A linear drop of 24V to 12V is not practical. It's converting the excess voltage into HEAT.
Not necessarily. Consider the
LM9076 datasheet. This 150mA LDO can handle a nominal 40V input (no limit on duration) and can handle 55V for longer period and even 70V for transients of 10ms or less (1% duty). It comes in an SOIC housing version that dissipates heat through the circuit board traces. And because of its currently handling, and the fact it is simpler, less costly, and less noisy than a switcher would be, I am considering the LM9076 to power my 3.3V MCU. Normally, the MCU and all my other circuitry will draw no more than 2mA -- and that's without even sleeping the MCU at all. With all my other circuitry (but NOT including a relay) would draw about 90mA. A 150mA regulator will give me much needed headroom. So when installed in a 24V truck, you are basically dropping 28V (with the engine running) down to 3.3V. So it is possible without resulting in a fireball of heat.
What I am considering now is using this Hongfa
HFD27/003-H 3V relay (High Sensitivity version with coil resistance of 60Ω). The relay coil can be triggered by as little as 2.4V or as much as 7.0V. I would trigger the GND side of the relay coil using an output pin on my PIC16F1508 MCU via NPN transistor (and yes, I will have a 1N4004 across the relay coil to suppress spikes), and I can use the PWM module of the PIC to send 20kHz or even 40kHz at a 50% or 60% duty cycle to reduce the current to the relay after waiting 0.5s or so for the relay to switch. At a PWM 100% duty, it's Ohm's Law:
I = E/R
I = 3.3V/60Ω = 55mA (without PWM)
And yes, it will indeed draw 55mA because don't forget the voltage is 3.3V. (Again, it's Ohm's Law.)
Now with PWM, I could drop that current draw and therefore the heat dissipation down. And technically I do have enough headroom on my LM9076 to accommodate the 3V relay's 55mA even without PWM. But I think PWM is important because this will be a normally open relay used to cut two signal wires coming from the PUSH start switch in a vehicle and the relay will need to be powered the entire time the vehicle is being driven. For trucks, that could be 10 hours straight.
I started this thread to see if there were alternatives to using my 3.3V rail to power the relay though. The reason is because even with the 1N4004 diode across the relay coil, I'm concerned about having it on the same 3.3V rail that powers my PIC. Maybe it would be fine. Thoughts?
Of course, powering the positive side of the relay coil would eliminate any noise concerns for my MCU. And I could use the 12V of that same relay in such a case. But as you can see from the relay's datasheet, the 12V relay cannot handle anything above 27.7V, and a 24V truck with the engine running will see 28V. So I could not use that for both 12V and 24V. And if I used the 15V relay instead, note that it's lowest voltage is 12.0V, which means anything lower than 12V simply would not switch the relay.
Anyway, the current handling of that Hongfa relay is 2A which is really more than I need. I think a max of 500mA or perhaps even 300mA would suffice. But there is no economical solid state relay that would suit my application. They are all pretty outrageously priced.
Again, the reason I even need a dual-switch relay is to break 2 signal wires coming from the PUSH Start button (switch) of a vehicle, which will result in starter immobilization. Cutting something else is not optional. I need to cut those two wires. But again, they are signal wires not seeing more than 12V, and not passing more than a couple hundred milliamps.
Further thoughts in light of this would be appreciated.
THANK YOU!