series resistance for relay control

I am using a SPDT relay with a diode to enable/disable the vehicle. I am controlling the relay from a TTL transistor switch rated for 30V. My relay is normally closed, so there is a constant 12V on the switch. After a few days, my switch is failing - always on - relay triggered - car disabled. I swap out the transistor and it works again. Would adding a series resistance before the switch help? If so, what resistance? Thanks! Cheryl

I seriously doubt the relay is harming the switch assuming you're feeding 85 or 86 from the relay, coil draw will be about 1/2 an amp.

Do you have the diode installed correctly? What is the part number for the transistor you are using? Just because it is rated at 30vdc doesn't meant it can handle the current.Kevin Pierson

The diode is already attached to the Hella relay. The transistor is a MOSFET N-channel rated at 60V/16A. The relay control is connected directly to the drain of the TTL MOSFET on my PCB. I control the relay by applying voltage to its gate. I'm trying to use a PCB without modifying it. I would have designed in a series resistor to take the load when the relay was "on".

The relay coil is a resistor that is already limiting the current through the mosfet. That is a very large transistor, there should be no load concerns at all. I run relays in commercial products through 200mA NPN transistors (with diodes across coils) for years and years with no issues. Are you running a standard automotive relay (Bosch style)? They should pull about 160mA.Kevin Pierson

Yes, I am using an automotive relay. When I have the transistor off, my relay control signal stays at 12V. Since the transistor is rated at 60V, I assume it can live quite happily with 12V across it (no current). When I turn the transistor on, I am drawing just over 100mA. However, there is still 3-4 volts across my transistor. I would think I would want to see no voltage across a transistor when it is on. So I was thinking that I am burning out the transistor by trying to dissipate power (100mA over 3-4V) through it. So I thought I should add a resistor in series - before or after the transistor - to dissipate the power properly. If that makes sense, then what value resistor? Too low would still hurt my transistor. Too high might not provide enough current through the coil. ? Thanks so much for your help! Cheryl

The TTL (5V) does not have enough voltage to turn the MOSFET fully on, hence the heat. Better off using a Darlington transistor, else a lower Vgs FET.

Why did I say 1/2 an amp? It's nearer 200mA. We've discussed this before, this is the sort of thing I'm glad Oldie and K.P. have answered, I was right in fact but the theory side leaves me in the dust!

is the transistor getting hot to the touch (hot enough to burn you)? you may consider addind a heat sink if heat is a concern. depending on the drive circuit you may be able to swap in an npn transistor and get away from the mosfet. adding resistance in series will limit the current to the point the relay will no longer turn on. since you can not fully saturate your transistor you are most likely operating at the low end of the voltage range (hence the lower then normal current draw from an auto otive relay). how is the drive circuit set up? what package is your mosfet? what is the part number of the mosfet?Kevin Pierson

I'm using the Infineon MMBT3904LT1XT npn trasistor. (Sorry, my schematic says MOSFET.) I'm not too worried about heat, since this relay should only trigger (or not trigger) when the starter wire is hot. So it is more a concern of burst currents. However, the transistor is hot, not quite enough to burn me. I've blown two of these transistors after having them installed on a vehicle for less than a week. With no series load, I am seeing about 3V across the transistor with about 120 mA current. With 39ohm series, I see 0.6V across the transistor with about 100mA current. Adding more resistance would lower the current through the relay coil. Is there a minimum current draw through the relay coil for it to work? Thanks again! Cheryl