Hi there:
I do not know what I am doing. Is they any simple way to detect 12v dc current presence in a wire ( induction ) without interfering with that circuit what so ever to trigger a relay. In many of the newer cars, there is data transmitted in the circuit, so changing resistence, etc is a problem. Just want to see if I can add secondary lights to my high beam circuit. So when the cars existing hi beams come on I can trigger some secondary lights to come on without splicing into that circuit. Any help would be much appreciated.
I’d think you could use a resistor divider circuit of sufficiently high values to to detect this without messing with the signal to the light to any appreciable degree. Is the problem here more one of not wanting to cut into existing wiring or something else?
Keep in mind that the electrical environment in cars is pretty messy and noisy, complete with pretty nasty negative voltage spikes, plus 12 volts is only nominally what they run at.
I should explain that it is a projector beam and both low and high beam come from the same bulb
If the projector runs off AC you can make a short extension cord that breaks out the hot lead and use one of these to measure the ac current with the bulb OFF
and then again with it ON and use the AC voltage to calculate the difference in the power and then recalculate for 12V to get the dc current.
no , I have no problem digging into the harness. My understanding is that with this particular truck (2013 dodge ram) are now using a network , and also, you cannot even put a brighter bulb as the ECM will detect a difference in resitance and give you a warning. So , as I am told by several different sources, I cannot change anything in the circuit. As for the mention of a resistor differnce, can you explain more in layman terms. I am also looking into the reed switch as someone mentioned. Thanks for all you input.
This is and will only get worse as the cars andvance become more of an issue. Surprised no one has marketed something for this problem.
raschemmel:
If the projector runs off AC you can make a short extension cord that breaks out the hot lead and use one of these to measure the ac current with the bulb OFF
and then again with it ON and use the AC voltage to calculate the difference in the power and then recalculate for 12V to get the dc current.
By projector I think he means hi beam and dip which are dc I think
A resistor divider circuit consists of two resistors connected in series, end to end. The input voltage is connected to one end of the pair and the other end goes to ground, normally. You tap the connection between the two resistors. The voltage you get will be in proportion to the values of the two resistors. For example, use two 10k ohm resistors and your tap will get half the input voltage, use 10k and 90k ohm resistors and you'll get 90% of the original signal. Add up the values for the two resistors, divide that by the voltage, and that's how much current you'll drain off the original circuit. For example 12/(10k +10k) = less than a milliamp, negligible for a circuit powering headlamps.
What you'll want to do is select resistor values that will produce a voltage high enough to trigger the input line, but not so high that it will damage it.
Also, consider using a diode to protect your circuits from those nasty reverse-voltage spikes I mentioned. These may or may not be a problem in newer cars but they certainly were in my 69 Opel. The field collapse associated with any motors in the car, such as a fan or starter, even the started solenoid, when they are turned off, are to blame. This blew out all of the 12v LEDs on my instrument panel.
Is they any simple way to detect 12v dc current presence in a wire
Maybe what the OP is really asking is not how to measure the current, but simply how to detect the 12V on the load. If this was all he wanted to do I would have expected him to ask how to detect voltage instead of how to detect current. Detecting current without measuring it is the same as measuring the voltage, which is as simple as the explanation in the previous post. Detecting current using induction is quite a bit more involved.
Thank you all so much for the info. I think I will personally splice into the wire and trigger a relay and see what actually happens to the system. If it fails or gives me a warning. If so, then will pursue one or more of these options. I will let you know the result. I am just looking to detect voltage. Sorry for my lack of knowledge. And the idea of only bleeding off 1 millamp for a trigger sounds good. I will do more research, and keep you posted.
leeswreackcreation:
no , I have no problem digging into the harness. My understanding is that with this particular truck (2013 dodge ram) are now using a network , and also, you cannot even put a brighter bulb as the ECM will detect a difference in resitance and give you a warning. So , as I am told by several different sources, I cannot change anything in the circuit. As for the mention of a resistor differnce, can you explain more in layman terms. I am also looking into the reed switch as someone mentioned. Thanks for all you input.
This is and will only get worse as the cars andvance become more of an issue. Surprised no one has marketed something for this problem.
It appears that the data circuit loading is the issue. Any data used can only be used to turn on a relay to actually carry the large current. If you connect into the circuit on the switched side you have either a Gnd or 12v applied. Either side needs to have a voltage divider as the voltage of either will be 12v at some time.
Remember that 12v is nominal and will get over 14v.
In the noisy environment of a car though im a bit dubious.
Reducing the resistors may be a good idea depending on the sensitivity of the current sensing on the switch, but a cant think its going to be that seneitive
Safety warning though disconnect the battery first.
A short while soldering the 10 k resistor could allow many amps to flow the only fuse being the headlamp.
