Comparator circuit issues.

[robetere]

I am trying to use a comparator that will switch when the reference voltage drops below 6V.
The voltage I want to trigger this is the voltage between the test points (not between the test point and ground.)
I have attached a picture of my latest circuit and a zip of it in multisim any help will be greatly appreciated.

 

[crutschow]

I don't understand what you mean by "between the test points". One of the test points is just the supply voltage. Do you want to put a load between the two points? What are you trying to accomplish?

 

[MrAl]

I am trying to use a comparator that will switch when the reference voltage drops below 6V.
The voltage I want to trigger this is the voltage between the test points (not between the test point and ground.)
I have attached a picture of my latest circuit and a zip of it in multisim any help will be greatly appreciated.

Hi,

You might try a differential amplifier to detect the voltage difference and convert it to a single ended output, with the output going to your inverting input of the comparator.
You might want to use high value resistors for the amplifier to keep the input high impedance also.

 

[robetere]

The circuit is designed to test components.

The circuit has been designed to give a constant current of 98 μA accross the test points. (as long as it isn't open circuit)

The testpoints will be connected to a component it is deemed that if the resistance of the compontant is below 63kΩ then it is a "fail". This equates to 6.17V (63kΩ X 98μA).

That is why I want the comparator to compare the reference voltage 6V with the voltage accross the component between the testpoints.

 

[crutschow]

OK, now I understand what you are trying to do. Was that so hard?

Since you are using a comparator whose common-mode range extends to ground but not to the plus supply, I suggest you change the circuit so that the measured device can be connected between the constant-current source and ground. To do that, change Q1 to a PNP and turn the circuit upside-down so that the transistor emitter resistor and zener diode are connected to the plus 9V supply with R1 going to ground. Then you put the test component between Q1's collector and ground and use the comparator to monitor the voltage across the component to ground. Make sense?

 

[alec_t]

Your op-amp circuit has a fixed bias voltage on the non-inverting input but no variable input. The inverting input is floating. So the amp will do nothing useful.

 

[robetere]

Thanks crutschow that worked well.

 

[crutschow]

Your op-amp circuit has a fixed bias voltage on the non-inverting input but no variable input. The inverting input is floating. So the amp will do nothing useful.

That was the reason for his post. He didn't know how to connect the comparator (not an op amp).