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I should have asked eBay first

A project log for Mains protection box

Work more safely with mains power

Yann Guidon / YGDESYann Guidon / YGDES 04/23/2019 at 11:080 Comments

The last logs address some issues, such as the "over-current protection" (OCP ?), but here I address another issue : I need output meters

As I age, I get lazier and I finally made the effort to look up on eBay. I was not disappointed. OF COURSE there are countless panel meters, LED, LCD, galva... Some are even combined : Volts and Ampères on a dual display, Watts and Watt-Hours on quad display and even frequency and power factor for the latest trending gadget.

I don't need all those bells and whistles (at least for this particular project, wink) and I'll add a combined V+A meter on the DIN rail for the convenience :

However the combined meter can't be connected at the output of the variac because it is rated for 80Vac input, when the variac can go down to 0V.

One main application/use for the "mains box" is to slowly ramp up the variac, starting from zero, while examining all the parameters of the Design Under Test, so I need to go below 80V. However the combined meters have a common wiring and steal the operating power from the current it measures.

The solution is of course to use classic, non-combined, meters, so they can be power by the isolation transformer, but can measure the voltage and current after the variac.

I found a classic module, based on the venerable ICL chip, that can measure down to 0Vac and uses an external power supply, though many questions remain. For example, how are the supply and input isolated, how many digits are effective and why can't I find a similar device to measure current ?

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