voltage ramp rate at output of ignition switch

[tom2378]

Hi All,
the voltage at the input of the ignition switch, terminal 30 is 12V , I turn the key and the voltage at the ignition swith output climbs to 12V in 10mS. Half a second later both voltages drop as the engine is starting.

Question : why does it take 10mS for the voltage at the ignition switch output rise to 12V?

I presume the ignition switch is low resistance and the voltage at the ignition swith output does not fall indicating that the output voltage is not limited by a large load.

 

[MintJulep]

10mS from what trigger?

What are you measuring with?

 

[Fabrico]

You are probably seeing the powering up of one or more electrical items in the car, like a clock, alarm, ECU, fuel pump, etc. This should show up as a quick dip in voltage on the input side of the switch also.

 

[tom2378]

If I put an oscillascope on the output of the ignition switch I see that when the switch closes the voltage at the output of the ignition switch rises from 0 to 12V in 10mS.
The ramp is steeper at the start getting to 5V in around 1mS. The battery voltage itself falls by 200mV as the ignition switch output rises.
When the engine cranks both voltages fall to 6V and then ramp up to over 13V as the alternator kicks in.

What I would like to understand though is the initial 10mS rise time. Am I guranteed to have such a slow rise time or could it be much faster ( say 10uS ) as I had expected.

The ignition switch should be a low resistance say 0.05 ohms so even a 1mF of capacitance on the ignition switch output would have given a rise time of 50uS.

 

[Fabrico]

Is there some kind of problem with it that deserves further analysis?

 

[IRstuff]

Would you expect ALL the battery current to run through the ignition switch? I'd expect that the ignition switch powers a power relay that provides current to the rest of the vehicle.


TTFN

 

[Fabrico]

Assuming we are talking about a car...ignition switches typically power everything except the fuel pump, headlights, radiator fans, electric seats, A/C clutch, and horns. Some cars further separate the ECU, wipers, windows, sunroof, and blower motor(s). That still leaves a ton of lights, at least part of the dashboard, radio, trunk latch, etc, etc, going through the switch.

 

[MintJulep]

Anything electronic probably has some sort of filter on its input that will add inductance and capacitance to the circuit.

Wireing resistance is not zero.

 

[carnage1]

Tom your forgetting the obvious, the ignition switch is ot the only resistor in the circuit when you turn on the ignition switch, so even though your switch only has 50 mohm of resistance (seems a bit low id guess 200 mohm myself) you have lots of other resistors bumping up your time constant. Mr Thevinin would be dissapoined Also a standard time constant is only 63% of charge.