David, It's not necessary to run the fan unless the radio gets hot. The most reliable and practical method of controlling a fan is also the simplest and cheapest: a snap-action thermo-switch mounted on a heatsink fin. I use a Cantherm switch from Digi-Key that starts the fan at about 120 degrees Fahrenheit. It's a good idea to select a DC-operated fan that is specified for low EMI, such as those made by Panasonic. Some DC fans generate a lot of conducted and radiated noise.
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of drwoolweaver Sent: Wednesday, May 16, 2007 8:05 PM To: [email protected] Subject: [Repeater-Builder] Cooling fan relay circuit? My club has several ACC controllers and none have provided for cooling fan control. Does anyone have a simple circuit to turn on a fan with PTT and then keep it running for xx minutes after PTT is released? The repeater PTT line goes to ground on transmit. Most of the NE555 timer circuits that I have seen will not work in this configuration. Delay on drop out. DODO. Thanks de David K5RAV
