Programmable Electronic Load - Analyse the Summing Node Zero Point

I get excited about small things but the image below is the result of controlling our electronic load and a power supply in a LabVIEW script.

Two times the PSU is set to a certain voltage to mimic load current, then one of the load's ADCs sampled to validate that.

First time the output is set to 0 V and the load should report 0 amps. The second time to 0.02V and the load should report 0.4 A.

There are several reasons why the returned values aren't spot on, but that's not the purpose of the automation. The automation is there to reveal the errors and give a repeatable test case.

This is how it looks like in the LabVIEW run-time.

I still have to learn how I can make the 'set PSU then sample" block reusable and how to make good loops. Still it's satisfying to know that all tools are up to the game and that it's just skills that are lacking.

A day of studying and the flow looks significantly cleaner. No loop yet.

The upper flow is the load, the lower one the PSU.

The blocks in the load flow are custom made: Programmable Electronic Load - Write a LabVIEW Library part 1: Initialise Block

I'm preparing the driver board for automated measurements.

I'm going to measure the actual voltage that arrives at the current sense (A). I can't rely on the precision of the voltage that's delivered by my power supply.

The second masurement is after the opamp with gain of -7.8 (B).

The last sample is the voltage that goes to the ADC A, after the opamp with -1 gain (C).

I hope that by having a fixed measurement setup I can avoid the impact from me holding the PCB while measuring and influence of bad probe contacts.

It'll also allow for long time measurements.

If the input voltage is a bench PSU (sorry, I'm a bit slow to catch on at times), aren't you just measuring its noise.

You've got a few millivolts of fluctuation in the readings at the output. Divide that by the gain of eight and that equates to something like half a millivolt at the input. You'd be lucky if your PSU was cleaner than that (particularly if it has a lot of automation stuff inside it).

Anyway, do the test with lots of readings. It will be interesting to see if you get a scatter plot with a wide, fuzzy band stretching across it.

One way to quickly test if you are measuring the PSU noise would be to put a larger resistor in series with the current sense resistor, scale up the voltages appropriately, and then see if the variation drops by the attenuation factor.

jc2048  wrote:

 

If the input voltage is a bench PSU (sorry, I'm a bit slow to catch on at times), aren't you just measuring its noise.

 

You've got a few millivolts of fluctuation in the readings at the output. Divide that by the gain of eight and that equates to something like half a millivolt at the input. You'd be lucky if your PSU was cleaner than that (particularly if it has a lot of automation stuff inside it).

 

Anyway, do the test with lots of readings. It will be interesting to see if you get a scatter plot with a wide, fuzzy band stretching across it.

Yes, definitely. I don't have the sense resistor or other power-side components yet.

What I'm trying now is setting up repeatable measurements so that I have the learning curve behind me when the components arrive.

I'm still learning how to loop and log in LabVIEW. Once I've mastered that, I'll try to do this exercise.

One way to quickly test if you are measuring the PSU noise would be to put a larger resistor in series with the current sense resistor, scale up the voltages appropriately, and then see if the variation drops by the attenuation factor.

I can't do that with a precision component - it's ordered but no info yet when my components arrive - but I have a few low-resistance-reasonable-tolerance through-holes components that I can use for this exercise.

I have loops and logging working:

The program continuously steps through a given number of voltages.

The voltage step increment and the number of steps are given as parameters. When the number of steps is reached, the flow starts over from 0V.

In this example the PSU is set to 0 V, 0.02 V, 0.04 V, 0.06 V and 0.08 V. It loops until you push the STOP button or when a VISA error occurs.

Ah, simple data logging works too now. Just for one signal at the moment: the current measured by our load.

The result of two cycles logged to an excel file.

The first column is the PSU output value, as set by the flow.

The second column is the current that the load reports.

This is how a test cycle of 34 iterations looks like, filtered for a set voltage of 0.04 V:

Look at the temperature behaviour of the setup (power supply + eload):

I started sampling last night: 2074 * 6 measurements. Turned off the heat in the lab shortly after - that's when the slow ramp starts.

This morning I opened the lab window and it's rather cold outside. You see a steep change at the end of the sampling.

But which is drifting though

But which is drifting though