The Silliest Voltmeter Ever…

The year…. something like 2004.

The transmitter…. Harris Broadcast PowerCD, a funky Inductive Output Tube based system with modular architecture supporting multiple cabinets.

The silliness: much. Much, much silliness. So here’s a hilarious one: this board is right at the power input to each cabinet and watches for power supply phase loss and provides voltage metering outputs. Power in this case is 480v 3 phase delta and the metering circuits on this card measure the voltage between A and B, B and C, and C and A. The measurement circuit is uhhhh the CACA type. 😉

This Got Hot. The resistors’ markings bleached out from the heat!

So, here’s what you’re looking at. The three resistors are 15K ohm 3 watt and are wired in series with R27 (a PTC 0.15 amp polyfuse device) and the sensor coil inside that LEM current sensor.

The LEM LV25-P sensor is a Hall effect current sensor with galvanic isolation. It accepts 0-10mA and puts out a sense voltage proportional to the current input.

The resistors are dissipating 3.84 watts total on each phase, well within their rating. However, this rating was not assuming they’d be piled up like this and crammed on a board stuffed in an unventilated space in the cabinet!

The end result of this was that one of the voltage readings constantly jumped around and caused false alarms to fill the alarm log…. while we were trying to diagnose another issue.

Note the power reading at the top– that was the Other Issue. Ow. Signal go down the hoooooooole.

I’m entirely confused as to why such a roundabout approach was taken to this when a set of isolation amplifiers with one side being powered off a voltage divider or even a small transformer on each phase would have worked with less bill of materials cost and less heat, but uh

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