LEM voltage transducer

[seyyah]

lem voltage transducer

I have lem lv25-p voltage transducer. But the dahasheet isn't detailed, so i didn't understand very well. What's the function of the output or what's the relationship between input and output? Does this unit require negative supply for dc voltage measurement? Related to these questions how to choose input and output resistors? Again related to these, what's my measurement band? I made a simple connection according to the datasheet but i didn't connect negative supply since i would measure dc voltage as low as 12V. But output doesn't change with the input. It stays constant. Can anybody help me please? Or are there any application notes related to this?RM=100 ohms

 

[Marcel Majoor]

voltage transducer lem

First of all, this is an 'actve' device (meaning it has some electronics inside it). For this it needs a power supply (+-12 .. +-15V). So, it does need the negative voltage, even if you only want to measure DC voltages (this has to do with the internal electronics).
Think of the device as a -current- transducer (and not as a voltage transducer). This means that you have to pass current through the primary winding of the device. The R1 resistor in your schematic converts the voltage (0-12V) into a current of about 0..12 mA. Actually there is also a series resistor (the measure resistor) of about 100-400 ohm, which is in series with the 1K resistor, which gives about 1100-1400 ohm -> which means a current of 0-8.5 mA. The transfer ratio is 2500:1000, which means that at the secondary side there can flow 0-3.4 mA.
This seconday current is then typically converted into a voltage with a resistor (Rm). With your 100 ohm this would give 0.34V. In your schematic you have this resistor connected to the negative supply. This should be the 0V, and, as stated before, you should have a +-12V supply.
I would suggest a larger Rm resistor to get a somewhat higher output voltage (e.g. 1 or 2kOhm, which would give about 3.4 or 6.8V).
The input resistor R1 (1K in your example) should be based on the measuring voltage. The primary current allowed is +- 14 mA, so base your resistor on this value (the 1K value in your example was a good value in that respect).

 

[seyyah]

lv 25p connection

Thanks for info. I measured voltage at the output without negative supply but with some problems. First of all, i changed R1 to 2.2k and made the input voltage variable from 0 to 30V. And saw that the conversion ratio is exactly 2.2. I also measured an ac voltage. As you can guess, i couldn't see the negative side. Actually there is a dc offset but the peak value of the output is correct. Without any input signal there is 600-700mV at the output. When there is signal, this portion of the signal is trimmed at the output. Anyway i'll try with negative supply. I think, the problem will get solved with it.