I am looking for a cheap safety switch circuit and or sensor for a fan if it were to fail. So if the fan cuts off. Then it will switch off anything i want it to.

 

Some fans have a built in sensor that outputs pulses. You could have an LED one side of the blades and a photo sensor the other side. You could paint some of the bales white and us a reflective opto sensor. All these metods give string of pulses when the fan is running. use these pulses to triggere a retriggerable monostabe multivibrator qwith a pulse widthe setting greater than the time between pulses. Another metod is to use a thermistor in the aiflow. You would pass a current through it so it heats up a few tens of degrees. wihile it was in the airflow the temperature rise would be less than when it is in still air. If you monitor the voltage across it you will be able to tell if it is being cooled by the air flow. The voltage will go up or down depending on weather the themistor has a PTC or NTC characteristic.

Les.

 

There are duct air flow switches available, or it may be simple to fabricate with a simple micro switch and a small air operated vane attached.
Max.

 

There are duct air flow switches available, or it may be simple to fabricate with a simple micro switch and a small air operated vane attached.
Max.

I have looked into that method and seems reliable. But i will need something smaller as i am using for a computer size fan with limited space.

 

Some fans have a built in sensor that outputs pulses. You could have an LED one side of the blades and a photo sensor the other side. You could paint some of the bales white and us a reflective opto sensor. All these metods give string of pulses when the fan is running. use these pulses to triggere a retriggerable monostabe multivibrator qwith a pulse widthe setting greater than the time between pulses. Another metod is to use a thermistor in the aiflow. You would pass a current through it so it heats up a few tens of degrees. wihile it was in the airflow the temperature rise would be less than when it is in still air. If you monitor the voltage across it you will be able to tell if it is being cooled by the air flow. The voltage will go up or down depending on weather the themistor has a PTC or NTC characteristic.

Les.

it is for a computer sized blower fan that keeps batteries cooled. If the fan were to fail at any time the switch will shut down any charging.

 

You are not from Korea, are you? When i was working there, this is what i discovered:
https://en.wikipedia.org/wiki/Fan_death

 

I have looked into that method and seems reliable. But i will need something smaller as i am using for a computer size fan with limited space.

Some computer size fans have a tacho output. Air flow sensor vanes have to be carefully designed to avoid problems when the "dust bunnies" build up.

 

You are not from Korea, are you? When i was working there, this is what i discovered:
https://en.wikipedia.org/wiki/Fan_death

no im not

 

I have a White Rodgers steam humidifier that uses a fan safety device that is basically a hall effect sensor. It is kind of like a clamp on ammeter with a relay instead of a display.
The device won't allow the humidifier's heating element to operate unless the fan is drawing proper amps, thus proving it is on. If you reach lock rotor amps, the relay opens. (I noticed this when the blower motor seized.) Actually I installed two in series (safety nut that I am...). One is the White Rodgers part & one is a generic similar to those I mention in the 4th paragraph.

You can find the factory one by searching White Rodgers HSP 2600 Fan Safety Control Shut Off. I apologize but I could not find the part number or the trigger amps. (I think, if I remember correctly, that there is a pot inside where you can fine tune it.)

It operates at 24 vac. You open the lid and lay the common (non capacitor) lead for the fan motor through the device. You can lay it in twice if you need less amperage to trigger the internal relay. Again I apologize, but I can not remember what amps it triggers at, but my blower takes 5 amps. It also has a little LED on it that turns from red to orange to green as the fan amperage reaches the set point.

Generic ones are also available. There's one called Aprilaire 50 & Skuttle A50. And, they are available not connected to a humidifier brand for less money. They are a different configuration than the White Rodgers one, but the principle of operation is the same. Those aren't adjustable beyond looping the wire through the included bracket more than once. (In case your motor uses less amperage than the trigger point of the device)

The duct airflow switch that MaxHeadRoom mentioned are sometimes found under the term "Sail Switch". You have to keep the pivot joint very clean of dust build up or they won't sail. If it is a ducted blower, you can also get manometer switches where you put one sensing tube on the intake side of the blower and one on the discharge side. Many of those are very precisely adjustable for CFM requirements. Like the switch that Ian Field mentioned, dust and ice are troublesome with those.

