Questions About BJTs in My Circuit











up vote
2
down vote

favorite












I have an LED multiplexer circuit (inspired by https://www.instructables.com/id/Multiplexing-with-Arduino-Transistors-I-made/) in which I have set the resistor values to maximize the current through the LEDs (without exceeding the maximum forward current through the LEDs or the maximum power dissipation in any of the components). It looks like this:



current circuit



current circuit with I labels



The R1, R2, R3, C1, C2, and C3 pins are Arduino digital pins while the 5V pin is the 5V from the Arduino. I have this circuit assembled on my breadboard and it works as expected. (Though please let me know if there is anything atrociously bad about it).



What I want to do is add a potentiometer between the 5V pin and the collectors of the Q-RX NPN BJTs so that I can vary the LED light level from off (or almost off) to "full brightness". It seems to me that this should work in theory. When I replace the 5V pin with GND in my simulation, I get ~0 current through the LEDs, which is what I want:





However, when I test with a single LED enabled and I unplug the 5V pin from the Arduino, the LED dims some but is still fairly bright. I measure ".001 A" with my multimeter (I get 0 with it set it to mA). This must be from current flowing from base to emitter of the Q-RX BJTs. I have been trying to re-familiarize myself with how transistors operate, but I need some help here to understand this. A few questions:




  1. Am I trying to use the NPN BJTs completely incorrectly?

  2. Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?

  3. How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?


Reference:




  • LEDs: https://www.cree.com/led-components/media/documents/C512A-WNS-WNN-942.pdf


  • BJTs (2N3904): https://www.fairchildsemi.com/datasheets/MM/MMBT3904.pdf


  • Resistors: https://www.amazon.com/gp/product/B07J48VNR7



Side note: I just noticed that my simulation shows ~54 mA at the C1, C2, and C3 pins which exceeds the 40 mA Arduino pin limit, but that is besides the point.



Any help is greatly appreciated! Thanks!










share|improve this question







New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.




















  • Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
    – jonk
    4 hours ago










  • This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
    – Tony EE rocketscientist
    2 hours ago












  • I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
    – Misunderstood
    2 hours ago















up vote
2
down vote

favorite












I have an LED multiplexer circuit (inspired by https://www.instructables.com/id/Multiplexing-with-Arduino-Transistors-I-made/) in which I have set the resistor values to maximize the current through the LEDs (without exceeding the maximum forward current through the LEDs or the maximum power dissipation in any of the components). It looks like this:



current circuit



current circuit with I labels



The R1, R2, R3, C1, C2, and C3 pins are Arduino digital pins while the 5V pin is the 5V from the Arduino. I have this circuit assembled on my breadboard and it works as expected. (Though please let me know if there is anything atrociously bad about it).



What I want to do is add a potentiometer between the 5V pin and the collectors of the Q-RX NPN BJTs so that I can vary the LED light level from off (or almost off) to "full brightness". It seems to me that this should work in theory. When I replace the 5V pin with GND in my simulation, I get ~0 current through the LEDs, which is what I want:





However, when I test with a single LED enabled and I unplug the 5V pin from the Arduino, the LED dims some but is still fairly bright. I measure ".001 A" with my multimeter (I get 0 with it set it to mA). This must be from current flowing from base to emitter of the Q-RX BJTs. I have been trying to re-familiarize myself with how transistors operate, but I need some help here to understand this. A few questions:




  1. Am I trying to use the NPN BJTs completely incorrectly?

  2. Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?

  3. How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?


Reference:




  • LEDs: https://www.cree.com/led-components/media/documents/C512A-WNS-WNN-942.pdf


  • BJTs (2N3904): https://www.fairchildsemi.com/datasheets/MM/MMBT3904.pdf


  • Resistors: https://www.amazon.com/gp/product/B07J48VNR7



Side note: I just noticed that my simulation shows ~54 mA at the C1, C2, and C3 pins which exceeds the 40 mA Arduino pin limit, but that is besides the point.



Any help is greatly appreciated! Thanks!










share|improve this question







New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.




















  • Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
    – jonk
    4 hours ago










  • This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
    – Tony EE rocketscientist
    2 hours ago












  • I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
    – Misunderstood
    2 hours ago













up vote
2
down vote

favorite









up vote
2
down vote

favorite











I have an LED multiplexer circuit (inspired by https://www.instructables.com/id/Multiplexing-with-Arduino-Transistors-I-made/) in which I have set the resistor values to maximize the current through the LEDs (without exceeding the maximum forward current through the LEDs or the maximum power dissipation in any of the components). It looks like this:



current circuit



current circuit with I labels



The R1, R2, R3, C1, C2, and C3 pins are Arduino digital pins while the 5V pin is the 5V from the Arduino. I have this circuit assembled on my breadboard and it works as expected. (Though please let me know if there is anything atrociously bad about it).



