Newbie needs help! Relay diagram question

Thread Starter

merzatt

Joined Jan 15, 2009
43
Søren;112615 said:
Hi,

Try another 10k resistor from the transistors base to emitter (ground).
I have a feeling that the line you're driving it from is either 12V or floating (i.e. not connected). If this is the case, the pull down resistor should cure it.
Did you mean this?
 

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Thread Starter

merzatt

Joined Jan 15, 2009
43
I replaced the BJT with the MOS.

The circuit works on my desk. I am using 18v adapter on drain side and a 9v battery on gate side.

But after I plug the harness to the radio, it does not!:confused: Also the 12v supply measures around 3v at this point.

What is it?

Some possible stupid ideas from me;
a) maybe pin A and pin C do not share the same ground in the radio? should they?
b) I should not ground this to radio but some other outlet? does it make difference?
 
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mik3

Joined Feb 4, 2008
4,843
But after I plug the harness to the radio, it does not!:confused: Also the 12v supply measures around 3v at this point.
From where do you get the 12V for the power supply? At which point do you measure 3V and when?

a) maybe pin A and pin C do not share the same ground in the radio? should they?
b) I should not ground this to radio but some other outlet? does it make difference?
The ground should be the same for the microprocessor and the radio but you never know what happens. Try to connect the source to the same ground as the microprocessor. Also, MOS needs 5V between the gate and source to fully turn on.
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
From where do you get the 12V for the power supply? At which point do you measure 3V and when?
12v is between the Pin A harness and the ground when not plugged to the radio. As soon as I plug it to the radio, it measures 3v. This is new.

The ground should be the same for the microprocessor and the radio but you never know what happens. Try to connect the source to the same ground as the microprocessor. Also, MOS needs 5V between the gate and source to fully turn on.
MOS was getting more than 5v.

Both harnesses are plugged into the same head unit. Harness that has PinA is used for tuner, com ports etc and harness that has Pin C is for CD changer (separate 12+, ground and CD controls). May be CD changer power and ground was disabled for some reason when I was doing the test. Would that be the cause?
 

mik3

Joined Feb 4, 2008
4,843
Disconnect the radio from the microcontroller and try to see if the MOS is working, ie dropping the voltage down to 0V when it is on. Maybe you measure 3V instead of 12V because the radio draws much current from the microprocessor and the voltage drops due to the internal resistance of the microprocessor output.
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
I connected to the radio and tested again.

MOS is not doing what it is supposed to do (or doing same thing as the BJT did). Regardless of the voltage on the gate is below or above the 5v, voltage on the drain is 12v-gate voltage.

When the gate voltage is 0 (light off), the Pin C is 12v, so it does switch to day mode. But it does not drop the drain to 0 when there is >5 volts on the gate (lights on). I know, but that's what it does when plugged in to the radio. Only solution I can think of is lowering the 12v to something like 3-6 volts.
 

mik3

Joined Feb 4, 2008
4,843
Strange things!!
Disconnect pin C from the drain and measure the voltage between it and ground. Also, check if the MOS pulls the voltage down to 0V by connecting its gate directly to the 12V supply.
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
Strange things!!
Disconnect pin C from the drain and measure the voltage between it and ground. Also, check if the MOS pulls the voltage down to 0V by connecting its gate directly to the 12V supply.
Disconnecting the pin C did not change anything.

If I increase the voltage on Pin A to max 11.5 volts (using the illumination knob), Pin C voltage drops to 0.5v and radio switches to night mode. But that's because 12v-11.5=0.5v
 

mik3

Joined Feb 4, 2008
4,843
Why do you say 12-11.5?

Where is the illumination knob? If it is on the radio why it is controlling the microprocessor of the car?
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
Why do you say 12-11.5?
I meant 12v minus 11.5v -- output drops to 0.5 volts. Any voltage below 0.5 is enough for radio to switch to night mode.

Where is the illumination knob? If it is on the radio why it is controlling the microprocessor of the car?
Illumination knob is on the instrument cluster (not on the radio). The reason radio gets this voltage (from pin A) is buttons on radio that needs to be lit.

I jump wired the pin A to this circuit so I can use it as a signal. Pin A current is still going to the backlight lamps of the radio as it is going to the circuit.
 

mik3

Joined Feb 4, 2008
4,843
If you disconnect the 1K gate resistor and from pin A and connect it directly to the 12V power supply does the MOS pulls the voltage down to 0V on pin C?
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
How about going around the problem if we cannot beat it? If I change the 12v+ drain side voltage down to 4 volts? 4 volts may drop to <0.5 volts when light are on. What do you think? Enough to trigger the radio.

This is what I came up with. But I am not sure that this is a correct diagram.
 

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mik3

Joined Feb 4, 2008
4,843
No, because, as you said, the voltage on pin C won't rise to 12V and the radio won't switch to day mode. Increase the 10K resistance on the drain to 20K.
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
No, because, as you said, the voltage on pin C won't rise to 12V and the radio won't switch to day mode. Increase the 10K resistance on the drain to 20K.
I will try 20k and post the result.

Maybe radio is not very picky about high voltage.

So there is something wrong with pin A. When it is loaded the voltage drop and is not enough to turn on the MOS.
I measured the gate voltage at the MOS leg and it was very similar to pin A voltage.
 

Thread Starter

merzatt

Joined Jan 15, 2009
43
It didn't make any difference.

Still as the voltage on Pin A increases, voltage on pin C decreases the same amount. MOS is getting >= 5 volts at the gate but still not dropping the voltage on drain to 0.
 
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