A humidity interlock has been developed to prevent damage that could be caused by condensation of water on a delicate and expensive laser crystal that must be maintained at a temperature below ambient during operation. The humidity interlock is installed in conjunction with the laser temperature controller system. Whenever the humidity inside the laser housing rises beyond a safe level, the humidity interlock turns off power to a thermoelectric cooler on which the laser crystal is mounted.

The Humidity Interlock Circuit includes a relay switch through which power flows to a thermoelectric cooler. The switch is opened when the relative humidity exceeds a trip point.

The humidity interlock (see figure) consists of (1) a small, inexpensive, commercially available humidity sensor placed inside the laser housing, and (2) a control circuit. The sensor generates a voltage proportional to the local humidity. The sensor output is fed into a conditioning amplifier for conversion into a voltage indicative of the percent relative humidity (RH%). The RH% voltage is compared with a voltage representative of low RH% that is considered acceptable for safe operation of the laser and is designated the trip point. A technician can adjust the trip point by use of a potentiometer in the circuit.

When the RH% inside the housing is below the trip point, a comparator in the circuit activates a relay that closes a switch through which current flows to the thermoelectric cooler. When the RH% is above the trip point, the relay switch remains open, disabling the cooler.

Included in the interlock circuit is a two-color light-emitting diode (LED) that shines red or green, depending on whether the humidity is above or below the trip point, respectively. There is also a front-panel display device that indicates either the RH% or the trip point. The circuit includes an override switch that enables the cooler to operate when the humidity exceeds the trip point.

This work was done by Carlos Esproles of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category.


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Humidity Interlock for Protecting a Cooled Laser Crystal

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This article first appeared in the January, 2001 issue of NASA Tech Briefs Magazine.

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