Hoep This Helps & your project is a fun one!
Paul

 

I forgot one!
I've used this item a few on other, non fan related, projects:
http://www.crmagnetics.com/Assets/ProductPDFs/4395_a.pdf

It is quite adjustable for amperage & has a timer circuit in it. It's not as 'finished' looking as the ones in my earlier post, but that may not matter in your project.

I don't know how price compares with the ones in my previous post, but it may be worth checking into. eBay or Amazon may be the lowest price. Grainger does carry them, but $$$.

 

IF your fan does not have a tach output (mentioned twice, not answered), there are several ways to create an over/under signal based on fan speed.

One is to put a small resistor in series with the fan and interpret the voltage across it. I did not do this design, but a colleague did and it worked very well.

Much more common is to paint one of the blades white, or paint a target on the hub, or put a reflective sticker on the hub, use a reflective sensor to produce a pulse waveform, and use a comparator to create a good/bad signal. Less common is to use a broken beam method, where an LED is on one side of the fan and a photo transistor is on the other. This makes sense only if the fan is completely internal to the computer (one side is not mounted on a wall of the case) so neither of the optical components is outside the enclosure.

Once you have a pulse signal that is proportional to fan speed, a search for 'fan fail circuit' will yield many schematics.

ak

 

All options and ideas are greatly considered and appreciated. I will try all and figure the best option to use with less hassle. Thanks

 

If you want to get really creative you could attempt to do what the industrial versions do and use a small prox sensor such as the Honeywell ss400 series by drilling a small hole in the case and using the sensor to detect the rotor magnet.
You could seal it into the case with epoxy.
Max.

 

I've scrapped several old tape decks. They have a switch that doesn't have a hinge, so nothing to worry about keeping clean. The pressure to actuate these switches is so small that gluing a small aluminum foil on it and hanging it down from a vertical position should be more than enough to detect the movement of air. Perhaps later on today I'll dig out something and take a picture or two of it and post it.

 

Just glanced over and saw the board sitting there on my shelf. Grabbed it and took a picture of it. Did some drawing on it with paintbrush and hopefully you can see what I'm talking about. Notice the two switches on the far left side - you can see their shape. Sorry the picture isn't clearer but I've been having trouble with this camera and this is truly the best closeup shot I've done in a long time with it.

 

I worked with all sorts of fan failure sensors from the simple flag and microswitch; the differential pressure switch, and the metered air velocity transducer.

They were all problematic at sometime or another. In my implementation, there was a digital meter so you could tell when the sensor needed cleaning and the fault latched. In another, there was no meter and no latching so random cut-offs were common.

 

One problem I can imagine with my solution is that the switches can be noisy. As air buffets against the vane the switch will tend to move about. Partly wearing it out and partly making a lot of noise (switch bounce). Since this is a small fan and there's not going to be a lot of turbulence I'd imagine you could get some useful time with my switch. It may need to be debounced, but that's the easy part. I was thinking of just gluing a small disk of cardboard onto the face of the switch and see how much air flow (pressure) was needed to close the switch. The greater the area of the disk or vane the greater the pressure. Using too much pressure could promote faster wear. Too little pressure could result in premature shut-down.

I don't think my solution is the best. But it sure is simple. And sometimes simple works well enough for the task.

 

If you want to get really creative you could attempt to do what the industrial versions do and use a small prox sensor such as the Honeywell ss400 series by drilling a small hole in the case and using the sensor to detect the rotor magnet.
You could seal it into the case with epoxy.
Max.

One bank of computer fans I salvaged had a little square of self adhesive aluminium foil on one blade of each fan. A reflective opto sensor sent pulses to a little PCB with a comparator on it.

 

One bank of computer fans I salvaged had a little square of self adhesive aluminium foil on one blade of each fan. A reflective opto sensor sent pulses to a little PCB with a comparator on it.

That might work in a clean environment, the one I was referring to is used in CNC machines for cooling spindle motors etc. These can get quire messy with coolant splashing around and in the air.
Max.

 

it is for a computer sized blower fan that keeps batteries cooled. If the fan were to fail at any time the switch will shut down any charging.

if battery temp is your parameter, why don't you measure that?