Timeline for RGB Hub using MUX and PNP
Current License: CC BY-SA 4.0
22 events
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| Feb 24, 2023 at 1:12 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 24, 2023 at 0:01 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 23, 2023 at 23:46 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 23, 2023 at 23:20 | vote | accept | superN8 | ||
| Feb 23, 2023 at 22:17 | answer | added | Ste Kulov | timeline score: 2 | |
| Feb 23, 2023 at 3:24 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 22, 2023 at 23:05 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 22, 2023 at 23:01 | comment | added | superN8 | @SteKulov Yes I do have access to more (3) GPIO pins. I could even add another MCU if necessary. I've just been avoiding MOSFETs because I haven't spent as much time learning about them, but I am very curious to see your solution. | |
| Feb 22, 2023 at 22:57 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 22, 2023 at 5:34 | comment | added | Ste Kulov | Sorry for the confusion. I should have said emitter-base diode, since I'm referring to Q1. I don't know your LED voltage drop so it's hard to say if it will affect you much. Basically, the equivalent circuit would be 12V->190Ω->LED->Si-Diode->GND. Yes, using NPN transistors will trade emitter-base drop for saturation voltage which is much lower. With less drop, you get more headroom and less power lost. I can propose you a solution using N-channel MOSFETs which will be much better, but I need a bit more info: Are you able to spare an additional MCU GPIO pin, or do you only have access to one? | |
| Feb 22, 2023 at 2:03 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 22, 2023 at 1:41 | comment | added | superN8 | Thank you for the tips @SteKulov I have made edits to the question to reflect your input. I plotted the Power dissipation and it looks okay, but better when using an NPN instead. Could you elaborate more on the voltage head room? Are you referring to Q1 or Q3? | |
| Feb 22, 2023 at 1:38 | history | edited | superN8 | CC BY-SA 4.0 |
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| Feb 21, 2023 at 19:41 | comment | added | Ste Kulov |
One extra comment about the simulation. The LED symbol is using the default model D, which is a very basic silicon PN junction diode which will give you around 0.76V drop at 60mA. If you're going to rely on your simulation results for resistor and power draw calculations, you need to use a proper LED model which more closely matches your actual LED. Try picking from the built-in diode list and sort by type and find the few LEDs in the list. Try them out and plot the voltage across them to see which one works best for you.
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| Feb 21, 2023 at 19:36 | comment | added | Ste Kulov | I like your solution. To clarify, you're using a PNP emitter follower (i.e. common collector; i.e. class A amplifier) to get more current out of your multiplexer. Some downsides to this are the make-before-break problem, you lose voltage headroom due to the base-emitter diode, and you might be wasting power in the PNP (plot the power dissipation in your simulation to make sure you're not burning it up). If you're confident you can solve the make-before-break using a different mux, and the voltage headroom is not an issue, and the power wasted is not an issue...then I think you'll be OK. | |
| Feb 21, 2023 at 1:25 | comment | added | superN8 | Perhaps I should add that the spice sim did function ideally (barring the current spike) and I was able to control the LEDs using this NPN+PNP configuration on a bread board (single controller, no MCU, no mux) it does technically work, but are you suggesting it's not ideal? | |
| Feb 21, 2023 at 1:01 | comment | added | superN8 | @vir yes you are correct are about the intended behavior, as well as Q4 being inaccessible (Q2 is but for simplicity I'm happy to say it's not). I am however confused about your suggestion to use a NAND gate. Are you suggesting I should pass the Microcontroller signal through the NAND, and use the low side NPNs of the controllers to produce a NOT signal that would then pass into the base of the NPN/N-channel that youre proposing? Thats the only way I could think of since NAND gates seem to max out at around 40mA, and I'm not sure how that would be different from multiplexing | |
| Feb 20, 2023 at 22:31 | comment | added | vir | I believe the bases of Q2 and Q4 are inaccessible. That PNP transistor won't operate correctly on the low side. I think you should be able to use a low-side NPN or N-channel MOSFET and some creative wiring of a quad-NAND to translate the open-collector signals from the controllers and logic level signal from the microcontroller into a suitable control signal for the NPN/N-channel. | |
| Feb 20, 2023 at 21:24 | comment | added | jsotola | remove the mux ... connect Q2 and Q4 collectors together ... multiplex the Q2 and Q4 bases | |
| Feb 20, 2023 at 21:14 | comment | added | vir | Let's take a step back and clarify the intended behavior: you have two low-side switching controllers and you want to have one or the other controlling the same LED strip based on a signal from the microcontroller? | |
| S Feb 20, 2023 at 21:05 | review | First questions | |||
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| S Feb 20, 2023 at 21:05 | history | asked | superN8 | CC BY-SA 4.0 |