On 8/3/14, 12:48 PM, Doug Crompton wrote:

It can be done through IO but unless you have some objection running the fan at half speed (or so) all the time is a much easier way to do it.

Or a better way to do it (IMHO) is using a thermostat bolted on the heat-sink. I use a Digi-Key 723-1191-ND 113f Normally open thermostat that will cycle even a large 12v fan easily as it warms up, then shut off as it cools down. This has none of the energy loss from running it full time or via a resistor at half speed. In most cases it will need to be keyed down for 10 min + for it to even turn on, and then it will run 10 min to cool it down and shut off. It will then cycle about 5 on/5 off for the rest of the key up time. Of course this doesn't answer OP's question. Sorry :( 73's -- Bryan Fields 727-409-1194 - Voice 727-214-2508 - Fax http://bryanfields.net