A stand-up MRI uses voltage regulators to linearly control voltage without reflected or added harmonic distortion. MRIs from Fonar Corp., Melville, N.Y (fonar.com) let doctors scan patients in a weight-bearing state, in a recumbent position, or both. A stable power supply to the MRI's electro-magnets ensures reliability and image clarity. Tight regulation of the incoming line voltage is critical because it provides a stable input to the power supply. This in turn produces a tightly controlled dc output which powers the electro-magnets.

Fonar had to control the voltage to a few parts per million. Incoming line voltage is normally supplied at ±10% but it needed ±1%. Voltage fluctuations can have disastrous effects, including rescanning or restarting the whole machine. Output from the voltage regulator is sent to a power supply where it is converted to DC with a stability of 1 ppm.

Another consideration was reliability. The unit has to perform reliably on a daily basis with Fonar's low-impedance power source requirements. Linear rather than stepped regulation was required, because any transit voltage jumps would disturb the power supply, translating to output disturbance.

Fonar selected a regulator from Staco Energy Products, Dayton, Ohio, (stacoenergy.com) with a variable transformer/buck-boost transformer design. The buck-boost transformer reduces the work the variable transformer must perform, and multiplies the power rating of the transformer. The buck-boost also reduces the current per coil requirement of the variable transformer, which results in increased MTBF — a key measure of reliability.

The regulators withstand substantial current overloads with no additional harmonics or wave form distortions, have low impedance (less than 1%), high overload capacities (1000% for thirty (30) cycles), attenuation of electrical noise, and are impervious to frequency changes.