What I want to do is add a potentiometer between the 5V pin and the collectors of the Q-RX NPN BJTs so that I can vary the LED light level from off (or almost off) to "full brightness". It seems to me that this should work in theory. When I replace the 5V pin with GND in my simulation, I get ~0 current through the LEDs, which is what I want:





However, when I test with a single LED enabled and I unplug the 5V pin from the Arduino, the LED dims some but is still fairly bright. I measure ".001 A" with my multimeter (I get 0 with it set it to mA). This must be from current flowing from base to emitter of the Q-RX BJTs. I have been trying to re-familiarize myself with how transistors operate, but I need some help here to understand this. A few questions:




  1. Am I trying to use the NPN BJTs completely incorrectly?

  2. Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?

  3. How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?


Reference:




  • LEDs: https://www.cree.com/led-components/media/documents/C512A-WNS-WNN-942.pdf


  • BJTs (2N3904): https://www.fairchildsemi.com/datasheets/MM/MMBT3904.pdf


  • Resistors: https://www.amazon.com/gp/product/B07J48VNR7



Side note: I just noticed that my simulation shows ~54 mA at the C1, C2, and C3 pins which exceeds the 40 mA Arduino pin limit, but that is besides the point.



Any help is greatly appreciated! Thanks!










share|improve this question







New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











I have an LED multiplexer circuit (inspired by https://www.instructables.com/id/Multiplexing-with-Arduino-Transistors-I-made/) in which I have set the resistor values to maximize the current through the LEDs (without exceeding the maximum forward current through the LEDs or the maximum power dissipation in any of the components). It looks like this:



current circuit



current circuit with I labels



The R1, R2, R3, C1, C2, and C3 pins are Arduino digital pins while the 5V pin is the 5V from the Arduino. I have this circuit assembled on my breadboard and it works as expected. (Though please let me know if there is anything atrociously bad about it).



What I want to do is add a potentiometer between the 5V pin and the collectors of the Q-RX NPN BJTs so that I can vary the LED light level from off (or almost off) to "full brightness". It seems to me that this should work in theory. When I replace the 5V pin with GND in my simulation, I get ~0 current through the LEDs, which is what I want:





However, when I test with a single LED enabled and I unplug the 5V pin from the Arduino, the LED dims some but is still fairly bright. I measure ".001 A" with my multimeter (I get 0 with it set it to mA). This must be from current flowing from base to emitter of the Q-RX BJTs. I have been trying to re-familiarize myself with how transistors operate, but I need some help here to understand this. A few questions:




  1. Am I trying to use the NPN BJTs completely incorrectly?

  2. Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?

  3. How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?


Reference:




  • LEDs: https://www.cree.com/led-components/media/documents/C512A-WNS-WNN-942.pdf


  • BJTs (2N3904): https://www.fairchildsemi.com/datasheets/MM/MMBT3904.pdf


  • Resistors: https://www.amazon.com/gp/product/B07J48VNR7



Side note: I just noticed that my simulation shows ~54 mA at the C1, C2, and C3 pins which exceeds the 40 mA Arduino pin limit, but that is besides the point.



Any help is greatly appreciated! Thanks!







transistors led bjt multiplexer






share|improve this question







New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question







New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question






New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









asked 5 hours ago









nabelekt

111




111




New contributor




nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






nabelekt is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












  • Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
    – jonk
    4 hours ago










  • This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
    – Tony EE rocketscientist
    2 hours ago












  • I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
    – Misunderstood
    2 hours ago


















  • Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
    – jonk
    4 hours ago










  • This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
    – Tony EE rocketscientist
    2 hours ago












  • I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
    – Misunderstood
    2 hours ago
















Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
– jonk
4 hours ago




Examine this to start out. Some is covered there. Also, a book by HP called "Optoelectronics:Fiber-Optics Applications Manual", 2nd edition, might include lots of useful information to help with this and into the future of similar projects.
– jonk
4 hours ago












This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
– Tony EE rocketscientist
2 hours ago






This design is hopeless since base current * hFE gain is shared by 1 or more LEDs. The only thing that might work with this Matrix is a regulated at Vf and low RdsOn logic level FETs (<1 ohm)
– Tony EE rocketscientist
2 hours ago














I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
– Misunderstood
2 hours ago




