Why is there voltage in circuite when kill switch engaged?

I've sought to document the wiring circuit in my car. I think its an accurate representation of what I have in place but for some reason the simulation shows voltages across all elements when the kill switch is engaged (the voltage should be zero).

Does anyone have any idea what I'm missing?

by zippityzappity
May 16, 2022

The switch is on the low side. So the voltage is high at the load, but without (significative) current, unless there is a short (touching the circuit, as example). Same as the case of a copper wire attached to the + side of a battery, but hanging like an antenna, it will be at the same voltage than at the battery. You may even add a resistor at the wire, since V = RI still hold, but with I = 0, the voltage drop across the resistor will be zero.

That is one of the reasons to place the kill switch at the high side, as we do for a PMOS. (Rather than like for a NMOS, as it is actually). It is safer against accidental shorts since there are less wires and components at a high voltage waiting to be discharged.

by vanderghast
May 16, 2022

Simplified circuit:

by vanderghast
May 16, 2022

Interesting and thanks for the explaination. That makes a lot of sense. The application here is a kills switch in a racing car. Interestingly, many online guides suggest placing the kill switch on the low side. I think this is to provide for the jump functionality, but it does seem like your suggestion of putting it on the high side would otherwise be safer...

by zippityzappity
May 16, 2022

Having the switch on the low side has the problem that if a human touches the denuded wire or a component, while being ground with the other hand/feet, then this human being creates a close loop to allow the current to flow.

Hand-held voltmeters are not a problem since by themselves, they don't give a path to ground. A non-hand-held oscilloscope can provide such a path though (through its own ground). In a dry environment, it is considered that there is no severe problem when much below 50V, but that is for healthy adults with "intact" skin.

by vanderghast
May 16, 2022

Also note that in the case of a car accident, gazoline leak could also connect a component to ground, which could create a new problem for a circuit with its kill switch on the low side.

by vanderghast
May 16, 2022

Thanks again. I've revised as follows and all is working as intended and I think I've addressed the point you raised regarding the kill switch being on the high side.

by zippityzappity
May 16, 2022

1 Answer

Answer by Iceburg66

Question - Is the kill switch NO or NC to create the required function?

I see a circuit that the kill switch connects the negative side of the battery to ground, there by allowing the battery voltage to be applied to the circuit or but not discharging it in reverse action. I assume the simulator recognizes the charge like capacitance.

+1 vote
by Iceburg66
May 18, 2022

It can but unlikely be a regular momentary switch (NO or, here, would be an NC) since they would require maintaining the pressure to ensure the working (for a NO switch) or the stopping status (NC switch), which is an unsafe measure in the case of critical scenario which is logical to assume for a kill switch. It is quite probable that the kill switch will also prevent the operator to re-arm the circuit for some time, but that would depend on the safety measures provided by the maker of the switch.

We cannot really presume what every simulator does, for an open branch of the circuit, but "we" can see it as a huge resistor (tera-ohm or more) which, like for the case for air or for a MOSFET (here, it would be a NMOS), allowing some small current leaking through it. Any connection also adds capacitive effect, but unless you are in extremely high frequency, we don't add the small cap (around 2 picofarad, take or add).

Last note somehow related to our case, if you allow me, about the expression: "there is no resistor". That may imply a 0 ohm resistor (no resistance) or that may imply a missing component, like here, which is a tera-ohm resistor. That expression can thus denote two diametrically opposite status. Ambiguity is to be avoided.

by vanderghast
May 18, 2022

I just saw another way to make a kill switch with a Normally Open momentary switch: You force the button down with a clip or with a cable and, in emergency, you pull out the clip (pulling it with a cable attached to it) or you cut the cable so that, in any cases, the button climbs up in its open position. So, you shut off the circuit and then could not immediately re-arm, unless you press the button itself, and maintain it. It is purely mechanical. Possible problem: the clip does not fall off, or the spring, of the switch, has lost its strength due to a long time kept down, and does not push back the button.

by vanderghast
May 18, 2022

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