Calculating Size of Resistor needed?

[Adjuster]

Someone just mentioned the dreaded Safe Operating Area. It's a valid enough point, but I think a TIP41 is man enough to handle a 500mA relay on a 12V supply, provided you don't forget the coil transient protection.

TIP41 looks good for up to 40V continuously for currents up to 1A. See Fig.3 on Page2 of the attached data-sheet.

 

[SystemTheory]

I decided to try 5Spice and ran this saturation model of the TIP41C with success. A load resistor RLoad = 24 ohms will draw 0.500 amps at 12V, so I put this in as the coil resistance.

R1 = 330 ohm 0.5 watt rating 5% tolerance.
R2 = 3.3k ohm 0.25 watt rating 5% tolerance.

Base current is just 0.034 amps with the switch closed (shown as 12V battery). Power dissipation is about 0.38 watts in R1, so use 0.5 watt rated component. Power dissipation in the saturated transistor is a few tenths of a watt. When I set the input battery to zero volts the transistor turns off and a nano-amp current flows in the load resistor.

You can use the same values of R1, R2, and D1 (diode) for both the 0.300 ampere (40 ohm load) solenoid and the 0.500 ampere (24 ohm load) solenoid and the transistor should saturate properly in both cases.

The only cautionary issue is that simulated saturation voltage was quite low, less than a tenth of a volt, and I am not sure why the datasheet has such a high saturation voltage listed. It is probably because the collector current is 6A under the spec sheet saturation test. Yes, I think that's why, and with collector current at less than 1/10 that value in the solenoid design the simulator is probably fairly accurate.

 

[Wetmelon]

SystemTheory:
Hurray! My calculations were rather close, then! I suppose this clears me for breadboard stage testing :P

Adjuster:
From what I know of the car's electrical system, we get (surprisingly) little transient voltage. Our main problem is electrical/magnetic fields created by the alternator...

OTOH, if you would like to explain to me how to include protection in the circuit without throwing off the calculations too much, feel free :)

 

[Adjuster]

Surge Protection.

Unless the solenoids all have with internal anti-back emf measures (check, don't guess), you need to include a flywheel path for each coil. As a minimum, connect a diode like 1N4002 etc. in parallel with each coil with the cathode (bar end of symbol) to positive supply and the anode (triangle end) to the collector.
The coils will turn off more briskly if you can add a resistor in series with each diode, of value roughly equal to the coil's resistances (say, 24 and 39 ohms) and rated to carry the coil current briefly.

For protection against external transients, consider adding a Transient Voltage Suppressor between Collector and Emitter of each transistor, something like 1.5KE33A (aka 1N6283A), might fit the bill - see attached datasheet.

The above measures will have no effect on your calculations so far.

Finally, there is something that has to be said. FOR YOUR OWN SAFETY AND THAT OF OTHERS, PLEASE ENSURE THAT YOUR DESIGN IS CHECKED BY A COMPETENT PERSON BEFORE IT IS TESTED IN PRACTICE. However sensible any advice you obtain from this forum may appear to be, you don't really know where it comes from. Neither do we know exactly your situation. So do yourself a favour, get it checked by someone you can be sure to be competent.