+ Post New Thread
Page 1 of 3 1 2 3 LastLast
Results 1 to 20 of 46
  1. #1
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    See attached circuit. Circuit is designed to have the bulb light, when a very low amperage momentary switch is pressed. In the final circuit, the bulb will draw 4 to 5 amps for a short period of time when current is applied. Hence the need to protect the low amperage momentary switch.

    I am stumped by the actions of the SCR, which after the initial triggering of the circuit, continues to remain closed, anode to cathode, even when no current is applied to the component.

    I have separately tested this component, and it passes those tests. Do I just have this wired in correctly? Thanks!
    Click image for larger version. 

Name:	12th Test.jpg 
Views:	34 
Size:	224.2 KB 
ID:	143849

  2. #2
    Super Moderator
    Points: 26,100, Level: 39
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    7,866
    Helped
    974 / 974
    Points
    26,100
    Level
    39
    Blog Entries
    5

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    The SCR is only acting as expected: Once the gate has been triggered, its output terminals will only stop driving if the load current is zeroed or if a negative voltage is applied to the gate.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



  3. #3
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Thanks! Silly question, but how do I zero the load or apply negative voltage to the gate, if not my removing the power source (Battery)?



  4. #4
    Super Moderator
    Points: 241,738, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    41,978
    Helped
    12769 / 12769
    Points
    241,738
    Level
    100

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    The answer is simple: Don't use a triac or scr for DC circuits.

    Only special GTO (gate turn-off) SCR are able to be turned off by a negative gate voltage. A more realistic option would be a second SCR with series capacitor connected in parallel to the primary switch. But that's interesting for high voltage, high current switches. For your application, a MOSFET would be the obvious solution.



  5. #5
    Super Moderator
    Points: 26,100, Level: 39
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    7,866
    Helped
    974 / 974
    Points
    26,100
    Level
    39
    Blog Entries
    5

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    In addition to FvM's recommendation, going a little further: Considering its battery powered application, and supposedly the efficiency would be an important aspect to consider, choose a MOSFET with the lowest Rds(on) possible, in such a way that the voltage drop on it is as small as possible; as a reference, a BJT transitor would produce a voltage drop of ~0.7v, therefore any MOSFET with smaller product resistance x current would be suited.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



    •   Alt9th January 2018, 18:19

      advertising

        
       

  6. #6
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Ok thanks!

    But, I am in a quandary with that approach. The MOSFET's I have seen only supply output power while there is power supplied on the input side. I tried that and it did not work for this application. Reason is because the duration of the input power (momentary switch) is so short that it does not provide enough time for the bulb to fully light.

    That is why I went with an SCR in the first place, because its power/current production is not dependent upon the amount of time that the input power/current is supplied.

    Hope this makes sense?



  7. #7
    Super Moderator
    Points: 26,100, Level: 39
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    7,866
    Helped
    974 / 974
    Points
    26,100
    Level
    39
    Blog Entries
    5

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    That is why I went with an SCR in the first place, because its power/current production is not dependent upon the amount of time that the input power/current is supplied.
    The current drained by the gate side of the MOSFET is virtually null, so you could turn the states of the momentary switch keeping it On or Off for the amont of time you need with no concern on the power consumption of the coin cell.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



  8. #8
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    That is the rub. There is no ability to modify the input switch or the output bulb. They are what they are. Which means the solution must be achieved by modification to the circuit itself......



    •   Alt9th January 2018, 19:04

      advertising

        
       

  9. #9
    Advanced Member level 4
    Points: 6,154, Level: 18

    Join Date
    Dec 2015
    Location
    Madrid, Spain
    Posts
    1,101
    Helped
    267 / 267
    Points
    6,154
    Level
    18

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    A more realistic option would be a second SCR with series capacitor connected in parallel to the primary switch.
    Or shunt the SCR with a low value resistor. It is not as robust as the reverse voltage source and obviously not for high power, but it is another way.



  10. #10
    Super Moderator
    Points: 26,100, Level: 39
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    7,866
    Helped
    974 / 974
    Points
    26,100
    Level
    39
    Blog Entries
    5

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Quote Originally Posted by btvarner View Post
    That is the rub. There is no ability to modify the input switch or the output bulb. They are what they are. Which means the solution must be achieved by modification to the circuit itself......
    Try adding a capacitor in series with the momentary switch, this would perhaps produce a negative voltage to the gate of the SCR, but this would be nothing more than workaround, far from the suited solution.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



    •   Alt9th January 2018, 19:29

      advertising

        
       

  11. #11
    Advanced Member level 4
    Points: 6,626, Level: 19

    Join Date
    Jun 2015
    Posts
    1,171
    Helped
    275 / 275
    Points
    6,626
    Level
    19

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Hi,

    An alternative, that uses a few more parts and possibly power (to a degree) is a latching toggle pushbutton using transistors. This link to the mosiac industries page of such circuits is a good place to get ideas using MOSFETs for this. The page and the mosaic website weren't working just now, not sure why, but give it a try or in Google Images for the schematics if the page stays down a long time and if they could be an alternative to the SCR version.



  12. #12
    Super Moderator
    Points: 26,100, Level: 39
    andre_teprom's Avatar
    Join Date
    Nov 2006
    Location
    Brazil
    Posts
    7,866
    Helped
    974 / 974
    Points
    26,100
    Level
    39
    Blog Entries
    5

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Why simply do not just plug a mosfet into the MOC and NTE footprints and if so add some aerial wiring? It would not do that much work.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



  13. #13
    Advanced Member level 4
    Points: 6,154, Level: 18

    Join Date
    Dec 2015
    Location
    Madrid, Spain
    Posts
    1,101
    Helped
    267 / 267
    Points
    6,154
    Level
    18

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Quote Originally Posted by btvarner View Post
    There is no ability to modify the input switch or the output bulb. They are what they are. Which means the solution must be achieved by modification to the circuit itself..
    Just to get things right, you can not modify the circuitry from the left of the MOC, regarding post #1 schematic, but Yes you can modify the circuit from the right of the MOC ?



