Ca3140 amplifier output voltage problem

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engineer1000

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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?
 

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.
 

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
 

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.
 

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