An instrument for determining a characteristic of a biological fluid or a control comprises a radiation-reflective surface, a first source for irradiating the surface, a first detector for detecting radiation reflected from the surface, and a mechanism for determining the characteristic based on the detected radiation. A slide holds a sample of the biological fluid or control the characteristic of which is to be determined. The slide has two opposed walls transparent to the radiation. The first source and first detector are disposed adjacent a first one of the two opposed walls and the radiation reflective surface is disposed adjacent a second of the two opposed walls. A system is provided for supplying power and regulating supplied power from one of multiple power sources to the instrument. The system comprises first and second power sources, first coupling circuitry for selectively coupling the system to the first power source, second coupling circuitry for selectively coupling the system to the second power source, and circuitry for comparing a voltage supplied by the first power source to a voltage supplied by the second power source and for controlling the first coupling circuitry to decouple the system from the first power source when the voltage supplied by the second power source exceeds the voltage supplied by the first power source.