Occasionally you come across some unusual situations when setting up measurement systems. The author once had to set up a system to register the vibrations and strain supposed to be present in a contractor that operated at a voltage of 25 kVAC. One of the biggest problems with this project turned out to be the power supply for the measurement system. Since it required a power of about 30W it wasn’t possible to use batteries since the system had to operate for many hours at a time. A logical solution would seem to be to use an isolating transformer, but still… 25 kVAC means a peak voltage approaching 40 kV, and on top of that you would have to include a safety margin. In addition, everything that is connected to high voltage lines should also be able to withstand lightning strikes! Consequently, the isolation should be able to cope with a test voltage of 150 kV, which is a lot to ask of the isolating material.
After extensive research, no supplier could be found for a transformer rated at 50 W, 230 V primary, 12 V secondary and an isolation of 25 kVAC. Because of this, a dynamic system had to be used that unfortunately suffers a bit from wear and tear. This system consists of a 50 W 3-phase motor connected up via an isolating drive-shaft to a 30 W generator (a 3-phase servo motor that was used as a generator), which provides the power for the data logger and associated electronics.
Because a 3-phase generator was used, the voltage obtained after full-wave rectification (via D1 and D4 to D8) already looked good, also because the revs of the generator were fairly high. The secondary supply can, therefore, remain fairly simple. The main supply of 9 VDC is stabilized by IC3, an LM317T. From there it is fed to a few small DC/DC modules (IC1, IC4, IC5), which supply voltages of +5 V, +30 V and -9 V, which are required by the other parts of the circuit. IC2 (LM566, a voltage controlled oscillator) makes LED D2 flash when the supply voltage is present.