×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

voltage ramp rate at output of ignition switch

voltage ramp rate at output of ignition switch

voltage ramp rate at output of ignition switch

(OP)
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.

RE: voltage ramp rate at output of ignition switch

10mS from what trigger?

What are you measuring with?

RE: voltage ramp rate at output of ignition switch


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.

  

RE: voltage ramp rate at output of ignition switch

(OP)
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.

RE: voltage ramp rate at output of ignition switch


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

RE: voltage ramp rate at output of ignition switch

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



RE: voltage ramp rate at output of ignition switch


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.

RE: voltage ramp rate at output of ignition switch

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.

RE: voltage ramp rate at output of ignition switch

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 wink Also a standard time constant is only 63% of charge.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources