# Power Supply with High Voltage Isolation

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.

Power Supply with High Voltage Isolation Circuit Diagram:

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!

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.