Calibration: Unable to adjust current

[0xPIT]

While in calibration mode the voltage can be adjusted, the current can not.
Rotating the encoder has no effect.

[Arachnid]

Can you try a factory reset and see if the issue persists?

[0xPIT]

Thanks, a factory reset did the trick.

However, after successful calibration, the current is still totally off.
I've set the R:P to 500mA, but it draws only 276mA, the PSU is not in I-Limit.

[Arachnid]

I suspect this is an issue with the opamp offset calibration, which is the remaining problematic part of the calibration procedure. If you open a serial console to the R:LP at 115200 baud, try 'cal O' and see what value it reports. Then try adjusting it with different (slightly higher or lower) values by specifying 'cal O num' to see if it resolves the issue.

We're still working on this one remaining calibration issue, which is tough because the unit I have to develop on doesn't exhibit the issue. :)

[0xPIT]

Hm, looking at it again, it seems that the actual current measurement is quite OK (836mA R:Pro vs. 840mA on my HP 34401A) but the set current is 1.00A.
Is this related to the calibration issue?

I'll try the serial calibration when i have more time on the weekend.

BTW, thanks for your quick replies

[bluepup]

Same for me for small currents.
From set 0 to 280mA there is no current flowing at all.
At 300mA set it only shows 54.2mA where ma multimeter shows 62.6 (at 9.9V)
At 400mA set it shows 194mA where my multimeter shows 202mA (at 9.68V due to burden voltage? not measured real voltage at device port)
600mA set shows 479mA and mm shows 488mA
700mA set shows 606mA and mm shows 610mA
at 1.2 A all seems to be the same ore lose enough.

edit: 2nd 300 was 400

[Arachnid]

Yup, the issue solely affects the opamp trim calibration, which results in these sort of large offset errors. Measurement of voltage and current is generally fine and calibrates well.

I'm working on improving it, but I'm still waiting to get my hands on a problematic unit. In the meantime, manually setting 'cal O' on the serial console is the easiest way to resolve the issue.

[nealzip]

I noticed something interesting. I did the Cal and Display did not agree with my Rigol Meter and Power Supply but the Meter and PS did agree. Sorry I did not capture how much it was off. However after the Re:Load warmed up It appears to match very closely to the Meter and Power Supply ~.01 amp. Heat sink temp at 107.0F.

[0xPIT]

Could this also be a linearity issue?
The divergence between set and measured current gets smaller as I increase current, it's OK from 1.4A until 2.2A, when it starts to diverge again.

[Arachnid]

@0xPIT What you're describing sounds like a gain error. It's linear, but one of the two has a gain that's slightly incorrect.

[spapadim]

Just got it yesterday (thanks!), updated to 1.7 and did calibration (at 10V with PSU, leads were about 0.2Ohm, or ~0.6V total voltage drop @~2A).

At smaller setpoints, the actual current draw is quite off, and at higher setpoints it fluctuates.

If it helps, this is what I observe:
https://docs.google.com/spreadsheets/d/1qxPyTRGG4gnoDKzO7djDICAdJrdo5naOwW_tRr00W1s/edit?usp=sharing
This is with the ReLoad warmed up (draw is a few milliamps higher when cold). Also, FWIW, there were instances I observed a non-monotonic change in actual draw (i.e., increasing the setpoint would make actual draw go up, then down, then up again), but I did not observe this behavior in this test.

Edit: Calibration was with ReLoad relatively cold, now that I think of it...

Hope this info is relevant and maybe helps -- thanks!

[Arachnid]

Hi Spiros,

Thanks for the feedback. There's still a couple of calibration issues we're
ironing out. The most notable relates to the opamp offset trim, and is what
is causing the 0 observed current issue you're seeing.

If you connect to the R:LP via serial at 115200 baud, entering 'cal O'
(capital o) will give the current trim value, and 'cal O ' allows you
to set it to any value between 0 and 63. Tuning that upwards somewhat
should allow you to cancel out the offset error.

The oscillations you observed - were they at low frequency, or high
frequency?

-Nick

On Wed Oct 08 2014 at 5:08:25 PM Spiros Papadimitriou <
notifications@github.com> wrote:

Just got it yesterday (thanks!), updated to 1.7 and did calibration (at
10V with PSU, leads were about 0.2Ohm, or ~0.6V total voltage drop @~2A).

At smaller setpoints, the actual current draw is quite off, and at higher
setpoints it fluctuates.

If it helps, this is what I observe:

https://docs.google.com/spreadsheets/d/1qxPyTRGG4gnoDKzO7djDICAdJrdo5naOwW_tRr00W1s/edit?usp=sharing
This is with the ReLoad warmed up (draw is a few milliamps higher when
cold). Also, FWIW, there were instances I observed a non-monotonic change
in actual draw (i.e., increasing the setpoint would make actual draw go up,
then down, then up again), but I did not observe this behavior in this test.

Hope this info is relevant and maybe helps -- thanks!

—
Reply to this email directly or view it on GitHub
#28 (comment)
.

[spapadim]

Thanks, I'll try.

Low frequency -- wasn't exactly scientific on this, just eyeballing display on PSU and noting minimum and maximum observed. The ReLoad display also jumps around, but at a smaller amplitude (I'd say single-digit mA range).

FWIW -- I only have a toy scope with me now (so take these with some grain of salt), just a quick-and-dirty set of screencaps:
https://drive.google.com/folderview?id=0Bzh3Mupk84uIdkFlbk9jd0xpeW8&usp=sharing
Channel 1 is @reload terminals, Channel 2 is @psu terminals. Setup was big mess of wires, so a bit of noise, and this is just over the 0.2+0.2Ohm leads.

These seem to suggest ~40ms period, but that's weird... can the PSU display capture that? If I can find my power resistors, I can repeat with those in series, to see if there is any longer periods (the tiny voltage drop makes it very hard to get a good trigger...).

[spapadim]

Found power resistors, added screens with a 10.1Ohm resistor in series (PSU voltage set to 10V always). There also seems to be a component with ~0.2sec period (better visible in the first capture). Not sure if that tells you anything (I'm by no means an expert, quite the contrary :). Thanks!

[Arachnid]

This appears to be a bit of oscillation induced by the software feedback loop intended to keep set and actual current in agreement.

I'm closing this issue since the original issue no longer applies, but I'll open a new issue for the oscillation.