I would go with the Cree C503D-WAN. Better efficiency so you can lower the current or have a greater range of brightness. Use scan frequency and on pulse width to control the brightness. You are not using a constant current so the max pulsed current will be higher than 25 mA (C512A) 20 mA (C503D). Pulsed current can go as high as 100 mA with either of these Cree LEDs. I would go along with @Jasen on using current limiting resistors, whereas some believe it's okay to use the Aurdino pin as the current limiter. And Tony's FET is a good idea as well.
– Misunderstood
2 hours ago










1 Answer
1






active

oldest

votes

















up vote
2
down vote













First off your matrix has no current limit on the LEDs you need to add resistors, where you add them depends on how you intend to use the matrix. That instructible has these resistors in the "anode" circuit vetween the collector and the LED anodes. that's suitable for when you turn the rows on one at a time and then light some LEDs on each row.




Am I trying to use the NPN BJTs completely incorrectly?




Not completely incorrectly, but you need to remember that a BJT also looks like a pair of diodes when it's not amplifying signals.



so with "V+" disconnected the BJT will pass the current that arrives through "R-Rx" on to the LED
with the BJT grounded via a resistor the B-C diode of the BJT will divert some of the R-Rx curren to ground causing the LED to dim.




Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?




Q-Cx can generate negative voltage on the collector due to internally generated photocurrent thus making enough voltage to light the LEDs. I don't think any simulators will simulate this very minor transistor effect.




How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?




Use PWM to dim the LEDs. I see a matrix:so I'm guessing you've figured out how to scan a matrix. when you want the LEDs dimmer just do each row (or column) faster and then wait for a while before doing the rescan.



you probably don't need trasnsitors on both the rows and columns which ever. is active in parallel doesn't need transistors (but does need resistors), the microcontroller can make enough current to light the LEDs, but where the currents group together you need the transistors.



R-Cx seem to be very low resistance, those resistors would be better used between the collector and the column. put 1K resistor in their place (on the base of the Q-Cx)






share|improve this answer























    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["\$", "\$"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("schematics", function () {
    StackExchange.schematics.init();
    });
    }, "cicuitlab");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "135"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    convertImagesToLinks: false,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: null,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });






    nabelekt is a new contributor. Be nice, and check out our Code of Conduct.










    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f411244%2fquestions-about-bjts-in-my-circuit%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    1 Answer
    1






    active

    oldest

    votes








    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    2
    down vote













    First off your matrix has no current limit on the LEDs you need to add resistors, where you add them depends on how you intend to use the matrix. That instructible has these resistors in the "anode" circuit vetween the collector and the LED anodes. that's suitable for when you turn the rows on one at a time and then light some LEDs on each row.




    Am I trying to use the NPN BJTs completely incorrectly?




    Not completely incorrectly, but you need to remember that a BJT also looks like a pair of diodes when it's not amplifying signals.



    so with "V+" disconnected the BJT will pass the current that arrives through "R-Rx" on to the LED
    with the BJT grounded via a resistor the B-C diode of the BJT will divert some of the R-Rx curren to ground causing the LED to dim.




    Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?




    Q-Cx can generate negative voltage on the collector due to internally generated photocurrent thus making enough voltage to light the LEDs. I don't think any simulators will simulate this very minor transistor effect.




    How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?




    Use PWM to dim the LEDs. I see a matrix:so I'm guessing you've figured out how to scan a matrix. when you want the LEDs dimmer just do each row (or column) faster and then wait for a while before doing the rescan.



    you probably don't need trasnsitors on both the rows and columns which ever. is active in parallel doesn't need transistors (but does need resistors), the microcontroller can make enough current to light the LEDs, but where the currents group together you need the transistors.



    R-Cx seem to be very low resistance, those resistors would be better used between the collector and the column. put 1K resistor in their place (on the base of the Q-Cx)






    share|improve this answer



























      up vote
      2
      down vote













      First off your matrix has no current limit on the LEDs you need to add resistors, where you add them depends on how you intend to use the matrix. That instructible has these resistors in the "anode" circuit vetween the collector and the LED anodes. that's suitable for when you turn the rows on one at a time and then light some LEDs on each row.




      Am I trying to use the NPN BJTs completely incorrectly?




      Not completely incorrectly, but you need to remember that a BJT also looks like a pair of diodes when it's not amplifying signals.



      so with "V+" disconnected the BJT will pass the current that arrives through "R-Rx" on to the LED
      with the BJT grounded via a resistor the B-C diode of the BJT will divert some of the R-Rx curren to ground causing the LED to dim.




      Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?




      Q-Cx can generate negative voltage on the collector due to internally generated photocurrent thus making enough voltage to light the LEDs. I don't think any simulators will simulate this very minor transistor effect.




