continuity check after mosfet

Discussion in 'The Projects Forum' started by mwawrzyniec, May 7, 2014.

  1. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    ghThis is a simple example of what I am trying to do and the problem that I am trying to overcome:

    This is arduino based, attached is an 8 bit shift controller, each out pin of the shift controller is running to a mosfet working as a power switch for 8 jindividual LED's.

    What I am trying to do is a continuity test to check and see if there is an LED properly connected without supplying enough power to noticeably light up the LED.
     
  2. #12

    Expert

    Nov 30, 2010
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    Sounds like testing the igniter on a model rocket. Try sending 100 ua through the LED and measure the resulting voltage across the LED.
     
  3. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    Thanks for the fast reply. I am attempting to function test each led individually.
     
  4. wayneh

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    Sep 9, 2010
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    I would argue that a "function" test requires producing light, as that is the basic function of an LED. But otherwise the proposed test, to look for a Vf when a small current is passed, makes sense. I suppose you could refine the test to require the Vf to be within an acceptable window.
     
  5. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    What would I use to produce 100 uA from a 5v power supply from an arduino?
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A 47k resistor?
     
  7. #12

    Expert

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    A bit less than that. If you have a white LED that needs 3.2 volts to light up, the resistor would be 18k to get 100 ua with the remaining 1.8 volts.

    Of course, I'm assuming that LEDs break over like a well behaved zener diode would.
     
  8. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    So here is what I came up with, please keep in mind this is my first electronics project. I omitted the connection to my shift registers. They will be connected to my arduino. I am attempting to build a launch controller for fireworks. The panel will be fitted with LED's for now so I can function test my circuit. The 9v will power the Talon ignighters (http://www.pyrouniverse.com/forum/showthread.php/32881-Talon-firing-current-data) when 5V+ARDUINO_1 is supplying 5V, alternatively when 5V+ARDUINO_2 is supplying 5V, the 74HC165N will be able to store the individual continuity checks.

    Will the 74HC165N will be burnt out during normal operation?

    View attachment controller.sch
     
  9. #12

    Expert

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    What do I open a .sch file with?
     
  10. mwawrzyniec

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    May 7, 2014
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  11. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    Here is a screen shot. The 9v power is at the top. Right below is ARDUINO_1. Right below that is ARDUINO_2 Screen Shot 2014-05-10 at 1.48.05 PM.png
     
  12. #12

    Expert

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    So, the question is: If I apply 9 volts to the data input of a 74HC165 that has a supply voltage of 5 volts, will it hurt the chip?

    I think the answer is "yes".
    The 74HC inputs have diode protection and they will burn out.
    Their normal input current is 1 microamp.
    The RCA book I'm looking at says the RCA QMOS devices have a 20 ma current limit, but I have no way of knowing if you're using RCA brand QMOS chips of if this book from 1989 is still the standard for these chips.

    Simple answer? Don't over-volt the inputs.

    Are the LEDs the square boxes right of center?
     
    Last edited: May 10, 2014
  13. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    The square boxes are 2 pole terminals that will have LED's in them for testing. If I put a resistor after the terminals will this solve my issue or is there a better way to handle this?
     
  14. #12

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    A resistor won't fix it. The simple way is don't apply excess voltage to the inputs.

    Besides this, I think you have missed several ideas. First, LEDs need a current limiting resistor or they will burn up. Second, the LEDs need a current path to ground and you didn't provide it. Third, LEDs are the most reliable light devices ever invented. Why do you want to test LEDs? To see if they are plugged in?
     
  15. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    The LEDs are there to test the circuit during beta testing. The 2 pole terminals are there to connect Talon clips. I am using the LED's because the talon clips are burned up during use. When I put an LED into the terminals I will attach a resistor to them. My goal is to be able to test if they are plugged in. Is there a better way to do this?
     
  16. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    I added a diode to each line, attached a common ground with a resistor after. I also added a MOSFET to the ground controlled by the same power source as the MOSFET that controls the 9V power. I believe this may solve my issue.

    Screen Shot 2014-05-10 at 3.56.09 PM.png
     
  17. #12

    Expert

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    Here's an idea. Add a resistor to limit the current to 100 ua, provide a limiting zener diode so you don't blow up the chip, and measure the results. If the input is high, the LED is connected and current is flowing. If the input is low, the LED is not working.

    ps, that's a 4.7 volt zener diode but it's hard to read with all those dots on the page.

    There are a lot of ways to do this. Yours might work just fine.
     
    Last edited: May 10, 2014
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  18. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    I was having some trouble to get this circuit to work, I have attached a schematic of the basic for my initial function test. The shift register is being controlled by an arduino. I measured the gave at 5v. When I run my code, i get LEDs placed in the terminals to light up, however when I switch them out to my talon clips they are not working. I measured the voltage after the mosfet "IRLB8743PBF" and got 1v. The mosfet should be fully saturated with the 5v at the gate? Does anyone have any suggestions?

    Thanks in advance,

    Screen Shot 2014-06-03 at 11.05.47 PM.png
     
  19. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Your fet is set up as a source follower instead of a switch, so the source will be a threshold voltage lower than the gate. Move the load to the drain side.
     
    mwawrzyniec likes this.
  20. mwawrzyniec

    Thread Starter New Member

    May 7, 2014
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    I looked at the data sheet, this is how it looks on my breadboard. If i flip the drain and source then the power is always on even why there is nothing connected to the gate?

    View attachment New Bitmap Image.bmp
     
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