder ( between pins) that said the photo would not be clear enough
    ive recreated with 5 no L293d and 3no lm708s and get identical readings and problems so dont think its a soldering problem
    I dont understand why the power feed from 16 is looped thru push button + to pin9
    is the drawing fundamentally wrong?

  9. #19
    Quote Originally Posted by rjsutton View Post
    Doddy
    im not using any breadboard etc im soldering directly to the L293d pins i dont physically have the space to have a breadboard
    Ive been super careful to keep the heat down and not shorting with the solder ( between pins) that said the photo would not be clear enough
    ive recreated with 5 no L293d and 3no lm708s and get identical readings and problems so dont think its a soldering problem
    I dont understand why the power feed from 16 is looped thru push button + to pin9
    is the drawing fundamentally wrong?
    Pin 16 (VCC1) is connected directly to Pin 9, according to your diagram. Pin 9 is the 3,4 enable pin - if HIGH (5V) then the outputs 3,4 (pins 11, 14) are enabled. if Low then outputs 3,4 go high-impendance (Read: OFF). So what is concerning you?

    The other contact of the switch (marked CCW in your diagram) goes into the pin 10 (input 3, "3A") which drives through the driver chip one side of the motor to either +VCC2 or GND.

    Describe the behaviour of the 5V supply - does it drop to 3.8 V in response to a button press?

    EDIT: Just a thought as I was about to leave the house... wiring directly to pins, as far as I understand from your description you're completely disregarding the thermal requirements for the L293D - read the datasheet.
    Last edited by Doddy; 07-08-2020 at 06:48 AM.

  10. #20
    No it just drops to 3.8v when its connected to 16
    i thought the complete circuit 16-cw-ccw-9 would cause problems . obviously its fine
    the buttons are pressed separately CW produces a current of 8.21 when pressed from pin 14 to motor and .12 from 11
    Cww doesnt produce anything. what SHOULD occur. would the current from pin 14 be .12 be 8.21 from 11 ? would this create a ccw situation at motor?

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