Thread: How do I get my ACC +12v line to drop to zero

Originally Posted by SAScooby

Hi All

I have been having huge problems with my Switched positive in that it doesnt drop to zero quickly when I switch the car off, it drops to 3.6V quite quickly, now this messes my shutdown controller around (and I have tried several) what can I do to the switched positive line to solve this ?

I have tried small resistors etc and all to no real joy, some worked better than others but as my battery voltage changed so it became unreliable.

What about a bigger resistor ? will this help if placed between the switched positive and the shutdown controller ? any idea of the value of the resistor needed ? or is it a case of trying different values until the shutdown controller "sees" 0V on switched positive ?

Just use a 12V relay. I'm shure it will switch off at a 3.6V level.

tried the relay, using gnd on pin 85 ign on on 86, so the coil is energised when ign on, then I took the ign on from 86 to 30 which is the common, so when the car is off the relay is normally closed, when ign on the coil energises the relay allows current through and the shutdown controller then works, however this relay which is a 40A 12v one doesnt de energise the coil.....

what about a light or fan powered by this circuit, to bring the voltage down quicker ?

Throw in a little 12v LED with a resistor. That should be enough of a load to drop it.

I think the relay solution mentioned earlier should work as quicky. If you say the relay still doesn't switch off at 3.6v, you can try this.
Measure the coil resistance of the relay with a multimeter. (i.e put the multimerter accross the pins of the relay that power the coil. I'm not sure of the pin numbers in my head, but just read them off the casing)

get an equivalent or as close as possible resistor to that of the relay coil.

put it in series with the relay. (12v)+---(resistor)-----[relay]----gnd.

this should divide the 12volts going to the relay into 1/2.
So if the acc switch is on, then u will get half the value to power the relay i.e 6v instead of 12v.

if the acc is off(and u get that 3.6v), the relay should see 1.8v. The relay should not make it on @ 1.8v, except that relay is on crack.

Oh also that resistor might be hard to find @ radioshack, if you cant get it,
check jameco.com, digikey.com, allelectronics.com.
Just look for similar value like +/- 100ohms.

But are u sure the relay is still on@3.6v?

Originally Posted by SAScooby

Hi All

I have been having huge problems with my Switched positive in that it doesnt drop to zero quickly when I switch the car off, it drops to 3.6V quite quickly, now this messes my shutdown controller around (and I have tried several) what can I do to the switched positive line to solve this ?

I have tried small resistors etc and all to no real joy, some worked better than others but as my battery voltage changed so it became unreliable.

What about a bigger resistor ? will this help if placed between the switched positive and the shutdown controller ? any idea of the value of the resistor needed ? or is it a case of trying different values until the shutdown controller "sees" 0V on switched positive ?

You could use a pull-up resistor- I'm not sure what ohm value or you could just use a 7805 5-volt regulator. If the voltage dropped below 5V the output of the regulator would definetly drop to zero.

thanks for the help guys

I am going to try the resistor idea (and if the resistor doesnt work then a light, as a load on the circuit)

that regulator is too complex for me !!!

You could try putting a 2.7V (or even 3.6V) zener diode in series with the ACC line to the shutdown controller. This will drop the voltage by the respective voltage. However, a potential issue is this: with ACC at 12V, the voltage at the shutdown controller will be 9.3V - it MIGHT not be high enough for your shutdown controller.

Find out whats the minimum voltage needed to trigger the shutdown controller before it sees the ACC as a "LOW" voltage.

You did say the voltage goes to 3.6V quickly? If lets say currently your shutdown controller need 1.5V or below before it consider it as a "No ACC" voltage. Of course this can take forever to go below 1.5V if theres some big capacitor in the ACC circuit somewhere. You can make its minimum trigger voltage of 6V by inserting a voltage divider.

In this case as soon as the ACC voltage goes below 6V the shutdown controller will shutdown and not at 1.5V or below

EDIT :

Basically make the shutdown controller goes off when its input goes below 6V.

Argg im too tired to explain now...good nite

Originally Posted by dexy

I think the relay solution mentioned earlier should work as quicky. If you say the relay still doesn't switch off at 3.6v, you can try this.
Measure the coil resistance of the relay with a multimeter. (i.e put the multimerter accross the pins of the relay that power the coil. I'm not sure of the pin numbers in my head, but just read them off the casing)

get an equivalent or as close as possible resistor to that of the relay coil.

put it in series with the relay. (12v)+---(resistor)-----[relay]----gnd.

this should divide the 12volts going to the relay into 1/2.
So if the acc switch is on, then u will get half the value to power the relay i.e 6v instead of 12v.

if the acc is off(and u get that 3.6v), the relay should see 1.8v. The relay should not make it on @ 1.8v, except that relay is on crack.

Oh also that resistor might be hard to find @ radioshack, if you cant get it,
check jameco.com, digikey.com, allelectronics.com.
Just look for similar value like +/- 100ohms.

But are u sure the relay is still on@3.6v?

I would not do this because since the 12v relay is only getting 6v, it will refuse to engage at all. The resistor should have a 470uF cap in parallel with it (mind the polarity) so that when the 12v is first applied, the cap needs to charge and dumps 12v into the relay enough to energize it.