Say I have an MCU switching a couple SSRs using the ubiquitous ULN2803 at 5V on the "common" pin, but I'd like to switch a 12V mechanical relay with the same chip as well. Checking the equivalent circuit for the IC, it is basically a low-side switch so I can switch whatever voltage within specs right?
The "common" pin only act as a return path for the flywheel diodes integrated within each driver "channel". The 12V relay will have its own flywheel diode hooked to the 12V supply.
The only pitfall I see is that maybe the integral flywheel diode would switch on before the external diode and thus dump the back-EMF energy from the 12V relay coil to the 5V rail... But then is it any different than the back EMF from a 5V relay anyways? Most 5V relays operate on higher current than their 12V equivalents, back EMF might even be higher at 5V, if I memory serves back EMF in a coil is dI/dt not dV/dt....
But if it is a problem I can use a schottky diode for the relay and it will turn on first due to its lower forward voltage. Am I talking non-sense here?