Timeline for Driving an LED with a P-MOSFET or op-amp as a current limiter and no resistor
Current License: CC BY-SA 4.0
31 events
| when toggle format | what | by | license | comment | |
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| Aug 18, 2023 at 17:15 | vote | accept | BobaJFET | ||
| Aug 18, 2023 at 16:57 | answer | added | hacktastical | timeline score: 0 | |
| Aug 18, 2023 at 16:55 | history | edited | BobaJFET | CC BY-SA 4.0 |
typos
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| Aug 18, 2023 at 16:32 | answer | added | AnalogKid | timeline score: 0 | |
| Aug 18, 2023 at 16:26 | history | edited | BobaJFET | CC BY-SA 4.0 |
adding engineering applications for doing this as requested
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| Aug 18, 2023 at 16:05 | comment | added | BobaJFET | @KevinWhite Scratch that.. I just added the reason for doing this. | |
| Aug 18, 2023 at 16:01 | history | edited | BobaJFET | CC BY-SA 4.0 |
deleted 337 characters in body; added 39 characters in body
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| Aug 18, 2023 at 15:55 | comment | added | BobaJFET | @KevinWhite I added an example to illustrate why this might be needed. | |
| Aug 18, 2023 at 15:54 | history | edited | BobaJFET | CC BY-SA 4.0 |
added 343 characters in body
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| Aug 18, 2023 at 1:00 | comment | added | Kevin White | @BobaJFET - I know multiple ways of solving the problem depending upon the requirements and constraints. I still feel that you should give the engineering reason for not using a resistor - and not just stating 'Adding a resistor is not an option'. Personally I would almost certainly not use just a single resistor if the requirement was to control the LED by shorting it to ground. A depletion mode FET is not a terrible solution but they have extremely wide unit to unit variations and would be difficult to guarantee availability. | |
| Aug 17, 2023 at 23:42 | comment | added | BobaJFET | @KevinWhite You can use a transistor like Bravale suggested. | |
| Aug 17, 2023 at 14:09 | comment | added | Kevin White | @BobaJFET - That is not an adequate response - all the proposed solutions use resistors! | |
| Aug 17, 2023 at 10:49 | comment | added | Velvet | If you use a current source, you can power the LEDs without resistor. That's what many LED drivers do. | |
| Aug 17, 2023 at 10:41 | comment | added | BobaJFET | @KevinWhite Because the question is about how to power an LED without a resistor. Suggesting using a resistor is not productive at this point. | |
| Aug 17, 2023 at 6:29 | answer | added | Bravale | timeline score: 2 | |
| Aug 17, 2023 at 5:44 | answer | added | Simon Fitch | timeline score: 3 | |
| Aug 16, 2023 at 23:58 | comment | added | Kevin White | As others have said why not just use a resistor? For example replace the MOSFET in your first diagram with a resistor, that would work (depending upon your supply voltage). Also please put reference designators in schematics so we can refer to components unambiguously. | |
| Aug 16, 2023 at 20:12 | answer | added | Cristobol Polychronopolis | timeline score: 1 | |
| Aug 16, 2023 at 18:37 | comment | added | Chris Knudsen | "The LED is located somewhere outside of the device in such a way that a resistor cannot be added. It cannot be added inside of the device either..." But yet an IC can be added? Why do you surmise that adding an IC easier/less complicated than using any other component type, especially discrete parts? I'm not challenging your position; I just don't understand it. | |
| Aug 16, 2023 at 18:23 | history | edited | Null♦ | CC BY-SA 4.0 |
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| Aug 16, 2023 at 18:17 | review | Close votes | |||
| Aug 20, 2023 at 1:10 | |||||
| Aug 16, 2023 at 18:08 | comment | added | BobaJFET | @Jens The LED is located somewhere outside of the device in such a way that a resistor cannot be added. It cannot be added inside of the device either because the cathode side of the LED is also wired to a control pin which gets pulled LOW when the LED cathode gets pulled LOW. I'll try to draw a similar diagram to show this. | |
| Aug 16, 2023 at 17:57 | comment | added | winny | @Jens I can find a high side equivalent but the resistor part makes no sense. Even OPs suggestion of an opamp contains resistors if you take a look at the chip design inside it. | |
| Aug 16, 2023 at 17:13 | comment | added | Jens | @winny AP2502 is a low side driver and contains resistors. | |
| Aug 16, 2023 at 16:43 | comment | added | BobaJFET | @ChrisKnudsen I can’t give the entire circuit. But the question could be narrowed down to just “is it possible to limit an LED’s current without using a resistor?” | |
| Aug 16, 2023 at 16:13 | comment | added | winny | Why? I can’t think of a simpler solution but ok, try BCD AP2502. | |
| Aug 16, 2023 at 16:06 | comment | added | Chris Knudsen | This feels like a gap in understanding. You're willing to add an IC, but not a single common discrete resistor. Please enlighten us with the totality of your circuit and limitations. I'm concerned that this is an XY situation. en.wikipedia.org/wiki/XY_problem | |
| Aug 16, 2023 at 15:54 | comment | added | BobaJFET | @winny As stated in the question, a fixed resistor is not an option. no dimming nessesary. just on-off | |
| Aug 16, 2023 at 15:45 | comment | added | winny | What is the range? With fixed input and Vf, all you need is a fixed resistor. Is on-off ok or do you need dimming? If yes, analog or PWM? | |
| Aug 16, 2023 at 14:20 | history | edited | BobaJFET | CC BY-SA 4.0 |
added information
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| Aug 16, 2023 at 13:58 | history | asked | BobaJFET | CC BY-SA 4.0 |