Electronic – Where does the 1V voltage come from

circuit analysiscircuit-designnpnvoltagevoltage measurement

I have a four PLC with many digital I/O connected similarly to the schematic below. If I use multi meter and measure the voltage between the two points in the schematic, why is it showing 1V? Isn't it supposed to be 0V? The relay coil is not connected. The measurement point is between psu 24V+ and relay's NO. The measurement device is Kyoritsu DMM type KEW 2200R

Schematic

PLC's schematics.

PLC

Best Answer

What model number Hioki meter, which DC volts measurement range are you using, and do you have any special measurement modes enabled on the DMM like "high input impedance"?

DC voltmeters that have very high input impedance (>= 100 MΩ) can and do measure residual/static/floating potentials left on metal bodies as if those bodies were charged plates in a capacitor.

For example: When measuring DC volts, the DT428X handheld meters have an input impedance of 1 GΩ on the 60 mV and 600 mV ranges, and the 323X HiTESTER benchtop DMMs have an input impedance of "greater than 100 MΩ" on the 200 and 2000 mV ranges.

:: EDIT 1 ::

I don't think the KEW 2200R meter is causing this. Its input impedance is 11 MΩ for the DC Volts measurement function, which likely won't indicate a persistent 1 V reading if it's connected to a small metal mass with residual charge on it.

If it's easily done, remove the relay so that you can perform out-of-circuit tests on it. With no power connected to the relay's coil, perform three continuity tests (or ohmmeter measurements) across the switch's three terminals, i.e., test the

  • input-output path for the normally-closed (NC) signal path (there should be continuity (less than one ohm)
  • input-output path for the normally-open (NO) signal path (there should not be continuity (infinite resistance))
  • NC output pin to NO output pin (there should not be continuity (infinite resistance))

:: EDIT 2 ::

With everything connected normally but powered OFF, try a resistance measurement from the relay's NO contact to 24V- and 24V+. It should be infinite resistance. If the resistance is infinite, try the following:

  1. Configure the DMM to measure DC volts.

  2. Using jumper wires, connect a 10 kΩ 1/4 W (or higher wattage) resistor between the relay's NO terminal and ground (24V-).

  3. With the system turned OFF, connect the DMM across the 10 kΩ resistor to measure the voltage drop across the resistor. Nominally, no current should be flowing through the resistor; therefore, the measured voltage drop should be approximately zero.

  4. Apply power to the system.

  5. Observe the indicated voltage across the resistor. It should be approximately zero volts.

  6. Turn off the power to the system.

  7. Using jumper wires, reconnect the 10 kΩ resistor between the relay's NO terminal and 24V+.

  8. Repeat steps 3 – 7.

In step 8 if a voltage drop is measured across the resistor with the system powered ON, there's continuity somewhere between the relay's NO pin and ground. It could be the PLC software is actuating the relay (COM->NO), or the relay's socket is damaged, or a bug got under the relay socket and died and is now a resistance between the relay's NO pin and ground, or ??? (other).

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