Pull-down resistor AC voltage to DC ground

[Solara]

Hi. What I would like to do is install a pull down 1.5kohm resistor between a/c voltage source )from a variable reluctance sensor that is 2-10v with 25v spikes) And the ground plane of a 12v automotive system. The question is whether the AC voltage to the ground will harm other electronics that are grounded to that same plane. I am trying to limit the voltage input to the ecu.
Thanks

 

[Harald Kapp]

It depends how your AC source is connected to the chassis ground. Typically all electronic circuits in a car refer to chassis ground somehow. And looking at the noise that is present in a car's electric system, some additional noise from a sensor will not be noticeable.
A resistor alone will not help to limit spikes, however. You'll need a voltage limiting element such as e.g. a bidirectional suppressor diode or a pair of back-to-back connected zener diodes.

 

[Solara]

I should have said peak instead of Spike. This here is the intent. When you say it matters how it is attached to chassis ground what do you mean? There only one way to ground

 

[Harald Kapp]

Your intended circuit will not only attenuate the peaks but also the "normal" signal. That is most likely not what you want to achieve as it will change the sensor readings.

There only one way to ground

The grounds of different systems can be coupled in a variety of ways:
- AC coupled (e.g. via a capacitor)
- DC coupled with high resistance for AC (e.g. by an inductor)
- DC coupled with low resistance for AC (w.g. by a direct connection)
- not coupled at all (at least for practical purposes - a small capacitance and a very large resistance will always be present although these may not be relevant for practical purposes)

 

[Solara]

I do intend to divide the entire signal, not just the peaks. If I attach this circuit directly to frame ground will the ac voltage harm the 12v dc system?

 

[Harald Kapp]

No

 

[Solara]

Thank you Harald Kapp!