24VDC Fan Failure Indication Circuit

[me_guitarist]

fan fail circuit

Anybody know how to make a circuit for fan failure indication?

This circuit need to have an output mostly a relay will be great.
Input & fan is 24VDC.

Thanks in advance.[/i]

 

[FvM]

24vdc relay triggering circuits

If the fan has no speed sensor output, a thermal airflow sensor is the usual technique. It's standard equipment, available e.g. from fan manufacturers.

 

[me_guitarist]

fan detector circuit

I use the normal 4" exhaust fan so it has nothing. I need this circuit to trigger a relay when fan failure.

 

[kender]

24v dc fans

If you want to roll your own air flow sensor, here are articles that describe how to do it:
https://www.edn.com/archives/1996/031496/06di3.htm
**broken link removed**

P.S.
There are 2 distinct hazards:
1. Obviously, if a fan stops running, the components that it has to cool will overheat.
2. If a rotor is stalled (e.g. something is stuck into the blades), the motor can overheat and catch a fire.

 

[GrandAlf]

24 vdc fan

I remember doing this many years ago. I had a small vane attached to a sensitive microswitch, and placed it in front of the fan, even running slowly it manage to move the vane and operate the switch. Did not obstruct the airflow too much. I suppose you could even use a small magnet on a vane, to operate a reed switch. Only low current though. The above of course depends on whether you have the room to mount such a device. Have you actually instead considered monitoring the temperature of whatever you are trying to cool.

Added after 3 minutes:

There is one problem I forgot to mention, you need to give the fan enough time to run up to speed, before checking any switch.

 

[banjo]

24v dc fan with run monitoring

I had a similar problem several years ago. We solved it by mounting a small thin board with a photosensor behind the fan. The fan we were using had a metal disk in the center with a notch in it. The photosensor was IR and consisted of an emitter and a detector in one package. These parts are made to bounce IR light of an object and detect the reflection. As the fan turned, we got square wave out of the sensor. Frequency of the waveform told us how fast the fan was turning. No waveform or too slow a waveform indicated fan failure. If your fan does not have an metal to use as a reflector, you can easily make one out of metal tape. I have also had good luck with just white painted stripes as the reflector. Place the paint or metal tape on the center hub of the fan.

 

[me_guitarist]

Any idea using a transistor to trigger the relay?