voltage operated relay for automotive

[mark krawczuk]

hi, after a schematic that will turn on a relay at about 13.2 vdc and stay on , until voltage drops to about 12.8 then turn of ., basically want relay turn on when engine running , relay of when engine not running , not when ign turned of.

anyone help ?
mark k

 

[AnalogKid]

Relay manufacturer and part number, or coil voltage, current, and resistance?

ak

 

[mark krawczuk]

Relay manufacturer and part number, or coil voltage, current, and resistance?

ak

hi, bosch automotive 12 vdc relay coil resistance about 75 to 100 ohms.

 

[AnalogKid]

This sounds like a job for a single voltage comparator with +/-16.67% hysteresis. This can be done with a voltage reference and something like 1/2 of an LM358 dual opamp with small power mosfet on the output to drive the relay. Many comparators have open collector output, which would need a pul up resistor for this application. The opamp's totem pole output stage is better suited.

ak

 

[shrtrnd]

I'd go with Analogkid's advice. You can't practically rely on the relay itself for your application. When your battery gets older, you might inadvertently drop below your 12.8 voltage threshold for a short period
during crank-up, and that would probably cause you unwanted problems with your relay activation. Better to stick with a regulated voltage circuit from your battery to power your relay, rather than the actual voltage
coming out of the battery (which can drop when you turn the starter).

 

[Colin Mitchell]

You need a zener diode and some ordinary signal diodes, a few resistors and a transistor.