IC generate voltage when not connected

[Electroman87]

when i isolate an Op Amp or a 7805 IC they generate their own voltge which actualy effects the curcuit itself. Please give me some help.

[lacy]

I think I see what your refering to. I would like to look into this a little deeper. It would help with my analysis if you could post your circuit file configured in the manner in which you see this voltage if that is possible.

[Electroman87]

Well when the ignition is switched off the circuit still has volatage and when I isolated the components they still had power.

[lacy]

I can see exactly what you see Apparently there are errors in the op-amp models that are contributing to inaccuracies in the simulation of these components. If this is the case (as I suspect it is) then this will have to be addressed by NI..

I could find no faults in your circut that could possibly contribute to this. My analysis showed that the voltage is being generated at the power pin of the op amp itself with no power being applied to it. I even went one step further and disconnected it totally from power.  I think it has to do with this amp being modeled using the EWB 4 3-terminal model (which in my opinion is outdated and needs to be removed from the model list in favor of the manufacturer's models). In this model it has internal voltages that are set for operation without having power terminals. This actually causes a voltage to be present on the power pins at all times. It is also my opinion that all the op-amp models need to be checked and corrected where necessary.

Other than this glitch it seems your circuit is working. You may have to adjust the 200K Pot to something like 1K in order for it to swing properly.  The 200K Pot's resistance drops the voltage too quickly between settings. A lower value will not drop quite as much between increments. This is what I would call the sensitivity of the pot. Higher values are more sensitive to minor changes than a lower value..   

I hope this has been of some help.

[Electroman87]

Well i need it to have a slow drop because it is supposed to shut off the Light (which will be a relay) when the voltage drops below 12.80 volts. Its a battery saver circuit that i designed for my ATV so it would shut off an amplifier when the voltage droped to far.

[lacy]

About this slow drop . I was just seeing the voltage in the simulator at the wiper of the pot drop from 13V to 3V from 100% setting to 99% setting.  I used the 1K pot run it down to about 40% to 50% setting and the voltage dropped from 13V to about around 9V and then the LED would shut off at 9V. This is with the other pot set at 50%. If you try the 1K pot in the circuit you will see what I was refering to.

 I was assuming that you were just using this pot to simulate the battery drain and that it wasn't really a part of the completed circuit.

[Electroman87]

that is actually a simulation error. at 50% the voltage has to be half the voltage. One of multisims many aparent errors. but thank you anyway

[lacy]

No it wasn't a simulation error. It was a mis-statement error on my part. I did run it down to 50% and the voltage did drop from 13V to 5V. I just combined 2 statements together that should have been separate. I ran the pot down as far as 50% but the LED went out at approx 10V instead of 9V. I ran the pot through its entire range to see the voltage change at the wiper to see how quickly it dropped from one pot setting to another and try to get the slowest transition possible between the settings.

I will try and be more careful of how I state things. Sometimes my brain works faster than my typing and I usually mix my thoughts into one statement.

[Electroman87]

I added a transitor using it as a switch to turn on the circuit so there is no current draw through the ignition. But there is a voltage loss accross the transistor. I think it might be multisim yet again but i dunno.

[lacy]

I see a voltage of -4.051mV from ground to the emitter of transistor Q2. When I disconnect pin 3 of the  op amp and regulator from this portion of the circuit the voltage drops to 1.243uV. So I am thinking that there is leakage in the models that is contributing to this and upon looking at the model for Q2 there is leakage listed in the model information. I would also assume based upon what I see is that the op amp will also have leakage because it uses transistors in  its model as well. This may not be the only explaination, but it is the only one that makes any sense. I guess the components are modeled to be less than perfect so as to more accurately represent the real world components which are not perfect.

I am assuming that what I have said to be accurate based solely on my observations. This is not to say it is 100% correct and that this is the only reason for this behavior, but just what I see when I analyze this circuit.   (My disclamer for the day.)