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comparator question

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bonanz

New Member
Would it be "okay" to use a voltage divider from a regulated 5v source as a reference voltage for a 339 comparator since its input impedance is ideally infinite? Also is there anything else i need to consider if i just want to output a "high" if the input is below the reference, just the Vin the Vref and a pullup resistor on the output to get TTL level output?
thanks for any suggestions or comments
 

stevez

Active Member
I've spent a little time learning about comparators. Next steps for me are to try and learn about hysterisis or differential (lots of meanings for both words). I'll be doing that with a breadboard, scope, etc.

What I am worried about is having the comparator toggle back and forth when the reference and the inputs are very close. I understand that I can add some components (resistors) that will increase the difference. What I will be doing is measuring how much my input voltage varies, given the conditioning, and adjust the differential so that my circuit doesn't sit there and chatter - hopefully remaining sensitive enough for the intended use.

Your application may be such that the differential is not of concern.
 

bmcculla

New Member
A voltage divider should be fine because of the high impedance input like you said. One potential problem is if you have ripple on your power supply this gets into you reference voltage which can cause problems.

Zener Diodes are a good way to generate fixed reference voltages.

Stevez
if you're worried about the output toggling a lot when the inputs are close hysterisis is exactly what you need. Hysterisis adds a little positive feedback (resistors from the output to the + terminal) to cause the output to switch at a different voltage.

Brent
 

bonanz

New Member
thanks a lot for your replies,

isn't the lowest Vz a zener can have like 2.4 or something? what if my reference i want to make is lower than that? will just putting a resistor in series with it work?
 

Sebi

Active Member
Use a silicon diode(s).The forward voltage is 0.7V,or a red LED about 1.5...1.8V. But for comparator circuit also need a stable and noiseless supply so the resistor divider (or poti) can work properly.
 

Faiyaz

New Member
What you exactly need is a Schmitt trigger ckt (using opamp) with positive feedback. The feed back will act as your trigger Vth and Lth via a feedback resistor network and also provide the necessary hysteresis.
The ckt will be available in any book dealing with opamps
 
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