series resistance for relay control

Probably a formula but for an automotive relay with a coil of abut 80 ohms, in practical terms it's about 110mA. Required voltage around 7.5 volts.

That transitor is only rated for 100mA. Use a bigger or Darlington transistor. I'd suggest with at least 500mA or 1A (Ic = Collector) current.

The part number you posted is rated at 200mA. Is that correct? Above when you said it was a Mosfet you said it was rated at 16A. A 200mA transistor should be enough to drive an automotive relay, especially in a low duty cycle application. I would add a second diode across the coil of the relay - the inductive spike created by the collapsing of the magnetic field in the relay when power drops off is far more destructive to a transistor then pulling the current you are pulling through it. Perhaps the diode is either defective or not connected across the coil the way it needs to be. Again, I would also look at the drive circuit of the transistor. The base is most likely driven by a 5vdc signal - is there a resistor on the base? A base resistor will control the ability of the transistor to enter saturation and get rid of the voltage drop across the transistor that you are seeing. If you want to add a resistor in line, which most likely won't help you, you will have to determine the minimal amount of current needed to turn the relay on. One way to do this is with a 12vdc power supply and a potentiometer - start with the potentiometer set to the highest resistance possible and slowly turn it down until the relay turns on. Measure the resistance of the potentiometer and start there. You will need to make sure you wattage rating is sufficient as well.Kevin Pierson

I will replace my transistor with a Darlington. It needs to be the same footprint. (3 lead TO-252 D-PAK) Any suggestions? Currently I have a 1K resistor in series to the transistor base which is driven by a uC running at 3.3V. Since the base will stay at 0.7 V, that means my Base current is 2.6mA. I don't seem to have any problems with my relay triggering, so that seems to be OK for the transistor I am currently using. Would I need to adjust it for a Darlington transistor? (My schematic/BOM calls out a MOSFET n channel 60V, 16A TO-252 D-PAK part, but what I have on my board is the part noted above. Sorry for that confusion, I was not the original designer.) I tested this, I had 12V on the relay input and toggled my switch every 3 seconds for 24 hours. No problems. But what I hear Kevin saying is that the relay input (the starter signal) going on and off is what is probably killing my transistor. Correct? So I should stress test my design by flipping the starter off and on. (The switch is either always on or always off.) Does that make sense? Would the starter signal be floating or grounded when the key is out or in the ignition state? Thanks SO much everyone! Cheryl

The base current should be the output (Ic) divided by the gain - eg, design for 250mA (12V/.25 => a 60 Ohm relay coil as typical for many automotive relays) and if gain is 800, then 250/800 = .31mA. That may mean 2.6mA/.31ma = 8.4x you present base resistor value depending on you base-ground resistor value (if fitted).     But overdriving the base (current) should not matter unless its max base current spec is exceeded. However others are better to advise - I've forgotten my transistor design stuff. The starter should be GND when not cranking as the starter solenoid else its relay's coil provides a path to GND. A reverse biased diode across the relay coil (or collector to emitter) might be needed to prevent relay spikes damaging the transistor. I don't know about equivalent footprints though they should exist. (The data I saw was for your transistor a dual-pak SMD footprint. But whether SMD or leaded, ensure the correct order for b, s & e (even if it means bending leads).)

I've tried a Darlington transistor to switch off the starter as was suggested. Here is the sequence I am seeing... 1. Switch is off (gnd applied to transistor base) 2. Relay allows starter to start vehicle 3. Switch is turned on 4. Vehicle starts on first attempt. 5. Vehicle cannot start on all subsequent attempts. #4 is what is unexpected and undesirable. Could the Darlington transistor be causing an extra cycle of 12 Volts on the switch before it allows the relay to switch? Thanks again, Cheryl

What vehicle? Wouldn't be a Prius by any chance?

No. Actually, I don't know what vehicle my guy is testing it in. Should it matter?

Hybrids such as the Prius have a different "ignition" cycle and this may give the result you just had.

Can you explain more on how this would happen? My test car is a 2003 Acura TL Type S