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I have a DC/DC converter outputting 5.18 VDC @ 25% load (1 Ohm resistor). If I measure the voltage at the green nodes it fails out of tolerance, if I measure the voltage at the red nodes (before the line resistance) it passes tolerance. (See Figure 1)

Figure 1

The tolerance is 5.18 VDC +/- 0.155 VDC.

There is a line resistance of 0.1 to 0.5 ohms on each output, the wiring is pretty long in this case. I understand there is a voltage drop proportional to current flowing, at this case there is about 1.75 A flowing at 25% load.

1.75A * 0.5 Ohms = 0.875 VDC Voltage drop for each output wire. I get why that is failing.

However when I am measuring the voltage output at the red nodes, it is in tolerance, but the same line resistance 0.875 VDC voltage drops are still connected to the load resistor because the current is still flowing through them to get to the load (in parallel with the DMM). So why does this pass? Wouldn't the line resistance add to the load resistance in either case?

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  • \$\begingroup\$ you already have drawn your wires as resistors - that's how they act - as resistors. Make sure you use thicker wire and good low resistance connections to lower the wire resistance. \$\endgroup\$ – miceuz Jan 2 '14 at 22:18
  • \$\begingroup\$ With 5.18VDC +/- 0.155VDC you're asking a 3% tolerance on your main power supply (I assume), that's quite low. So, if the final system won't have the long wire, you should try to have the best similar setup, or if you can't, you can make some calculation on the voltage drop to explain that the test will pass in real condition (but for a 3% precision that's gross). \$\endgroup\$ – zeqL Jan 2 '14 at 23:34
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The voltage drop is in the wires - if you measure at the red nodes, you are measuring before the wires, so you won't see the voltage drops.

At the red nodes, you are measuring the battery voltage directly - the voltage you measure there won't depend on voltage drops elsewhere in the circuit. (for the pedantic, the voltage at the red nodes may vary with load, depending on the characteristics of the DC-DC converter.)

If the DC-DC converter permits remote voltage sensing, you can solve the problem by making the converter sense the voltage at the load, rather than at its output. This would require another pair of wires between the load and DC-DC converter.

An easier solution may be to move the DC-DC converter close to the load.

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    \$\begingroup\$ @zacharoni16: It's not being ignored. At the red points, the wire resistance is part of the load. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 2 '14 at 22:28
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    \$\begingroup\$ @zacharoni16: It's only regulated at the red nodes. The voltage at the green nodes will vary based on wire resistance and current. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 2 '14 at 22:34
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    \$\begingroup\$ @zacharoni16: The voltage at the red nodes will vary only due to the resistance to/of the sense element and to the regulation output. It will follow Ohm's law. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 2 '14 at 22:43
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    \$\begingroup\$ Why not measure each element in the circuit? Voltage drop from first red to first green, first green to second green, second green to second red. Maybe you will get a better feel for what is happening. \$\endgroup\$ – C. Towne Springer Jan 2 '14 at 23:09
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    \$\begingroup\$ @zacharoni16: To see how, or if, the output voltage of your DC-DC converter will vary with load, you will have to study the datasheet for that converter. The output voltage of a well regulated DC-DC converter will vary very little with load. (I probably shouldn't have added that "for the pedantic" sentence.) \$\endgroup\$ – Peter Bennett Jan 3 '14 at 0:02

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