increase pull voltage on 12volt automotive relay?

[jwhinerman]

Hello,

I have a situation with headlamps in an autombile. When the car is running , and the headlight switch is in the off position, the ecu sends 9volts to the headlamps to dimly light them during the day. When the headlamp switch is turned on, the voltage increases to the full 12 or 14 volts. I need to trigger a relay from this headlamp wire, however I would only like the relay to pull when the headlamp switch is on ( in other words, have a min pull voltage of say 10v or so) The bosch style automotive relays I have always used have a pull voltage of about 6volts or so. Currently I ran a variable resistor in line to the relay, and dialed it in to only switch with the higher voltage. I would like to find a more reliable or "fixed" way of doing this. I can not find a 12volt 30amp relay that has a pull voltage this size. If anyone can make a suggestion, I would appriciate it.

Thank you

 

[VisiGoth]

If the current drain to drive the relay is not too high, you could eplace your resistor with a zener of the voltage drop you need. You could also build a poor man's power zener. This ciurcuit is a little sloppy, but may be more accurate than a resistor. connect a zener from collector to bae of an NPN. Us a small (high Z maybe 2k or 10 kOhm) resistor base to emitter. Now the collector to emitter sort of looks like a zener. It is close to the zene voltage plus 0.7 volts B-E or so.

 

[OperaHouse]

I don't think either method would work very well. Any relay that would pull in at 6V would not drop out till about 2V. Since the headlight is likely PWM, it will have an AC output. A capacitor feeding an AC bridge on a relay would give an indication when the dimming is in effect. Add that in series with the headlight power to eliminate the power when dimming is on.

 

[itsmoked]

If you are trying to determine if the headlights are on as apposed to daylite running is there any other lights that you can tap into say running lights that aren't on in daylite running mode?

 

[wagnerc]

U can combine a zenner and a relay to get the effect. Connect the light line to a zenner with say 12V breakdown. The zenner should then connect to the coil of the relay. This way when the light is "on" the zenner will be energized and the relay will be energized. Then when the light is "off" the zenner will no longer be energized since it won't allow a mere 6V to pass and consequently the relay will also no longer be energized. This all of course assumes the light line is feeding DC.

Chris

 

[wagnerc]

er, 9V.

 

[BrianR]

Try a solid state relay designed to be triggered by 5V logic and obtain the 5V via a resistor/zener.