HP 34401A Stable reading 100Hz ripple, powered by 60Hz wall outlet. - Page 1
Forum section: electronics / testgear

author: earthwormjim
author_rank: Newbie
timestamp: June 06, 2014, 07:29:01 pm
content: Hello,I'm pretty new to using this multimeter, I am used to just using a handheld fluke.The 34401A that I am using is powered by 120Vac/60Hz. I'm using it to measure the current and voltage output of a constant current power supply I'm testing. The issue is if the power supply that I'm testing is being run by a 50Hz source. The output current ripple is 100Hz, rather than 120Hz. I want to get a nice stable measurement of the mean current and voltage, the ripple on the current is pretty high, 30-50%.The 34401A can't seem to give a nice stable reading unless I use slow 6 digits in the resolution. This would be OK, but slow 6 digits takes forever to stabilize, it really slows my testing down when I have to test multiple load conditions, multiple input conditions and multiple power supplies. I don't need this precision, I just need quick, stable readings.I'm guessing the 34401A is using the line frequency for its syncing/triggering, and 100Hz is just an odd multiple of 60Hz. Is there any way to get the 34401A to sync up properly with what it is measuring? Do I have to use an external trigger or is there a setting I'm just missing? Seems rather odd that an internal, constant clock isn't used instead.

author: robrenz
author_rank: Super Contributor
timestamp: June 06, 2014, 07:48:29 pm
content: IMO the 50Hz supply to what you are measuring is irrelevant. You are seeing the real variation in voltage plus noise at 6.5 digits. You should be able to select the number of power line cycles that the meter uses to integrate to reduce noise but it will obviously take longer.

author: earthwormjim
author_rank: Newbie
timestamp: June 06, 2014, 08:30:45 pm
content: Quote from: robrenz on June 06, 2014, 07:48:29 pmThere's ripple on the current, but it should be able to give a stable reading of the mean value. It does so when the power supply is driven by a 60Hz source (120Hz ripple). The output is not moving around, it's a constant 100Hz or 120Hz ripple, with no variation between cycles. I can see it on an o-scope.The variation in the readings isn't trivial either. On a 500mA driver, the readings bounce around between 495mA and 505mA

author: IanB
author_rank: Super Contributor
timestamp: June 06, 2014, 09:15:56 pm
content: What you are seeing is the normal mathematics of signal sampling.If you have ripple on the voltage you are measuring it is an AC+DC signal, and to measure that reliably you should use an RMS measurement mode that can integrate over the AC part of the signal.If you have the meter in a DC measurement mode it is expecting a steady DC voltage. You won't get good measurements if there is an AC component to the voltage.

author: earthwormjim
author_rank: Newbie
timestamp: June 06, 2014, 09:43:41 pm
content: Quote from: IanB on June 06, 2014, 09:15:56 pmI just don't understand why it reads correctly when the power supply and the multimeter have the same frequency AC source.

author: jpb
author_rank: Super Contributor
timestamp: June 06, 2014, 10:15:37 pm
content: Quote from: earthwormjim on June 06, 2014, 09:43:41 pmA lot of DMMs sync their measurements to the power line cycles (if you look at the information on later Agilent DMMs they state this). So if the power line and what you're measuring are the same frequency and you're doing a slow measurement of say one sample per power line cycle then you might be sampling the ripple at the same point in the cycle each time so get a steady reading. It won't be a correct reading of course as you won't be measuring the average - for that you'll need to do fast measurements taking samples many times over a cycle.

author: earthwormjim
author_rank: Newbie
timestamp: June 06, 2014, 10:32:08 pm
content: Quote from: jpb on June 06, 2014, 10:15:37 pmOk, I guess that is what the meter is doing. I had searched the literature on this DMM, couldn't find anything about line syncing, but if its in their later meters, they probably were doing it on their older ones.Thanks for the replies, I'll have to see about using the sync output on my power source.