Electronics > Projects, Designs, and Technical Stuff
Did I mess up? SSR control
<< < (3/4) > >>
Ian.M:
There's nothing wrong with an emitter follower if you need to source more  current from a MCU pin, don't want to invert the signal, and can tolerate the 0.7V Vbe drop.  However, as Wraper points out, your circuit needs decrapifying.   When the boards come, do not populate D1, C1, R2, R5, and fit zero ohm jumpers for R1, R4.

56 ohms should work well for R3 giving a LED current of about 23mA., which is pretty close to the recommended 20mA.   I wouldn't go much higher for R3 as the efficiency will drop as the LED ages.

Zero999's calculation neglects the internal voltage drop in the ESP8266 output, which when sourcing close to 12mA is going to be significant.  IMHO its too close to the bleeding edge - if you choose a resistor to satisfy the constraints If>=10mA and Iout <12mA, you should be able to get it to work OK on the bench in a shirtsleeve environment, but you wont have any margin for LED ageing and low temperatures, and at high temperatures you are likely to draw excessive current from the EP8266 I/O pin, so reliability will be poor.
antonmi97:
Thank you for the advice guys, you made this much easier and faster for me. Appreciate it  :-+
Zero999:

--- Quote from: Ian.M on April 06, 2019, 11:01:04 am ---Zero999's calculation neglects the internal voltage drop in the ESP8266 output, which when sourcing close to 12mA is going to be significant.  IMHO its too close to the bleeding edge - if you choose a resistor to satisfy the constraints If>=10mA and Iout <12mA, you should be able to get it to work OK on the bench in a shirtsleeve environment, but you wont have any margin for LED ageing and low temperatures, and at high temperatures you are likely to draw excessive current from the EP8266 I/O pin, so reliability will be poor.

--- End quote ---
Good point, but the output voltage drop/resistance isn't specified, which makes that approach more difficult, as it would have to be derived experimentally.
Ian.M:
Yes.  I didn't take the time to look for ESP8266 output characteristic curves, and am not particularly surprised to be informed there's no official data.

If the emitter follower transistor has a min. hFE of 100, the worst case load on the pin will be less than 2% of its max current rating so one can be certain that the pin will swing rail-to-rail under load.
Therefore, with the transistor, a simple analytic solution with appropriate tolerances will be sufficient.
antonmi97:
So the outtake of this would be to:
1. Short the Base to the gpio pio and remove 10k resistor conecting the base and emitter.
2. Not use the voltage divider and put in a resistor limiting the LED current to about 20-25 mA with the 3.3V - the transistors Vce voltage drop (0.8V with the one I have)
3. Remove the diode and the bypass cap

This means that I can use my boards. Yay!
Anything else? Advice, suggestions?
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod