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On/Off switch using low oil press. sender

PostPosted: Tue Feb 07, 2012 10:37 pm
by brian
I am installing the Smart Alternator Regulator and have a question.
The low oil pressure switch in my system is normally closed until the pressure reaches a minimum level.
In the diagram below, the 12v will be active when the engine switch is on.
The problem is the power will also be on when I energize the glow plugs for 10 - 15 seconds.
So I came up with the following idea.
What value of resistor should I use?
My guess is a 1/4 watt 2.2Kohm.

brian

Re: On/Off switch using low oil press. sender

PostPosted: Tue Feb 07, 2012 10:41 pm
by brian
Oh yeah, the 2.2K resistor will allow for ~ 12.5v/2200 = 5.7mA

Re: On/Off switch using low oil press. sender

PostPosted: Wed Feb 08, 2012 12:23 am
by Coulomb
NOT!

Wire the On/Off to the ignition switch.

Re: On/Off switch using low oil press. sender

PostPosted: Fri Feb 10, 2012 2:20 pm
by brian
ah. the picture is incomplete.
the 12v going to the resistor is to be connected to the engine switch.
when the engine switch is off, there is NO VOLTAGE at the on/off terminal.

Re: On/Off switch using low oil press. sender

PostPosted: Fri Feb 10, 2012 3:30 pm
by Coulomb
NOT:

The regulator on/off has to be connected to 12V without any dropping resistor.

Re: On/Off switch using low oil press. sender

PostPosted: Fri Feb 10, 2012 10:13 pm
by brian
unless the input impedance to the regulator is very low, which is not the usual case for modern electronics, the resistor is not a dropping resistor as much as a current limiting resistor. The specification for the SARv3 requires 3mA typically on this input. The resistor will allow for approximately 5.6 mA. But i will not belabor the point. Wiring directly to the start switch is no big problem as long as while the glow plugs are warming there is no damage to the regulator.

cheers,
b

Re: On/Off switch using low oil press. sender

PostPosted: Fri Feb 10, 2012 10:41 pm
by Coulomb
There's also a dynamic impedance that comes into play.

I'm not going to tell you to wire the regulator any other way than what the drawing indicates. The drawing has been vetted by engineers and thousands of installations.