AM503 current probe DC drift? - Page 1
Forum section: electronics / testgear

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 01:40:03 am
content: How does the drift characteristic of a AM503 amplifier look like?I left one on an hour and then plugged some current probes in and zeroed it to some microvolts on a scope. After about an hour drifted 2.5mA and 3mAI think they can use a potentiometer replacement.Does anyone know what a spec is? I know their not for low currents like this but I just wanna know if there is some kinda drift spec.set to DC coupling with high bandwidth and lowest resolution on the 15A probes.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 04:22:43 am
content: 5.5 and 3.5ma offset now.I wonder if tis baking out humidity from the basement maybe. it keeps rising. I wonder if it will hit a rail eventually or stabilize.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 06:37:55 am
content: it seemed to stabilize nicely that would allow for short term measurements in the 100's of uA range eventually after a long time if you let it sit for like 4 hours.Now I am trying the P6042. This was mega drifty at startup. I think it has a lower noise floor then the AM503, but maybe more really low frequency noise.After I cleaned the probe it seemed to work better, that was dirty enough for a blackened q-tip. Will play with it for a while and see what it does.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 07:12:53 am
content: similarly, a P6042, has floated by 3.5mA (50mV/div) on the lowest setting since the last post, when zeroed with the front panel.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 08:12:07 pm
content: I am starting to think its temperature based. I left the P6042 connected ot scope overnight and when I woke up it was very near 0 where I wanted it.. and it was a flat line for a while. When I opened the door to let some fresh air in, started dancing all over the placebut before bed it was offset to 3.5mA. When I woke up it was approx 0, and after over shooting to 1mA it dropped down to -250uA. So it seems after a while it stays in the 1mA region.

author: alm
author_rank: Super Contributor
timestamp: December 29, 2023, 05:21:23 am
content: In 2010 I bought an AM503 + A6302 used, and measured the DC stability. Set to the highest sensitivity, after allowing time for warm-up, stability was in the order of 10s of mV output over several hours, which is equivalent to several mA when referenced to the input. Though I suspected this probe had been mistreated in its past, causing strain on the hall sensor. I've had to adjust internal resistors in the probe to allow it null. Before that it was off-scale regardless of the setting of the null adjustment. Attached is a plot from the only data I could find, measuring the output of the AM503 over time with a Fluke 189 set to log with the probe locked without anything in the jaws. I'm guessing the jumps were me moving the probe, but I can't find any notes or further details from that time. I think I also measured the voltage coming out of the probe and concluded it was the probe, not the amplifier, and I believe I measured another probe to be much better, but I can't find any notes to support that.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2023, 05:38:56 am
content: I wonder if baking it at like 80C would do anyhting

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2023, 07:05:47 am
content: but I think your probes are fine. Mine did this thing where once in a while it would drift alot. But that went away. It might be something as benign as moisture baking off the amplifier because it was probably not powered up for a long period of time for god knows how long.Like those big spikes, I think I had stuff like that too. I was just keeping an eye on it.I say you can measure small currents with it, if you are fast, and let it warm up, and wait 20 min after the zero pot, but I don't see a big reason not to just zero it on the scope and not use the pot. So long its within ballpark.Because each time you clip it on, you get a big offset.Also, try fiddling with the locking mechanism. I put some teflon PTFE grease on the probe head that goes into the sheath, so it runs a bit smoother. I almost feel like that big spike you see followed by stable flat line, its some kinda mechanical thing like the spring shifting or something.Actually, I am almost sure of it, you need to clean your probe head. The dirty 6042 was doing it ALOT, util I cleaned it good by scrubbing with a q-tip in IPA (anhydrous spray can). Then it was stable without the bumps.I thought it was like.. some kinda grime being displaced suddnely after it lost some.. corn starch like stiffness parameter.I found it bizzare becuase it was stable, then shot up by ALOT, then was stable again. Q-tip had grey residue on it.haha, I was so excited by these probes, then some how they went into disuse.Problby got pissed because I did not have a VHF power amp to test higher current at high frequency!What I did is put a q-tip in the hole for the wire, rotated it a bunch to compact it, then retracted the probe half way and spun the q-tip so it wipes the core, then blew it out with air.

author: alm
author_rank: Super Contributor
timestamp: December 29, 2023, 09:32:22 pm
content: I read on the TekScopes list / groups.io group that contact between the two transformer halves is very critical for low frequency response. Not so much DC offset, I'd think. But the hall sensor is a thin film that's glued to the ferrite and acts as a strain gauge, so it's plausible that mechanical damage induced stress. I doubt that it's moisture in the probe and wouldn't bake the probe given how sensitive the ferrite and Hall sensor. But maybe the amplifier is drifting due to moisture?I was only able tot test with a function generator, so I was indeed limited to low currents only except for DC.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2023, 12:41:34 am
content: well mine was doing this thing where it was stable and then I watch it shoot up like a few miliamps in a strait line then it was stable again it would do that until I cleaned the jaws. it looks kind of like your plotlike there was slow drifts and occasional big pemra offsets. the offset seemed to go away after cleaning. I also noticed if I clamp down slightly on the jaw it would do a offset.