      How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?




      Use PWM to dim the LEDs. I see a matrix:so I'm guessing you've figured out how to scan a matrix. when you want the LEDs dimmer just do each row (or column) faster and then wait for a while before doing the rescan.



      you probably don't need trasnsitors on both the rows and columns which ever. is active in parallel doesn't need transistors (but does need resistors), the microcontroller can make enough current to light the LEDs, but where the currents group together you need the transistors.



      R-Cx seem to be very low resistance, those resistors would be better used between the collector and the column. put 1K resistor in their place (on the base of the Q-Cx)






      share|improve this answer

























        up vote
        2
        down vote










        up vote
        2
        down vote









        First off your matrix has no current limit on the LEDs you need to add resistors, where you add them depends on how you intend to use the matrix. That instructible has these resistors in the "anode" circuit vetween the collector and the LED anodes. that's suitable for when you turn the rows on one at a time and then light some LEDs on each row.




        Am I trying to use the NPN BJTs completely incorrectly?




        Not completely incorrectly, but you need to remember that a BJT also looks like a pair of diodes when it's not amplifying signals.



        so with "V+" disconnected the BJT will pass the current that arrives through "R-Rx" on to the LED
        with the BJT grounded via a resistor the B-C diode of the BJT will divert some of the R-Rx curren to ground causing the LED to dim.




        Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?




        Q-Cx can generate negative voltage on the collector due to internally generated photocurrent thus making enough voltage to light the LEDs. I don't think any simulators will simulate this very minor transistor effect.




        How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?




        Use PWM to dim the LEDs. I see a matrix:so I'm guessing you've figured out how to scan a matrix. when you want the LEDs dimmer just do each row (or column) faster and then wait for a while before doing the rescan.



        you probably don't need trasnsitors on both the rows and columns which ever. is active in parallel doesn't need transistors (but does need resistors), the microcontroller can make enough current to light the LEDs, but where the currents group together you need the transistors.



        R-Cx seem to be very low resistance, those resistors would be better used between the collector and the column. put 1K resistor in their place (on the base of the Q-Cx)






        share|improve this answer














        First off your matrix has no current limit on the LEDs you need to add resistors, where you add them depends on how you intend to use the matrix. That instructible has these resistors in the "anode" circuit vetween the collector and the LED anodes. that's suitable for when you turn the rows on one at a time and then light some LEDs on each row.




        Am I trying to use the NPN BJTs completely incorrectly?




        Not completely incorrectly, but you need to remember that a BJT also looks like a pair of diodes when it's not amplifying signals.



        so with "V+" disconnected the BJT will pass the current that arrives through "R-Rx" on to the LED
        with the BJT grounded via a resistor the B-C diode of the BJT will divert some of the R-Rx curren to ground causing the LED to dim.




        Why does my simulation (I'm using OrCAD Capture Lite) show ~0 current through the LEDs when I can measure it at ~1 mA and there is obviously enough to light the LEDs?




        Q-Cx can generate negative voltage on the collector due to internally generated photocurrent thus making enough voltage to light the LEDs. I don't think any simulators will simulate this very minor transistor effect.




        How can I adjust this circuit to achieve my goal? Should logic level MOSFETs be used instead?




        Use PWM to dim the LEDs. I see a matrix:so I'm guessing you've figured out how to scan a matrix. when you want the LEDs dimmer just do each row (or column) faster and then wait for a while before doing the rescan.



        you probably don't need trasnsitors on both the rows and columns which ever. is active in parallel doesn't need transistors (but does need resistors), the microcontroller can make enough current to light the LEDs, but where the currents group together you need the transistors.



        R-Cx seem to be very low resistance, those resistors would be better used between the collector and the column. put 1K resistor in their place (on the base of the Q-Cx)







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 4 hours ago

























        answered 5 hours ago









        Jasen

        9,7401428




        9,7401428






















            nabelekt is a new contributor. Be nice, and check out our Code of Conduct.










            draft saved

            draft discarded


















            nabelekt is a new contributor. Be nice, and check out our Code of Conduct.













            nabelekt is a new contributor. Be nice, and check out our Code of Conduct.












            nabelekt is a new contributor. Be nice, and check out our Code of Conduct.
















            Thanks for contributing an answer to Electrical Engineering Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.





            Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


            Please pay close attention to the following guidance:


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f411244%2fquestions-about-bjts-in-my-circuit%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            A CLEAN and SIMPLE way to add appendices to Table of Contents and bookmarks

            Calculate evaluation metrics using cross_val_predict sklearn

            Insert data from modal to MySQL (multiple modal on website)