[Amps] 8877 blower requirement -airflow switch

[Hugh Duff]

I saw a variation of this on the 'net. It uses the filament of a #47 
light bulb (with the glass carefully broken and removed) as a
temperature sensor. A constant current source heats up the filament and 
a comparator measures the resistance across it.
The filament resistance drops with cool airflow and rises without any 
airflow, causing the comparator to trip.

http://www.aaroncake.net/circuits/airflow.asp

It's pretty ingenious but I might just go with the microswitch-vane 
method to keep it simple.

Thanks for all the suggestions.

73 de Hugh VA3TO
www.va3to.com



>Date: Thu, 12 Apr 2007 08:03:27 +0100
>From: Steve Thompson <g8gsq at eltac.co.uk>
>Subject: Re: [Amps] 8877 blower requirement -airflow switch
>To: amps at contesting.com
>Message-ID: <461DD9BF.8090406 at eltac.co.uk>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>I've used a ptc thermistor - the sort that changes from low to high 
>resistance abruptly at a set temperature. Run some current through it so 
>it self heats, the airflow cools it so it stays below the trip point. 
>The thermistor is about the size of a match head, so the mechanics are easy.
>
>Many years ago a friend tried a variation on this - putting the sensor 
>in the output air so it tripped if the air got too hot, whether that was 
>from lack of flow or excess dissipation.
>
>Steve
>