Hello,
I would like to operate a 12VDC, single supply circuit, as a rail to rail window comparator (or within 200mv). The inputs would be from a single variable voltage source in the same range, 0-12VDC, one comparator would trigger when the input reached a high of >= 11VDC and the low would trigger when the same input signal reached <= 6VDC. Basically, only 1 output at most could be 'on' (either high or low), or neither in the case the input was between 6-11VDC. The output of each, independently would need to switch, with decent isolation, a 500ma circuit ( a 12VDC 400ma fan). The high would connect the fan direct to 12V rail and the low would disconnect it and break the circuit from ground. When neither are triggered, the fan would be controlled variably by a TIP142 emitter-follower in the range 6V-11V.
1. What would that window comparator look like in terms of supporting components to have those trigger points with adequate hysteresis for slow moving inputs?, and
2. what would the outputs be to switch a 1/2A load (fan), could it be a FET of sometype or relay, or be driven directly by the comparator?
3. Since the TIP142 that would normally regulate the fan voltage (6-11V) might have some Base-emitter current, is there any reason to break that circuit and bypass the TIP completely at the emitter to GND leg when the TIP is bypassed and either FULL on with 12V rail or full OFF by breaking the circuit - the goal being near-ZERO power dissipation in the TIP in those 2 bypass cases.
** Stupid Question? I am new to this forum, how do I post a drawing or pic? If I click on insert image all I get is the IMG/IMG tags. No call out for a file location to paste in.
Many thanks! You guys are the best!