Using Relays and fans with opamps/ comparators

Hi.
I have built many fan and relay control circuits with opamps and comparators with relative success. But I always wondered about one thing:
Can the relay or cpu type fan be powered from the same rail as the opamp or comparator supply pins?
I see many circuits online where the opamps are run off of a 78xx or zener regulator to keep the circuitry on its inputs noise free and stable. Then the current drawing component such as a fan or relay that is being switch on or off by the opamp via a transistor, is sinking its current from the positive rail BEFORE the regulator .
Is this a general rule? I would like to know so that I dont make the same mistake again.

If you want repeatable and predictable results from the opamp/comparator then it's good practice to give it a stable noise-free supply. Hence the use of a regulator. Relays and fans can cause voltage spikes and dips which the opamp/comparator wouldn't like.

I agree with alec_t.

JimB

Or you can use a chip like a TL431 which is like a comparator with its own built-in stable voltage reference. No need to regulate the supply voltage.

For example, here is a circuit that picks up a sensitive relay as the voltage V(ref) crosses 2.49V quite independent of the supply voltage as the supply voltage goes from 11V to 15V. The current through the relay coil (and hence the relay pull-in current) is independent of the supply voltage.

Note that the actual trip point of V(in) can be ratiometrically adjusted from 2.49V on up just by picking two resistor values (R1 and R2).

Circuit is much simpler (and likely more stable) than using an opamp or comparitor to drive a relay...

diy didi said:

Hi.
I have built many fan and relay control circuits with opamps and comparators with relative success. But I always wondered about one thing:
Can the relay or cpu type fan be powered from the same rail as the opamp or comparator supply pins?
I see many circuits online where the opamps are run off of a 78xx or zener regulator to keep the circuitry on its inputs noise free and stable. Then the current drawing component such as a fan or relay that is being switch on or off by the opamp via a transistor, is sinking its current from the positive rail BEFORE the regulator .
Is this a general rule? I would like to know so that I dont make the same mistake again.

Click to expand...

Some DC fans have pretty bizarre current demands. I have seen them make 78XX regulators oscillate unless relatively large caps were added. I suspect it is always good design practice to isolate the fan using a DC regulator and caps as well.

Also, by powering the load device from a point before the regulator, it keeps the load current out of the regulated path. Besides all of the noise conditions mentioned above, it prevents the load current from increasing the heat dissipated by the regulating device. This directly increases the long-term reliability of the regulator circuit.

ak