  14. #14
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Sorry to be unclear on that. I can modify anything in the circuit except,

    Left: The momentary switch itself
    Right: The bulb or the power supply (2 D Cell Batteries)



  15. #15
    Super Moderator
    Points: 241,738, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    41,978
    Helped
    12769 / 12769
    Points
    241,738
    Level
    100

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    But, I am in a quandary with that approach. The MOSFET's I have seen only supply output power while there is power supplied on the input side. I tried that and it did not work for this application. Reason is because the duration of the input power (momentary switch) is so short that it does not provide enough time for the bulb to fully light.

    That is why I went with an SCR in the first place, because its power/current production is not dependent upon the amount of time that the input power/current is supplied.
    This is achieved by the latching nature of the SCR switch. A trigger longer than some ten microseconds starts the current flow which isn't stopped before the load circuit is interrupted.

    Your requirements sound however confused. You say you want the pulse extension feature of the SCR circuit. On the other hand you want to stop the load current when the momentary switch is released. What does this exactly mean? You want pulse extension, but no latching effect? If so, how long should the extension be?

    In my view it's an arbitrary and bad idea to use a SCR switch for the intended purpose. It's not completely impossible to implement the behavior with SCRs, but much more difficult and needing more components than a MOSFET or BJT based circuit. Unless it's a special contest, it seems pointless.



  16. #16
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Hi FvM, yes it is a very special contest. Please follow the below links to images I have posted for clarification.

    Breadboard Layout Angle 1: http://brucevarner.com/picts/CurrentDesign_1.jpg
    Breadboard Layout Angle 2: http://brucevarner.com/picts/CurrentDesign_2.jpg
    Breadboard Attached for End Testing: http://brucevarner.com/picts/CurrentDesign_3.jpg
    Breadboard Attached CloseUp: http://brucevarner.com/picts/CurrentDesign_4.jpg

    Any currently produced devices designed to allow old flash units to be used with new cameras does not work in this application............ I have tested such units. I have also personally talked to the manufacturers, and they have confirmed this to me.
    Last edited by btvarner; 10th January 2018 at 00:05.



  17. #17
    Advanced Member level 5
    Points: 20,380, Level: 34
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    3,932
    Helped
    848 / 848
    Points
    20,380
    Level
    34

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Wait a minute. Do I understand this correctly?

    You connect the battery=> light off
    You press the switch, triggering the SCR=> light on
    You release the switch=> light on
    Light stays on forever, or until battery dies.
    You remove the battery=> Light off
    You install the battery=> light on

    This makes no sense. Shouldn't the SCR turn off once the current drops below several milliamps?

    It's still not clear how you are 'normally' expecting the light to turn off. It seems the only way is by removing the batteries.



    •   Alt10th January 2018, 01:42

      advertising

        
       

  18. #18
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    Hi Berry, you have it partially correct. Definations:
    "switch" as you used it to be the momentary switch or the switch that momentarily closes in the camera.
    "Light" as you used it to be the analog test bulb or the flash bulb in the flash unit.
    "Flash bulb" only stays on for milliseconds. When testing those, the "light" automatically goes off quickly as spent.....

    I connect the battery=> light off
    I press the switch, triggering the SCR=> light on (Test light stays on/Flash bulb burns & goes out)
    I release the switch=> Test light on / (Flash maintains current because when new bulb is inserted it wrongly, instantly ignites)
    Light stays on forever, or until battery dies.(Test light yes)
    I remove the battery=> Light off (Yes)
    You install the battery=> light on (Test light yes. New flash bulb will ignite instantly when installed)

    So, yes this is crazy. I thought at first that the flash had a malfunction, but the same thing happens with my test light & test battery.

    Therefore (In my opinion) the path somehow stays closed between the Anode & Cathode of the SCR...... Crazy right?



  19. #19
    Advanced Member level 5
    Points: 20,380, Level: 34
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    3,932
    Helped
    848 / 848
    Points
    20,380
    Level
    34

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    so you're expecting the SCR to turn off when the flashbulb burns out, right? Have you made any measurements? Like, what does the SCR gate voltage look like after you install the battery (but not the bulb).

    Does the SCR EVER turn off? How? What do you do to turn it off?



  20. #20
    Junior Member level 3
    Points: 156, Level: 2

    Join Date
    Nov 2017
    Posts
    26
    Helped
    0 / 0
    Points
    156
    Level
    2

    Re: Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

    I understand that when using the test bulb and battery, the light would stay on until the battery is disconnected. That makes sense. But yes, its seems that the expectation would be that with the actual flash bulb, after that flash bulb is expended and especially after it is removed from the flash gun, that current should stop because that action would break the circuit (anode to cathode). Thereby making the setup ready to do it trigger again upon closing the low side switch.

    But that is not what happens with the flash. I can even disconnect the flash from the circuit (Which should positively remove the current from the SCR) and the voltage reads 0, only to reconnect and have the multi-meter read that there still is again 3.1V across the circuit, before anything in triggered.

    The only true way to ensure that there is no voltage on the circuit after an initial triggering, is to physically remove the SCR from the breadboard, and reinstall. With that, all is ready for the complete process again..............

    I can understand that I might have something wired in correctly, but what I don't see is how the SCR can still be closed after the battery is removed, and remain so after the battery has been reinstalled? I assume that the MOC truly separates the trigger side power from the flash side power?



--[[ ]]--