Question about LM392N

[pblakeney]

Hi

I have more of a general question. Is the LM392N have an open collector for its output?

I am interested in using the comparator part of the circuit (Amplifier A).

If I place 12V on the V+ (pin eight) and I have the non inverting input voltage greater than the non inverting input voltage does that mean the ouput would be floating?

Let's say the Non Inverting input has 6 volts and the Inverting input has 5 volts and the output is connected to an Led through a resistor (say 680 ohms), would the LED light up?

If I think I understand this comparator my gues would be that it would not. And I would have to use a pull-up resistor on the output. However if I did this then wouldn't there be a voltage divider and the LED might not light up or glow only dim because of the pull-up resistor and even if I did find a pull-up resistor that would cause the LED light to glow normally wouldn't it be on all of the time unless the inverting input became greater than the non inverting input?

Am I understanding this correctly?

Actually what I am trying to do is have the output of a comparator pass to the input of another comparator (comparator A output being connected to the non inverting input of comparator B). If I understand this comparator correctly if the non inverting input on comparator A is greater than the inverting input of comparator A then I would have no signal at all for the inverting input of comparator B. If I use a pull-up resistor on the output of comparator A then the inverting input of comparator B will be high all of the time unless the non inverting input of comparator A falls below the inverting input of comparator A.

If this is indeed the case, would I be better off using an Op Amp with no feedback so that I could get the high output signal needed when the non inverting input is greater than the inverting input?

Thanks for any help of suggestions in advance!

 

[keith1200rs]

If I place 12V on the V+ (pin eight) and I have the non inverting input voltage greater than the non inverting input voltage does that mean the ouput would be floating?

Yes - it is open collector and the transistor will be 'off'.

Let's say the Non Inverting input has 6 volts and the Inverting input has 5 volts and the output is connected to an Led through a resistor (say 680 ohms), would the LED light up?

No.

I got lost in the description of what you were trying to do connecting one comparator to another, but it is not a normal way of using comparators and you need to be careful of the input voltage range of the comparators.

Once you have been through a comparator you have a digital signal so you should use digital logic to work with the signals. You can make a simple 'wired OR' with open collectors which is why they are used. This way you can make a window comparator with 2 comparators and no extra logic just by connecting the outputs together with a pull-up resistor.

Keith

 

[pblakeney]

Hey thanks Keith for your answers! That was a big help and what I expected.

Actually what I am trying to do is make a circuit that has two seperate batteries. There will be a main battery and a back up battery. Attached is a picture of the circuit.

the circle with a 2 in it means that it gets power from both batteries. Each battery has a diode connected to it and power is taken after the diodes so that one battery does not try to charge the other.

the voltage from the main battery (before the diode) is going through the ignition switch and to the inverting input of Op Amp 4.

Voltage is also going from the main battery (before the diode) and through a voltage divider circuit to help compensate for any voltage drop of the other input of the wire length from the ignition to the op amp.

Op amp 4 is primarily checking that the key is in the ignition and the switch is turned to the RUN position.

If the key is turned to the RUN position I want it to output 12 Volts or thereabouts from Op Amp 4. The 12 volts will then be regulated to around 6 volts through the 1k ohm resistor and 1N753 diode to the inverting input of Op Amp 5.

If the Main Battery (before the diode) which is connected to the non inverting input of Op Amp 5 does not have at least 6 volts (say it has 5 volts) then I want Op Amp 5 to output zero volts which will bias the PNP transistor BD912. BD912 will then energize a relay.

So to sum up I want to make sure that the key is in the ignition and turned to the RUN position and if the main battery does not have at least 6 volts then I want to have the Back up battery energize a relay.

I guess I would need to use regular Op Amps instead of voltage comparators to accomplish this?

Or do you see another more simpler approach? Thanks for any help!!!