I haven't set one up yet, but have picked up a few sliders wanting to add a kill switch to the circut of a mod. where would be the best place in the circut to place a slider to function how I want, but not have it carry a heavy current ?
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First, I am not a EE, or a serious modder type. This question would be more properly addressed by those wiser than I in this forum by making a separate thread for it. But the basics are simple: If your mod is pure mechanical, that is, raw power from from battery (or batteries, series or parallel) through the firing button to the atty and back to battery, then there is no place a small-current-capacity kill switch will work. In that mod, if you want a kill switch, it would have to be rated the same amps as the firing circuit.
If you are using a N-channel FET to control the firing circuit a small-amp switch-- tiny, really, it carries only a uA current, mostly a voltage signal-- can be placed between battery positive and the FET gate. A N-chan turns "on" when the gate it high (+V or batt pos) and "off" when the gate is low (-V or batt neg). This is the exact same lead you will use for the firing button to the FET to fire the mod. Just series the slider with the pushbutton and it doesn't matter which one comes first. When the switch is on *and* the button pressed, the mod will fire. When either switch is in the off state, it will not. (
WARNING: an N-chan FET requires a pull-down resistor to make sure the gate voltage is really low, and the FET completely off. Otherwise the mod might fire, or stay partly energized, unintentionally. This sounds odd, I know, but it's just nomenclature. An FET is not really a "switch" per se. More like a variable resistor. When it's supposed to be "off", resistance is really high and current can't flow. When it is "on," resistance is much lower and current can flow. But it's not a sharp cut off, there are
shades of off and on with a FET.) Please seek proper guidance from those wiser than I on exactly how to wire it and the proper resistance values and so forth.
If you want to use a P-chan FET for the fire switch, I have not figured out where to put the switch. A P-chan is backwards; it is "off" when the gate is high. This requires a SPDT (on/off) pushbutton, with +V to one terminal and -V to the other; the common leg then being taken to the gate of the FET. You wire the button so the gate is +V when the button is not pushed and -V when it is. Placing a switch anywhere in that circuit will turn the FET on, not off. The only dodge I have figured for a small amp shut off switch is to series an second, N-chan, FET between the atty and the battery -V side. I don't like that idea, because I am an electrician, not a EE, and I
hate low-side switching. In the electrician game, putting the switch between the load and the ground return is a big no-no. It works just fine-- series is series-- but it's viewed as unsafe because it leaves the load energized all the time and any ground path-- like through your friendly electrician-- will turn the load on. Admittedly, this is a much larger problem with a 600V, 3Phase AC, 150SHP motor than an atomizer running at <10VDC. But it just
feels wrong, and I have a hard time getting over it. It also does not obviate the tiny, tiny uA current the P-chan is using to stay closed. And adding the N-chan is low side switching anyway. Why add another one when one will do? You could put the slider in the battery negative feed of the pushbutton, but the gate might then "slide around" and get close enough to -v to turn on anyway. Any kind of trickle current to the gate to prevent it would prevent it from turning on at all, ever. So far, I haven't figured it out. Looks like my choices are N-chan low side switch or live without a a slider, or add an extra, low side, FET just for the slider and make the high-side FET redundant.
