Trip alarm ???

[elfvenlord]

heya guys

Today a friend of mine asked me to come up with a circuit that would be able to sound a buzzer when the fan in his computer(+7V 100ma) short circuited or broke down so that his computer didn't melt!

So its really a case of the blind leading the blind but hey I said i'd give it a go.So below is a circuit I came up with using the op amp to power the buzzer when the 7V -ve input drops to 0V(i.e the fan circuit is broken).

The 7V -ve is taken from the +V of the fan.
The circuit is powered by a seperate 12V source from another computer power lead.
The capacitor and MOSFET are there to provide a time delay(hopefully around a few seconds) so that when the computer shuts down the buzzer doesn't ......well buzz.

So can anyone tell me that Im either a complete n00b and I dont know what im doing or suggest an easier way this can be done.
Really appreciate it.

 

[zevon8]

Good start!

Only thing I would suggest is that you replace the 7 volt sense line input with a pot, a NTC thermistor , and a resistor. NTC thermistors are special resistors that will rapidly drop in value when the surroundings increases in temperature.

the problem with sensing the presence of the fans power supply is that you won't detect a stuck, or slowly turning fan. Most brushless fans don't die open circuit, but often will stop turning due to bearings, dust, etc. They often don't draw a whole lot more current when stalled or slowly turning either.

connect the pot as a variable resitor, in series with the thermistor then the resistor. connect the resistor end to ground , the pot end to the + rail, and the connection between resistor and thermistor to the input of the op-amp.

You will have to play with values, based on the range of the thermistor, and set the pot so that a rise in temperature above normal in the PC case sends the thermistor value plummeting, triggering the alarm. depending on the thermistor, you may only need a fixed value resistor instead of a pot.

 

[Phasor]

Good start!
the problem with sensing the presence of the fans power supply is that you won't detect a stuck, or slowly turning fan. Most brushless fans don't die open circuit, but often will stop turning due to bearings, dust, etc. They often don't draw a whole lot more current when stalled or slowly turning either.

There is also an electro-mechanical solution - we use a vane to detect air movement (admittedly, with much larger fans). The vane is connected to a microswitch. If the vane drops while the fan is supposed to be on, the microswitch closes the alarm circuit.

Perhaps this is suitable for you, perhaps not. Just another idea.

 

[elfvenlord]

Thanks for your help.I'll check if the electronics store have any sensors to detect air movement from the fan,and I think your right zevon8,It would be better to build a temperature switch using a thermistor.
Great stuff
thanks again.

By the way do you think that the capacitor and MOSFET will provide some sort of delay (out of interest to see if I got it right) :lol:

 

[Exo]

Most new computer fans have a tacho built in (3 wires in stead of 2).
It would be a lot easyer to just check the tacho to see if the fan is still running.

 

[zevon8]

Good point Exo, I guess you could just take the tach signal, smooth it with a small capacitor, have a bleed-off resistor, and set the pair such that when the tach pulses are too far apart, the average voltage will drop blow a threshold point and trigger the alarm. I would think maybe this could be done better with a comparator , using some hysterisis to prevent the alarm from cycling. Unused sections of the comparator could be used to buffer the tach signal, an oscillator ( alarm )/ buffer to drive the PC's built in speaker.

 

[Nigel Goodwin]

Good point Exo, I guess you could just take the tach signal, smooth it with a small capacitor, have a bleed-off resistor, and set the pair such that when the tach pulses are too far apart, the average voltage will drop blow a threshold point and trigger the alarm. I would think maybe this could be done better with a comparator , using some hysterisis to prevent the alarm from cycling. Unused sections of the comparator could be used to buffer the tach signal, an oscillator ( alarm )/ buffer to drive the PC's built in speaker.

You could just use a 555 as a 'missing pulse detector', basically a one-shot timer with the timing capacitor discharged by a transistor across it - fed from the tacho pulses, once the tacho pulses get too far apart the 555 times out.