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:

A diode D1 is placed in series with the coil to be forward biased when the output is (+)

This allows a small current to flow out of the output & into the relay’s coil when output is (+),
so the relay operates.

This also prevents a current from flowing from the relay’s coil & into the op-amp when the output is (-),
so the relay will not operate.

This also prevents a current from flowing into the op-amp when an e.m.f. is induced once the relay is switched off.
 
A diode D2 is placed parallel to the coil to be forward-biased when the output is (-)

This allows a large current to flow out of the the relay’s coil into D2 when an e.m.f. is induced,
once the relay is switched off.  
 
Net effect:

Only a (+) Vout will activate the relay.
A (-)Vout will have no effect.
The op-amp is protected from damage due to induced e.m.f.
 
Play around with this simulation I created on Falstad.com to understand better!
You can change V+ to see how the output of the op-amp affects the relay.

If the relay only operates with a negative output voltage:

A diode D1 is placed in series with the coil to be forward biased when the output is (-)

This allows a small current to flow out of the output & into the relay’s coil when output is (-), so the relay operates.

This also prevents a current from flowing from the relay’s coil & into the op-amp when the output is (+), so the relay will not operate.

This also prevents a current from flowing into the op-amp when an e.m.f. is induced once the relay is switched off.
 
A diode D2 is placed parallel to the coil to be forward-biased when the output is (+)

This allows a large current to flow out of the the relay’s coil into D2 when an e.m.f. is induced, once the relay is switched off.  
 
Net effect:

Only a (-) Vout will activate the relay.
A (+)Vout will have no effect.
The op-amp is protected from damage due to induced e.m.f.
 
Play around with this simulation I created on Falstad.com to understand better!
You can change V+ to see how the output of the op-amp affects the relay.

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

Read more about this here:

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