Comparator Input Connected to Supply Rail

I'm planning on using a rail-to-rail input comparator like this:

100Vmax+ | 5V R |\\| |-------- |+ \\____Vout R2 5V-- |- / | |/| gnd gnd

When R2 goes beyond 5V, the output goes H.

I might use this comparator

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Note 2 says the input terminals are diode clamped to the power supply rails and should be current limited to 10mA or less. ok..I can deal with that.

afaik..this circuit should work. No crazy behavior except perhaps a little glitchy around the equal input point for slow signals.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC
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A positive feedback resistor to the + input would help with the glitchy and add some hysteresis. make R > 20K. It should work.

Be careful that the +5 can sink the few mA going through the clamp diodes and not raise its voltage. This guy doesn not like more than 5.5V.

I'd do this only if backed into a corner (price or size).

How accurate does the trigger point need to be. if its loosy goosy (simple over voltage ckt. you could use a PNP and a NPN to get nearly the same thing.

Reply to
mook johnson

I'm using a r-to-r op amp to simplify the PCB design. The more I trim the circuit the faster the PCB layout time.. Alternates are to use an op amp with an internal reference or use an op amp with an external ref (R divider, shunt, zener etc..).

Tripping between 100V and 110V might be ok. The circuit will be getting air temperatures between 20C to 90C and I suspect that'll disqualify using a transistor due to too much Vbe change with temperature.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

This means (1) you're using your 5V as a reference voltage (so spikes on the power supply will hurt you), and (2) some current from input through R will flow into the +5V supply. The usual +5 regulated supplies do NOT tolerate bias conditions where they have to sink net current, you MUST have net total circuit load on the

+5 supply.

With 100v input, you don't need the gain of a comparator; a simple zener+transistor with a couple of bias resistors would do. Or, you can get real reference-and-comparator in so-called 'voltage supervisor' circuits. An old variant, the Norton current-differencing amplifier, would be perfectly suited to this application (see LM3900 for a quad of this item).

Reply to
whit3rd

huh....I'll watch out for that.

I think it's easier to design a comparator circuit than a zener+trans cct. Zener production and temperature variations and transistor production and temperature variations scare me :)

But I'll check out the LM3900.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

True, but using any (differential) transistor pair converts the problem to Delta Vbe and matching the temperatures between the transistors. .

Reply to
JosephKK

I don't know if it's just me or perhaps it's popular practice but I tend to gravitate toward an IC solution whenever a circuit has more than 1 transistor.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

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How about a TL431 and a transistor?

Reply to
gearhead

That's why 6.2V is so popular for Zeners; a transistor base-emitter junction and 6.2V Zener diode have, in series, zero temperature coefficient.

Most 5V supplies are only 10% accurate, anyhow (some get to 2%, like LT1083/LT1084/LT1085).

Reply to
whit3rd

Neato...I didn't know that..

Yikes.. I haven't looked yet at the tolerance of typical 5V voltage regulator ICs.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

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