Having voltage proportionally

Hi all,

I am an electrical novice and have a question that I have been unable to solve on my own. I need to measure the voltage of a drive system with a logger.

The voltage is nominally 48VDC and the drive systems is rated at 1500W (~30A).

My datataker DT80 can only measure up to 30VDC. Can I halve the 48VDC proportionally, so that I can measure with my logger?

If so, how? Much appreciated for assistance. Richard

Can you use a 10 to 1 scope probe to measure?  Do you have a bnc input? 

Resistor divider.

Normally you need to think about how much current the source can spare, how much the meter/logger needs.

With 30A on tap, you have an abundance. Two 10K resistors series across the 48V. Meter across the lower 10K, it should be half-voltage. 24V should read well on a 30V scale.

Yes, a 10:1 probe would work, and be less mental math, IF this datataker had an input just like the 'scope the probe was designed for. Something like 1Meg||30pFd input. As a bonus, the probe would take very little current from the source. But with 30A, that is not necessary.

Probe, $3-$300. Two resistors, 25 cents.

biosystems said:

Hi all,

I am an electrical novice and have a question that I have been unable to solve on my own. I need to measure the voltage of a drive system with a logger.

The voltage is nominally 48VDC and the drive systems is rated at 1500W (~30A).

My datataker DT80 can only measure up to 30VDC. Can I halve the 48VDC proportionally, so that I can measure with my logger?

If so, how? Much appreciated for assistance. Richard

Click to expand...

Hmm, this might sound simple but it might not be.

A very simple solution would be to use a simple voltage divider. For example, two 10K resistors would make 24VDC and carry ~2.4mA * 48V is 115mW so common quarter watt resistors would be fine. To minimize noise, the resistor divider should be at the logger end so that the high(er) impedance created by the divider (10K||10K = 5K) is as short as possible so as not to pickup noise. So you could just make a cable that connects to the drive unit and then at the logger end just solder your resistors directly to the connector and cable and cover with shrink tube.

However, before you connect the cable you should check for a difference in DC between the cable ground and the datalogger ground by temporarily connecting the grounds together with a resistor like say 1K and measure to see if there is any voltage across it. You might even put a scope on it to see what sort of ground noise you have. If it's a few mV or maybe 100mV max, that is ok (100uA). Otherwise, if you have a difference in DC between grounds, you could have a significant current running between the drive unit and datalogger and that would not be good for a number of reasons.

Another potential issue is if the hot lead shorts you could cause sparks or a fire or damage something. A fuse at the drive end might be a simple work-around for that.

The proper way to do this sort of thing for a precision permanent installation would be to use a differential input. Any datalogger worth it's weight in bytes should probably have a differential input in which case you should just use that because it solves basically all of the above mentioned problems and more. But the circuit would be a little different.

puff page
399 page(!) manual

There seem to be several models with various in-counts.

The basic input is 30mV scaled to 300mV or 3V, plus a 30V. This *appears* to be 10Megs. Inputs are multi-pin, so differential is possible. Inputs are relay switched, so cross-shorts are less likely.

I wish you had somebody On The Spot who could work with you. That box is probably too expensive to want to blow-up by ignorant accident.