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I have a large power supply with multiple DC output modules. 2 of the modules are connected, positive to positive, through a 1.1 ohm resistor (really, 2 2.2 ohm resistors in parallel). When I measure the voltages without the resistor, I have the correct voltages (3.65V and 5.35V) but with the resistor I measure 5.9V on the 5.35V module, and 2.8V on the 3.65V module. I am probing across the + and common terminals of each module. I was also curious as to see if the commons were corrected, and when measured, it reads 67.5 ohms (However, inside the machine when all hooked uo, the commons become connected.) So I suppose my circuit would look a little something like this (outside of the machine)...

schematic

simulate this circuit – Schematic created using CircuitLab

My question is, what is the exact purpose of this circuit (My guess is to share the current for the loads) Also note, the resistors are only connected to the 2 positive output terminals. And how should I properly measure the voltages?enter image description here (This goes to an Ultrasound Medical Device, the computer reads the voltages correctly, but am I probing wrong? Red probe is indicating the 5.35V module and black is 3.65V. The blue line coming off the terminals is the power resistors located somewhere else.)

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    \$\begingroup\$ Where is your ground? What resistance is the load? \$\endgroup\$ – winny Jan 12 '17 at 21:51
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    \$\begingroup\$ If commons are not connected, the entire setup makes no sense. \$\endgroup\$ – Ale..chenski Jan 12 '17 at 21:52
  • \$\begingroup\$ This is where it gets a bit tricky. The commons could be connected down the line, somewhere. But this as a stand alone unit right now out of the machine, they are not. As I stated, the commons measure 67.5 ohms across them. I suppose i could try connecting them to see what happens. \$\endgroup\$ – Rayray Jan 12 '17 at 21:59
  • \$\begingroup\$ I'm going to edit this, I made a mistake \$\endgroup\$ – Rayray Jan 12 '17 at 22:01
  • \$\begingroup\$ Check schematic has no load when internal open or else show common return joined and check V5.35 (-) to V3.65 (-) and only measure R when 0V between and off. Then measure 3 branch voltages when all hooked up. (you only checked 2) \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 '17 at 23:44
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Those 2.2 Ohm resistors in parallel are current sensing resistors would be my guess. By measuring the voltage drop across them, or using a current sense amplifier to measure the magnitude and direction of the current, a software subsystem could infer a great deal about both the 5.35V and 3.65V were on and operating within specifications. If both were on and regulating perfectly, you would the current flowing through the resistors would be exactly (5.35 - 3.65) / 1.1. If the 5.35V supply was off or under-performing you'd get current flowing in the wrong direction through the sense resistors as the 3.65V tried to supply both loads. And so forth.

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  • \$\begingroup\$ I've assumed that the grounds are connected, I agree with the other commenters that it doesn't make much sense if the two supplies do not share a common ground. \$\endgroup\$ – vicatcu Jan 12 '17 at 21:55
  • \$\begingroup\$ I went back and looked at the machine. They are connected inside the machine, but the barrel connector threw me off. I do, however, have to say, that the resistor pack has no additional circuitry, its just connected to the 2 terminals. \$\endgroup\$ – Rayray Jan 12 '17 at 22:01
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schematic

simulate this circuit – Schematic created using CircuitLab

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