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 contactor 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 30 W 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 trans-former, but still.25 kVAC means a peak volt-age 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 lighting strikes!
Circuit diagram :
Power Supply with High Voltage Isolation Circuit Diagram
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 was fairly high. The secondary supply can there-fore remain fairly simple. The main supply of 9 VDC is stabilised 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 volt-age controlled oscillator) makes LED D2 flash when the supply voltage is present.
Author :Jac Hettema - Copyright : Elektor
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