# PHY C21: Outputs of Op-Amp Circuits

We’ve looked at op-amps as comparators & amplifiers, so now let’s look at what their outputs may be connected to!

• Output devices
• Relays
• LEDs
• Digital meters
• Calibration

Let’s start off with an issue that op-amps face:

## Op-amps cannot handle output currents above about 25mA, or voltages above 15V. Higher values will cause damage.

Unfortunately, most output devices require currents way higher than 25mA & voltages higher than 15V.
Thus, we need a way to allow a small output current to control a larger current to be useful!

Solution?

# Relays

What is a relay?
An electromagnetic switch which uses a small output current to switch on a much larger current.
It operates on the basis of electromagnetic induction.

How do you use a relay as an op-amp output?

• The coil section of the relay is connected to the output of the op-amp
• The movable arm section of the relay is connected to an external circuit

However, using this setup causes a PROBLEM:

• When the relay is switched off, there is a sudden change in magnetic flux (from non-zero to 0)
• This causes an unwanted large e.m.f. to be generated across the coil
• This large e.m.f. can damage the op-amp

How do we overcome this? DIODES.
Here are 2 standard setups:

If the relay only operates with a positive output voltage:

If the relay only operates with a negative output voltage:

Next, let’s look at:

# LEDs

What are LEDs?
Light Emitting Diodes are… diodes which emit light (duh).

They require a very small output current (20mA) to light up, so they can be directly connected to an op-amp’s output.

How do you use LEDs as op-amp outputs?
Practically, they are useful as indicators in a circuit which has input sensors.
See the example we covered here for 1 such application.

Few things to keep in mind:

• for an LED to light up, it must be FORWARD-BIASED
• if it should light up under a (+) output, it must be directed from the output to the ground
• if it should light up under a (-) output, it must be directed from the ground to the output
• a high voltage & current will damage an LED
• Thus, a resistor must be connected in series to the LED to reduce the voltage & current passing through it

Last thing, let’s look at

# Meters

What is a meter?
A device which displays a reading based on an input sensor.
Ex:

• Voltmeter

Meters can be either digital or analogue.

Remember that flowchart from the electronic sensors post?
Here’s an updated version specifically involving op-amps as processors to a meter:

When using a digital meter as an output device, remember that CALIBRATION is important.

Why calibrate a meter?
Because the variation in output voltage of an op-amp against the physical quantity may NOT be linear (it rarely is!).

Here’s a general method of calibration:

• Plot a graph of meter reading against physical quantity: AKA a ‘calibration curve’
• Refer to the graph when deducing the value of a physical quantity based on a meter’s reading. This can be done:
• manually
• by marking an analogue meter directly with a scale
• by processing the output via another device