Ohm’s Law

There was talk about using Ohm's Law to calculate resistance for negative pulse unlock wire for car keyless entry that requires resistors.

[b]My question[/b] how can u use ohm's law (I = V/ R, i = amperes, v =  volts, r =  resistance) be used to get  the amperage drop across a neg pulse wire?

You know what R is because thats the resistor you are placing on it.

V=? its not 12v is it? if so, why would it be 12v.

The senario is having that [b]neg 200ma through a resistor than having the amperage drop. [/b]

Someone said that you can use ohm's law to figure out your amperage for that scenario above

Havent used ohm's law since highschool physics class

There is no such thing as amperage drop.. there's a law that states current is equal in all parts of the circuit..

Correct me if I'm wrong, but the door lock circuit you're talking about is a single wire and when a resistor is added to the wire it will either lock or unlock the door, depending on the value of the resistor?..

What I suspect they are doing is using a voltage divider circuit to determine whether to lock or unlock the door. Imagine that there is a 500 ohm resistor inside the control box. When you hook up a 100 ohm resistor to the wire there will be a specific voltage between the two resistors.. to calculate it, divide system voltage (12v) by total circuit resistance (500 ohms + 100 ohms = 600 ohms). The current in this circuit will be 20 mA.

Now, multiply the current with the 500 ohm resistor.. (0.02 * 500).. it's 10 volts. This means that the 500 ohm resistor will drop 10 volts, so the voltage at the point between the two resistors is 2 volts.

Using the 1000 ohm resistor, the total circuit resistance would be 1500 ohms, divided by 12 volts gives us a current of 8 mA. 8 mA times 500 ohms gives us a voltage drop of 4 volts, which means that the voltage at the point between the two resistors would be 8 volts.

So, if you write the software that says 'if voltage measured is lower than 4 volts, unlock the door, and if voltage measured is over 6 volts, lock the door. you can figure out what range of resistors would work.

Hope this helps.. ask if you have more questions..

Jim

That is correct no-use.. it's referring to a 97 Sebring Convert.. we bought one a couple years ago as a toy.. the remote start RKE alarm wasn't working properly so I took it apart and fixed it right.. Scotch blocks etc up under the dash.

I don't do car audio as a profession.. but I do work with electronics. I could put a bunch of letters in my sig. line, but I don't see a need to; I prefer to let my advice speak for itself. What do you call a med student that just barely graduates?.. 'Doctor'. The letters don't mean nothing.

Jim