Electronic – Symetric power supply from two isolated supplies

power supply

I'm trying to power a voltage transducer such as this one LEM CV3-1000

It needs a symetric +-15V supply.
I can't seem to find a +-15V supply on a DIN rail (for a reasonable price) and thought I'd do one out of two 15V supplies such as this one : MeanWell 15V

Now, I found this thread which seems to confirm that it is not a problem at all.

I'm just wondering, would a 30V power supply fit the purpose? There'd be no common ground though but would that be OK anyway? The output would be completely floating in that case, am I right?

Best Answer

The direct link to the datasheet is LEM CV3-1000.

enter image description here

Figure 1. From the datasheet.

Let's figure out the worst case current consumption:

  • If \$ R_L \$ is 1 kΩ (the minimum allowed) and \$ V_{PN} \$ is +700 V then the current consumption will be \$ I_C = 32 + \frac {700}{1000} = 32 + 0.7 = 32.7\ \mathrm{mA} \$. (At -700 V it will be 32 - 0.7 mA.)
  • The voltage out will be given by the 1000:10 conversion ration and so will be +7 to -7 V.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. (a) What you are proposing. (b) What is required.

  • In 2(a) there is no return path for R1 so the circuit can't work.
  • In 2(b) there is a return path.

schematic

simulate this circuit

*Figure 2. A work-around."

If you are stuck the circuit of Figure 2 should work. Assuming that the device will draw the same current on the positive and negative supplies there is probably little current on the 0 V pin other than the return current from R1. Set R3 to give 20 mA through D1 and you should be OK. Confirm that the voltage between 0 and -15 V is stable at both +700 and -700 V on the input.

Pay attention to the 5% tolerance on UC. A dual power supply would be a better option. Single to dual-voltage converters are also available.

Related Topic