OpenSprinkler › Forums › Hardware Questions › OpenSprinkler › Query with the firmware driving of the DC board › Reply To: Query with the firmware driving of the DC board
I have done some more digging into this today, since posting originally to you I noticed that the PCA9555A is also getting extremely hot like the 7805.
– I have a DC board that doesn’t have the PCA9555A fitted so I thought I would do some measurements on this first.
Initially I manually tested driving the bases of QB1 & QB2 HIGH individually, the current draw was as expected, when QB2 was HIGH an initial high current and quick decay as CB is charged. QB1 & QB2 LOW : 6.4mA, QB1 HIGH only : 23mA, QB2 HIGH only : 17mA (after CB charged). QB1 & QB2 both HIGH : 33mA.
– Measuring the above I went back to the board that had the PCA9555A fitted and decided to isolate QB1 & QB2 bases to P1_6 & P1_5 of the PCA9555A by cutting the tracks. Then I manually tested the driving of the bases of QB1 & QB2 HIGH again. All tested the same as when the PCA9555A wasn’t installed. The PCA9555A was also happy, even though its isolated on these pins at that time. A UNO driving the PCA9555A directly for the P1_6 & P1_5 pins behaved normally as well. I decided to measure the base current of QB1 & QB2 when HIGH. QB1 is ~7mA and QB2 is ~10.3mA. (Due to multimeter leads the voltage drop will also give slightly lower current measurements). On checking the PCA9555A datasheet the I(OH) is 10mA (I(OL) is 25mA). So it seems the PCA9555A maybe getting annoyed with this high base current.
– I decided to fit a current limiting resistor to each of the QB bases. For my transistors fitted I calculated and fitted a 4k22. Once fitted I get normal behaviour with the board now. Connecting the ESP board and fully testing the solenoid activation the currents are what I expected and what you are seeing as mentioned in your previous posting.
– I have attached the updated schematic of where the resistors are placed so that we are on the same page of where I have placed them. Also a picture of my board, just so that you can see what board I am using.
– Yes I also found out about the SMBJ24A’s, I have since rotated them on my boards.
– I’m not familiar with the evolution of your boards, do I have an older variant?