Hello,
I am doing the design of a board which contains a 12V relay, and a 5V microcontroller. The board should accept a wide range of supply voltage ( 12Vdc to 30 Vdc) and I am looking for a smart way of controlling the 12Vdc relay.
The 5V for the uC is made with a DC-DC converter.
The original idea was to read the supply voltage with the uC ADC and PWM the transistor that control the relay.
Unfortunately I need the ADC for other sensors, so I can't use it for this.
The idea I came up with is to use the inductance of the relay to make a simple DC-DC that limits the current in the coil to 45 mA (value from relay datasheet, FEME LCA 002 12 005).
Basically, when the voltage across the 15 ohm shunt resistor reach 0.7V (15*0.045=0.67), the lower transistor shorts the capacitor.
The voltage on the base of the top transistor goes to zero, and the transistor goes open. The current in the shunt stop, and the low transistor open.
The capacitor charges trough the resistor, until closing the top transistor again.
The current rise, until the voltage on the shunt goes >0.7 V again.
Tuning the size of the capacitor I could tune the switching frequency.
Something like this I saw for protecting a mosfet from short circuit.
Could this work?