Ca3140 amplifier output voltage problem

[engineer1000]

I'm using a CA3140 op amp as a comparator to switch a relay on and off. When I measure the output of the amp pin 6 I only get 2.8V unloaded, I was expecting it to go near the rail voltage of 5v. Does anyone know why this is? do I need a pull up resistor?

 

[Audioguru]

Why didn't you look at the datasheet for the CA3140 opamp?
It shows that its typical unloaded output high voltage is +3.0V and is less when it has a load to ground when its supply is 5V.
We don't know how much current the relay coil uses so we do not know if a pullup resistor will allow the relay to turn on and still allow the opamp to drive it low enough for the relay to turn off.

 

[KlausST]

Hi,

Not exactely related to your problem:

my recommendation:
If you need a comparator, then use a comparator
if you need an opamp, then use an opamp.

Why:
An opamp is designed to regulate it´s output in a way that the difference voltage of Vin- and Vin+ is about zero.
There are a lot of opamps that have problems when differential input voltage is more than some 100mV. (latch up, extended delay, even phase reversal = output swings to the opposite side than expected)

An comparator is designed to handle large differential input voltage without problems.

Klaus

 

[Audioguru]

A common comparator is in the LM393 dual and LM339 quad. They have open collector outputs so they cannot pull up a load and they are low power so their minimum output low current is onl;y 6mA when their output voltage might be +1.5V.