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Help required with an automotive circuit

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nbuuifx

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Hi, it's been a long while since I did any electronics - I still have bread boards and I think I've got a PIC programmer somewhere but I've forgotten just about everything I knew!


I want to modify a part of a car circuit and could do with some help working out how to do it, hopefully it's quite simple.

On the car in question there is a device which only runs with the ignition on, I want to keep this but also have a switch to operate it manually. I have identified that the device itself has a feed from the Alternator which provides 12v+ when the car is running, when 12v is detected on this pin it will allow the device to run (runs from a seperate 12v feed)

So for part one I just need to be able to apply 12v to this pin, do I need to put any protection in place so that I'm not sending 12v up the wire to the alternator? and making other things think the engine is running? I still want to keep the original system in place this will just be an override switch.

Secondly the said device also requires a small 12v motor to be running elsewhere on the car. So again I could do with applying 12v to a pin on the motor but again it will receive 12v at other times so I don't want to damage anything.

Then finally to confuse things even more, once this is tested and if it works well, then I want to add a second battery to run the device from, the device can use a bit of power and I don't want to drain the battery. The fuse rating is 25amp but to be honest most power would be used in the first 2 minutes. I want to add a second smaller battery and fit a split charge relay to allow the car to charge the second battery. I would probably change the main feed to the device over to this battery so it always uses the second battery, the 12v feed to trick it to think the alternator is running wouldn't matter as the draw on that would be tiny, but the small 12v motor could do with running from the aux battery during the time I want to force the device to be on.

So I would have two different 12v feeds to the one 12v motor, there is a possibility they could both be on at once.


Hopefully that makes sense and any advice would be appreciated, if I haven't made anything clear please just say what you need I could try and knock up a diagram if that would help?
 
Try a block diagram.

I am not aware of any output from an alternator which says the car is running. The big wire out of the alternator goes directly to the battery, so it has ~12.5V on it when the car is parked (i.e. the battery voltage), and it has ~14.4V on it when the alternator is producing output.

There might be a small wire going from the ignition key switch (or some computer in a newer car) which supplies an "enable" input to the alternator voltage regulator which turns on the alternator only after the engine has started. You could only steal a few mA from that wire, not power anything with it...

If your motor "load" can tolerate a slightly lower voltage (~12V instead of the 12.6V battery), you can use Silicon Schottky diodes to supply the motor from two different batteries...
 
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Thanks for the reply.

The alternator has a D+ output on it which is small wire which gives a low current output. IIRC this normally goes to the charge warning light. The charge warning light has two connections one from D+ and one from the ignition switch. So when the ignition switch is off the warning light gets 0v on both wires, turn the ignition switch on and you get 12v on the ignition wire and 0v on the D+ so the warning light comes on. Start the car and you get 12v on both wires so a potential difference of zero so the light goes out.

The device on the car uses this same D+ wire to sense if the car is running, this is already in place, what I want to do is trick the device into starting by applying 12v here but don't want the 12v going back up the wire to the alternator and the warning light. Is that where the diodes would come in?

I bench tested the motor a while ago on a car battery which had been sat around for a while so was probably only at about 12v and it ran fine, it's only a very simple 12v brushed bosch motor.

I'll try and draw up exactly what I'm after,

Thanks again for the reply though
 
I've attached a very basic diagram of what is currently there and what I think I want. For now I've excluded the 2nd Battery as it may not be neccessary and is easier without it.

The modified diagram on the right simply has a SPST switch added at the bottom with 12v going in one side (would be a fused connection) and 12v going out on the other side to two things, one is the motor and one is the device.

What I'm worried about is 12v then going to the ECU and 12v going to the alternator so I would need some sort of protection there but I wouldn't want to interfere during normal operation. The device has a low voltage cut out at 10.2v and an over voltage cut out at 16v. The motor is just a standard 12v Bosch brushed DC motor.
 

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Many alternators have a lead or terminal that puts out a tapped AC output from one of the phases.
Its primarily used for sensing that the engine is running or for an electric tachometer signal for some diesel engines.
On older vehicles it was sometimes also used to power the electronic choke control on the carburetor.

Without knowing the exact alternator is difficult to know what it has for outputs. Some only have two wires some have as many as eight!
 
I've already tested the alternator output it's a plain output which has the same voltage as the main B+ wire except when off it goes to zero rather than battery voltage.

It's not the alternator connection that concerns me it's the protection I need to put in place,

Thanks for the reply though
 
On standard alternators the small wire is the warning light / field current connection. It will be at around 2V with the ignition on and the engine stationary, and will come up to battery voltage when the engine starts.

Your device has a main supply, so it is highly likely that the control input to it will work fine when it is fed with nearly 12 V. A simple diode between the alternator and your device will stop current from your switch feeding current to the alternator.

(If you did feed 12 V into the warning light connection, it won't actually do any damage. The ignition warning light will come on, and you'll get about 2 amps of current into the field winding of the alternator. I know this because I had a diode fail short circuit in an alternator, which had the effect of feeding 12 V to that connection, and the ignition light was on whenever the ignition was turned off.
 
Thanks for the reply,

Here is a link regarding the D+ terminal:

Pelican Technical Article: Alternator System Troubleshooting


You may be right that it has 2v when the ignition is on but the engine is not running as that would still give a potential difference of 10v to light the warning light.


You say a simple diode will do the job, what kind of diode any particular spec?

Would I just cut the wire from the alt to the device an insert the diode inline with the new power supply on the device side of the diode and the diode pointing towards the device?

Then do the same for the 12v motor with another diode between the motor and the ECU?

Thanks again
 
Would something like an IN5402 diode be big enough, the datasheet says it handles 3.0 Amps of forward current, for the D+ line that will be fine, but for the Motor it may be a bit close.

The motor says it uses 16watts during normal operation which is about 1.5amps at most if the battery was a little low, but I wasn't sure if during startup it would draw more and blow the diode? Any advice on this?
 
Hi. Based on your description and circuit, you may want to put 3 diodes in, and possibly a capacitor or RC snubber across the motor. The diode between the switch and the device is needed if the motor and device are connected to the same terminal of the switch.

The diode(s) to the device can be any common diode you feel like (e.g. 1n4001/2/3/4/7) and the diode to the motor could be the one you suggested. If you're concerned about the diode heating up, connect 2 in parallel - but 1 should be fine.
 

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Thanks for that,

Is the extra diode just to make sure that the motor doesn't pull any current from the low current wire from the alternator and melt it?

Is there a better kind of switch that could be used to prevent the need for this?
 
Thanks for that
You're welcome.

Is the extra diode just to make sure that the motor doesn't pull any current from the low current wire from the alternator and melt it?
Yes. It can be a small signal diode if you wanted (e.g. 1N4148); while the 1n400x would still be fine.

Is there a better kind of switch that could be used to prevent the need for this?
Yes. If you use a double pole switch, just connect 1 pole to the motor and the other to the device input. e.g. N/O1 to device, N/O2 to motor, CMN1/2 to the battery.
 
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