Fan fail/jam

[anderino]

Does anyone have a fan fail circuit/detector so that if the fans get jammed or slow down then the system will shut off? I'll be using FB3, wondered if somehow the interlock could be utilised for this?

Also, anyone know where I can get a small internal PSU from, only need it to power the fans.

Cheers!

[shrad]

hi anderino

for the fan jam/fail detector, you could monitor the voltage drop of the fan if it is low voltage DC...

you'd just have to measure what the drop is when fully on, then measure it when jammed/not blowing, and then use a comparator or an opamp to compare a voltage reference set to the jammed value to the actual voltage drop of the fan

for small PSUs, you could look on ebay for a small switching PSU, or even a small laptop adaptor which gives 12~14Vdc... if the purpose is only to power the fan, a simple wall adaptor would do

[Things]

As shrad said, the fans will draw more when they can't move apposed to moving.

[heroic]

You could use PC style three-wire fans and use a 555 pulse-fail detector on the tacho output. This is a pretty simple circuit to set up. Like this:

http://www.ecelab.com/circuit-miss-pulse-det.htm

You'd take the output of this and use it to drive a FET which drives the coil of a relay that controls the power to the heat-producing components (lasers and galvos, I guess). The relay is configured as normally off; the output goes "high" when the fan is spinning, which turns on the FET which turns on the relay. When the fan is not spinning, the output goes "low" which turns off the power to your galvos and lasers.

Most PC type fans produce either two or four pulses per revolution, and they tend to spin at around 3000 rpm, so your pulse detector circuit should be set up to expect at least 70 Hz. When the fan's airflow is blocked, it will spin *faster*, not slower, so be warned that blocked filters will not trigger this sensor.

[laserist]

there are thermoswitches and fuses that will certainly do the job - a spring loaded flag in the air path with an optosensor would work. there are differential pressure sensors that would work. thermistors and thermocouples and bimetalic this and that... I sort of recall the old CDC 9766 disk drive (300 MB 650 lbs) used a resistor painfully under rated without the air flow to die and shut off power... the kiss answer is a thermoswitch.

[anderino]

Really appreciate all the answers guys, hopefully this will silence my paranoia about it going up in a fireball!!

[FourDee]

He wants a FAN-fail not a scan-fail