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| Voltage Divider Affected By Load |
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| free_electron:
i'm betting the ground wire is longer than the power wire ... with those currents, the drop in that wire 'lifts' the reference point of the voltage divider. hence the larger reading under load.. with these kinds of current it is very important to sense at the right spot so you don;t see the voltage drop in the cables.... |
| Johnex:
It's possible yes, but would just say 1cm or so make that drastic difference? Why would the voltmeter show the correct voltage at the exact same point too? |
| Brumby:
My first suggestion is also to establish 'star' connection points, which looks like this... All the connections are made at a single point with the conductors radiating out from that point - just like a star with rays of light shining out. |
| ArthurDent:
I think you have the answer and it is that where you sense the voltage makes a big difference. On one project I was working on we were generating a 20,000A pulse and had to measure it. The connecting 'wires' were 6 foot long heavy copper (1/2" x 3") buss bars but at those currents we could measure 1.5 volts drop or zero along the length of the bar if we changed the ground point for our instrumentation. Here is a link with the explanation and a helpful hint that a capacitor might be needed to keep the system stable and prevent oscillations. In most cases like yours the sensing should be right at the load because that is the voltage you are interested in. http://www.tmatlantic.com/encyclopedia/index.php?ELEMENT_ID=22008 |
| Brumby:
--- Quote from: ArthurDent on June 13, 2018, 03:52:05 am ---In most cases like yours the sensing should be right at the load because that is the voltage you are interested in. --- End quote --- Oops, yes. Move the star points from my diagram to the right, next to the load. :-[ |
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