Agilent 6811B AC Source/Analyzer (CC performance and general maintenance advise) - Page 1
Forum section: electronics / testgear

author: SebastianH
author_rank: Regular Contributor
timestamp: March 01, 2023, 08:07:10 pm
content: Hello,not too long ago I got an Agilent 6811B AC source/analyzer that seems to work fine. However, as usual I do some performance checks and I'm not so sure about the constant current mode. I didn't expect it to be nearly as fast as a DC supply CC mode, but at least for low power situations (first test: 60W bulb) it's sooo slow that I'm wondering whether I should have a closer look at this or whether it's completely normal. When the current setpoint is only (let's say) 5% below the measured current, it takes like 30 seconds until the source finally recognizes that the current is above the limit and the CC annunciator lights up. With larger differences it's significantly quicker. The control loop is slow too, but at least seems to settle after a similar time span, even if the calibration seems to be slightly off. Also there is some overshoot when the source goes into CV again (haven't properly measured it yet, <<1s, but the "flicker" made it quite noticable). Either I missed something or the manual simply doesn't state what dynamic performance I should expect, except some minor hints at a more slow-ish performance (also impedance-dependent).I'm curious to hear, whether someone has similar experiences with the source.Also, to learn from your experience: Is there anything except for usual suspects like electrolytics and the NVRAM that I should have a look at as a preventive measure?Thanks,Sebastian

author: TheSteve
author_rank: Unknown
timestamp: March 01, 2023, 08:45:24 pm
content: I don't know what the exact timing should be. I know when I've set my 6812B limit for close the current drain I was expecting it will instantly limit because of inrush of the connected device. I'd expect if it was super quick you'd never get your light bulb to light or you'd see it limiting during power on.When I repaired mine I did not change any capacitors but did replace the Dallas NVRAM with a FRAM device.

author: SebastianH
author_rank: Regular Contributor
timestamp: March 01, 2023, 09:23:55 pm
content: So did you just expect it to limit or did it actually limit instantaneously? As far as I've seen there is a separate inrush limit that can be set to a lower than default value (max. 40A in case of the 6811 and 6812B I think). I'm more concerned with the actual current limit/CC mode (effectively the CURR:IMM and CURR:TRIG values).Yes, I'll have to address the NVRAM issue at some point

author: danils
author_rank: Contributor
timestamp: May 31, 2023, 10:27:34 pm
content: Hello there!I was about to start a new thread about but, searching, I found this.I recently acquired a 6812B that exhibits the same behavior detailed by OP when CC mode engages.Testing for calibration, set and readback values are within specs (as detailed in the service manual)The transition between CV and CC mode takes a lot of time.Aside of this I experience the following:I connected a 230V 150W incandescent light bulb to output. Current measured is 0.676AIf I set CURR:LEV 0.6 the units stays in CV mode for several seconds. It turns to CC and, as per mode, voltage starts decreasing but very, very slowly. Voltage and current continue to decrease below current set point and after about one hour it stabilises at 0.584 (16mA below set point)If I connect a 230V 25W incandescent light bulb to output, current measured is 0.105A.If I set CURR:LEV 0.07 the voltage starts decreasing after turning into CC mode. Voltage and current continue to decrease below current set point and both goes to zero. At this point Unr annunciator goes on and after few second the output is disengaged with PROT annunciator on.The manual is quite vague about that, also in the user manual a testing procedure is detailed and it looks to disagree with the instrument behavior (at least mine and the OP one)makes me think that if i set 0.6A the voltage should decrease in order to keep the current drawn by the load stuck at 0.6A but it does not happen.At page 34, it writesAnd it makes me think it should be (almost) immediate and held to the setpoint.Two units exhibiting the same behavior makes me think it's not a fault but the intended operation.Could anybody explain me why this is the behavior? Also I do not understand, as per manual page 18, how a load could exhibit negative resistance i.e. a light bulb.@TheSteve could you be so kind to reproduce my experience on your instrument (and maybe post a video?)

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 22, 2023, 07:57:35 pm
content: Would be interesting to see the response using a normal resistor and at higher currents. An incandescent light bulb is a non linear load and has response delay (due to thermal inertia). Maybe the AC source detect the presence of a "delayed" load and perform the regulation slowly to avoid oscillations.Could the delay caused by the lightbulb heating/cooling up cause the AC source to momentary sense a negative resistance?

author: Gulftown
author_rank: Regular Contributor
timestamp: August 13, 2025, 02:36:01 pm
content: Hi,i recently got a 6811A and mine has the same behaviour.I tried it with a 20 Ohm high power resistor.But, when i use the peak current feature, the current gets clipped at the right point (checked with a current probe) until the CC regulation loop reduces the voltage enough, so that its a sine wave again.So i don't actually know its a bug or a feature.But the CC loop seems so slow, that when you disable the output and enable it again, the control keeps the current in check.The control need longer to reduce the voltage, the longer the output is disabled, like a pid loop without windup protection,

author: bateau020
author_rank: Frequent Contributor
timestamp: August 14, 2025, 08:18:08 pm
content: Not sure if it has the same cause, but the HP 66332A/6632B/6633B/6634B DC PSU series of the same era, has the same problem. There they even tell you to start the PSU in CC mode if you want CC mode. If you are close to the CC limit, it can take quite a long time before CC kicks in.Might be nice to upgrade the CPU board for higher ADC/DAC resolutions and better CC/CV switching, all while keeping the excellent analog section.