Timeline for Making a Darlington transistor (eg: TIP120) with D882 to drive 12V LEDs with 3.3V or 5V input RPi
Current License: CC BY-SA 4.0
18 events
| when toggle format | what | by | license | comment | |
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| Apr 23, 2021 at 21:46 | comment | added | D.A.S. | Your 1.5V to 1.6V Vbe clamp on Ch2 for your Rpi LVCMOS driver implies your Darlington gets enough base current but you are well exceeding the recommended 20mA or 25mA absolute max. (16mA just for Logic loads with noise immunity) possibly 50 mA. Not good for reliability as it may fuse open in time. Ch1 looks like 2.5 div. X 500mV/div = 1.25V same boat or worse... N.G. | |
| Apr 23, 2021 at 21:25 | comment | added | D.A.S. | Your noise is mostly measurement error with 1:1 probes and noisy ground loops. | |
| Apr 23, 2021 at 18:37 | comment | added | David Mikeska | Your connection in the picture is not the same, and there was something about the i2c pins that even though they are configurable to GPIO, they don't have the same current drive as the dedicated GPIO. | |
| Apr 23, 2021 at 18:36 | vote | accept | maxleb | ||
| Apr 23, 2021 at 18:15 | comment | added | JRE | I don't know how or where you measured what with channel 1 and channel 2. | |
| Apr 23, 2021 at 18:12 | comment | added | maxleb | @JRE, so is this why I get a noisy cycle on channel one cause it's requiring to much current? | |
| Apr 23, 2021 at 18:11 | comment | added | maxleb | @jonk, duty cycle will go between 0-100% depending on the color I will need (RGB) So currently at 100% duty cycle I get 1.5v - 35mA in the input and 11.13v - 238mA on the output so I get a 0.7v drop. Cutting in half at 50% duty cycle. So in my case, the voltage drop is not an issue as the led light up bright enough. | |
| Apr 23, 2021 at 18:09 | history | edited | maxleb | CC BY-SA 4.0 |
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| Apr 23, 2021 at 18:09 | comment | added | JRE | @maxleb: Raspberry Pi GPIO pins have a nominal 3.3V output. They are also not intended to pass more than about 16 milliamperes. Connecting the base of a large transistor to a GPIO without a resistor to limit the current could damage the Raspberry Pi. | |
| Apr 23, 2021 at 17:50 | history | edited | JRE | CC BY-SA 4.0 |
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| Apr 23, 2021 at 17:03 | comment | added | jonk | @maxleb You've already said that the TIP120 works well for your needs. This tells us that you don't need all of the +12 V supply potential because the TIP120 drops about 1.5 V of it. You are now in a position where you don't want to specify the TIP120 but want, instead, to design a replacement that meets additional requirements. At this point, I think we need to know more about the worst case current compliance needed and I think you need to re-evaluate the voltage drop (because in a new design you get to do that.) And what PWM rate are you using and what's the dynamic range of the duty cycle? | |
| Apr 23, 2021 at 16:47 | answer | added | David Mikeska | timeline score: 0 | |
| Apr 23, 2021 at 16:39 | comment | added | Unimportant | @maxleb Current is not expressed in Volts... | |
| Apr 23, 2021 at 16:24 | comment | added | D.A.S. | Both schematics are insufficient due to lack of base current limiting R missing and voltage rise when saturated on output. It is better to use open drain power FETs for interfacing 12V LEDstrips . or two stage inverting BJTs PNP+NPN with proper R values to reduce Vce(sat) <0.4V vs 1.7V @ 3A on your choices, you must specify load current and output max current you choose from Rpi. | |
| Apr 23, 2021 at 16:17 | comment | added | maxleb | Yes, they are limited to 5v. | |
| Apr 23, 2021 at 16:07 | comment | added | Unimportant | Unfamiliar with RPi: are those outputs current limited? If not you definately need to add a base resistor. | |
| Apr 23, 2021 at 15:51 | review | First posts | |||
| Apr 27, 2021 at 20:57 | |||||
| Apr 23, 2021 at 15:49 | history | asked | maxleb | CC BY-SA 4.0 |