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Power Line Circuits
Circuits designed by David Johnson, P.E.
Last Updated on:
Wednesday, April 28, 2010 03:53 PM
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- ISOLATED AC CURRENT
MONITOR
This circuit uses a small AC current transformer from Magnetek to produce an
isolated voltage proportional to the AC current in the primary winding.
The transformer contains a single turn primary with a low 0.001-ohm
resistance. It can easily handle 30 amps of AC current and provides at
least 500vac of isolation. With the components shown, the output AC
voltage is scaled so 1 amp of current produces 100mv of AC voltage.
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- UNPLUGGED POWER
CORD ALARM
Many medical devices, such a portable X-ray and ultrasound equipment, carry
their own batteries. However, the frantic pace of some hospital emergency
rooms might cause the device to be shoved off into a corner without being
plugged back into an AC power source to recharge their batteries. So, I
designed this circuit to emit an audible alarm whenever the device’s power
cord was unplugged from the wall for a period of time. The device was
designed to snap onto the outside of a power cord, where it senses the AC
electric field emitted. No direct connection to the internal wires of the
power cord is needed. I also included a low battery voltage monitor.
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