FAN-TASTIC CIRCUIT

Sorry about the title but 1 could not resist it (groan).

This circuit was used to control a fan fitted to the new GB3CW repeater. During testing of the repeater it was obvious that the PSU and TX module needed additional cooling, as it was likely to go into thermal shutdown when used in full waffle mode. These modules are generally not designed to be permanently on transmit for long periods and rely on convection cooling only. If the module went into thermal shutdown the output would drop to 5 watts or less. Perhaps being more effective than the 15?minute timeout option, it would make the repeater unusable after about 45 minutes depending on the temperature of the building.

The circuit is based on the common 555?timer chip with a time?delayed input from the TX keying line. It was found that if the fan was running from the start of a transmit period the module temperature stayed very cool. However if a temperature controlled circuit was used it would mean that a temperature rise had to take place before the fan would cut in. Taking into account the temperature rise in the building 'm mid?summer, it would mean that the fan maybe running for extended periods when the repeater was not being used. To avoid this the fan temperature setting would have to be set higher than 1 would like. 1 always try and design circuits so that there is as little temperature rise as possible to reduce failure due to premature ageing of components.

The fan start time needed to be delayed to prevent the fan starting when the repeater went into beacon mode. The fan is not required for the few seconds TX time of this mode, only when a QSO is in progress. If the start was not delayed, it would mean the fan running for 10 minutes, the fan over?run time, in every 15 minutes, the beacon timing period.

The final circuit is shown below.

Dave GW8GLO