Short circuit in PWM led dimmer with 555 & pot

Discussion in 'Technical Repair' started by outro, Mar 7, 2017.

  1. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    I'm quite new to electronics and I got this working already once, but now for some reason I get short circuit (variable PSU goes crazy with current, even when voltage <2)

    I don't expect anyone to find the problem from the pictures, but maybe help me to start debugging from the right place

    Multimeter on the 'beep' mode (signal?) beeps when I hold other in VCC and other in GND - so I guess it's a short circuit

    In short, VCC in red wire, GND in black, led gets GND from MOSFET center pin.
     
  2. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Didn't find edit button, but the beeping pins (555) are 1 5 8
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
    3,757
    1,963
    Welcome to AAC!

    Based on your pics the first thing I would look for is a solder flake or stray wire strand that has contacted something. Your soldering is not the best from the picture and many electronics problems can be traced to connections - either too few or too many i.e. shorts. Inspect carefully and reheat the joints. That will usually break inadvertent bridges plus some of the joints on the bigger pins near the bottom of the pic look cold i.e. not enough heat to cause the solder to flow smoothly to make the joint. You also can try dragging the tip of an X-Acto knife between the joints to remove hard to see bridges.

    Assuming the connections are correct and tidy, I';d look at the 470uF capacitor to be sure it is in the right way. A backwards electrolytic can work for awhile then fail short.

    Try measuring in the ohms mode without the beeper. Short the leads and note the reading if it's non-zero. That is your short circuit reading. Then probe around looking for a tiny increase in the reading. If you see one, you are moving away from the short. That doesn't always work but something to try. You may find that the problem is not a dead short (from a solder blob) but an overload due to a fried component that will show a low but not zero ohms reading.

    If your power supply can handle it, connect the circuit and set the current limit to max. Then see if anything gets hot while it's carrying the short current. Wear eye protection and cover the board so that if a cap or chip explodes, you don't get hurt.

    Failing all of that, the usual approach is to disconnect the things that connect to the power bus one at a time to see what clears the short.

    Good luck!
     
  4. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Thanks a lot for your response!
    I did pass through the joints with X-Acto knife, but this didn't resole the problem.
    This circuit board will be a good debugging practice and I will definitely try out your tips and hope to be successful.

    On a completely different note; what kind of wire should I be looking for? Seems like the stranded wire is not so suitable for soldering circuit boards? A quick look at eBay and everything seems very expensive compared to all other components sold there.
    Am I missing something?

    PS: Thanks again for the great response!
     
  5. JohnInTX

    Moderator

    Jun 26, 2012
    3,757
    1,963
    I use solid wire for boards like this.
    30ga. Kynar-insulated wire wrap wire for point to point and 26ga. solid for off board. The 26ga. is ok for point to point too but if you have a lot going on it kind of piles up. If going to something off board that flops around, sometimes I'll use stranded for flexibility. After stripping, tin the bare tip of the wire with solder to hold the strands together while you work with it.

    https://www.amazon.com/Insulated-Kynar-30AWG-100ft-Black/dp/B000PDJGP2

    Note: the link is representative. There is also some generic 30ga. stuff there but it is PVC insulated, not Kynar. PVC melts easily while soldering, Kynar not so much so stick with that.
     
    Last edited: Mar 8, 2017
  6. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Having some bad luck or bad skills, can't seem to get it right.
    Rebuilt the whole circuit and now the led doesn't dim?

    Any thoughts?
     
  7. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Replying to myself...

    Could the problem lie towards the reset pin? Always thought it could be left open.
    Somehow seems that the current skips the pot?
     
  8. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    I think it might have something to do with the PIN2.
    When I move the yellow and red cable going to pin 2 the lamp flickers, turns off, or goes full bright?

    I got it once work but I think something is not correct so it broke again
     
  9. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    I have started to go crazy, talking by myself but that's okay.

    I wonder if it's okay to merge the 2 diodes together in the potentiometer?
     
  10. Alec_t

    Expert

    Sep 17, 2013
    10,116
    2,468
    If you mean short the two diodes together then no. That will disable adjustment of the duty cycle.
     
  11. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Ahaa, how does this differ from putting them on breadboard in line?
    (As in the schematics)
    I try to explain as well as I can, hope you can understand.

    pin8]...
    pin7]resistor and diode -> and diode <-
    pin6]...
    pin5]...
     
  12. Alec_t

    Expert

    Sep 17, 2013
    10,116
    2,468
    Sorry, I don't understand. Any connection other than what is shown in the schematic will give a result different from what the designer intended. That design is a classic circuit known to work.
     
  13. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    Okay, I totally agree if you don't understand, so I show a working version of the breadboard. I'm very sorry I used Fritzing, I heard it's not very liked.
     
  14. Alec_t

    Expert

    Sep 17, 2013
    10,116
    2,468
    So you've solved the problem?
    The new pic shows the two ends of the pot shorted by the column of interconnected nodes in the breadboard. The pot would be ineffective at varying the duty cycle.
     
  15. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    I haven't actually.
    This is what I understood from the schematics, is this wrong? How should I change it?
    So simple circuit, yet so many hours and days spent on it, hehe
     
  16. Alec_t

    Expert

    Sep 17, 2013
    10,116
    2,468
    Make sure the wires from the two ends of the pot don't share a single column of connection points in the breadboard.
     
  17. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    You mean the ones touching the pot?
    I have one this last circuit as well as I can.
    The problem lies somewhere near the pin2, any ideas?
    If I move the pot sometimes it works, or shuts down, or is @full brightness

    edit
    Thanks for taking your time to response :)
     
  18. Alec_t

    Expert

    Sep 17, 2013
    10,116
    2,468
    Perhaps you have a poor connection in the breadboard socket.
     
  19. outro

    Thread Starter Member

    Mar 7, 2017
    43
    1
    I'm using the soldering board
     
  20. absf

    AAC Fanatic!

    Dec 29, 2010
    1,926
    546
    Hi outro,

    If you lay out your board properly on your donnut perfboard, it should look something as below...

    555 PWM.gif

    Allen
     
Loading...