I have 12 solenoids (12v rated) connected in parallel to two Darlington arrays (ULN2803A). Each solenoid has 88-ohm impedance. The power supply can provide a stable 12V with up to 2A current draw. (See below)
The solenoids are controlled with 20ms impulses. When all solenoids are activated, the current draw does not exceed the maximum (2A) and voltage stays stable (slightly above 12V).
All solenoids are connected with 0.5mm wires with up to 2-meter length.
When each of the solenoids is activated, they perform as intended. However, when 6 are enabled at the same time, one of them does not work.
As the solenoid works when activated on its own, there should not be mechanical issues and is rather a lack of power to activate it.
Why would it lack power if the operational voltage is supplied and the power supply maximum current draw is not reached?
- The inputs to Darlington IC are driven by 3.3V signals from an MCU.
- It is the same solenoid that fails.
- If 12 are driven at the same time, then multiple solenoids fail.
- All of them are relatively same distance from GND.
- (When 6 are driven, #2 fails)
EDIT2: After re-measuring voltages across the solenoids:
- When 12 are active - voltage across each is 10.8V
- When 6 are active - voltage across each is 11.1V
- When 1 is active - voltage across it is 11.4V
The voltage supplied by the power supply is 12.3V