# Symetric power supply from two isolated supplies?

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?

## 2 Answers

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

No, a 30V supply would not give you a reference for the output of the transducer, unless you created an artificial ground with extra circuitry. That could introduce errors in your measurement though.

You're better off using the two 15V supplies to make a true bipolar +/-15V supply with ground for the transducer.