Help with Lascar voltmeter

In my box of bits, I've got a Lascar DPM400 (http://www.lascarelectronics.com/pdf-usb-datalogging/data-logger0602353001277710769.pdf) voltmeter. This looks ideal for attaching to my leisure battery box that will be running my electric fence next year (starting tomorrow!)

However, I'm not sure how to connect it. I think I need to follow the supply voltage example in the datasheet but do I use the battery positive for the V+ and the negative for the V-, or should the negative be connected to COM? If not, is COM just left floating?

http://www.lascarelectronics.com/pdf-usb-datalogging/data-logger0602353001277710769.pdf

This device has multiple configurations. What do you actually want to measure? Is it the voltage of the supply being used?

Please give a full explanation of how you want to use it.

Weedpharma

However, I'm not sure how to connect it. I think I need to follow the supply voltage example in the datasheet but do I use the battery positive for the V+ and the negative for the V-, or should the negative be connected to COM? If not, is COM just left floating?

It says COM is connected to the negative input:

  1. REFLO Negative input for reference voltage (connected via link 3 toCOM).

I'd recommend trying the configuration that says:

Measuring a floating voltage source Split supply operation. of 200mV full scale.

And, make sure to use the recommended scaling resistors.

Read the datasheet CAREFULLY, and probably several times!

I have read the datasheet several times but don't fully understand it - hence posting this :roll_eyes:

The circuit will be built into a battery box and, powered on with a button, will measure the battery voltage to see if it needs recharging or not. I don't think I will be using the REFHI and REFLO pins (which aren't the inputs - they're reference voltages)

Guess I'll just play & compare to my multimeter

The input is a max of 200mV so make sure you use a voltage divider for the input. The data suggests 1M and 10k to measure 20v full scale for floating voltage input. (Measuring voltage that has no connection to the meter power supply.)

If you want to measure the voltage on the battery you are using to supply the power to the meter, use the third circuit (Measuring a supply voltage).

Weedpharma

Hi,
If you are using a 12V battery then the this should suffice.
The problem with this series of displays is that their power supply needs to be either isolated from the circuit being measured, or in this case the input pins biased with respect to the circuit being measured.

digitalVolt.jpg

Tom… :slight_smile:

Thanks, 3rd scenario it is. Off to play now. Happy New Year!

Well, had a play and not a lot of success, although some of that might be down to my cheap breadboard. I think I'll solder a more permanent setup. Instead of using a single 510K resistor, I'll be using a 470K and 39K in series - that's within the 5% tolerance of the resistors anyway (gold band)

In what way is it not working. If you have 12 volts on the input the voltmeter should read someplace between 533 to 618, using the 5% resistors shown. What values were you seeing? Dwight