Rigol DP832 sagging voltage - Page 1
Forum section: electronics / testgear

author: MattFL
author_rank: Contributor
timestamp: December 30, 2015, 06:25:06 pm
content: I'm trying to determine if there is a problem with my power supply, or if the voltage and current readout just aren't accurate sometimes.I've got the Rigol DP832 power supply and it has been great. But I used it to test an audio amplifier that I repaired, putting channels 1 and 2 in parallel for more current. The amplifier was amplifying a 35Hz sine wave. Even though I set the peak voltage to 14v and peak current to 3.2A, as the current demand of the amplifier approaches 3A/channel, the voltage starts sagging quite a bit. I had expected the voltage to remain constant until the peak current limit was reached. Is this expected for this power supply?I attached a screen shot, notice the voltage and current limits at the bottom (smaller numbers), and the actual readings on top:

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2015, 07:10:00 pm
content: The supply is limiting the current at any time, but the amplifier will have a non constant current, with some peaks going up to the limit or slightly above by getting some of the current from buffering caps. So you could reduce the drop by adding extra capacitance, but the supply seems to be just a little to small to avoid it.Some powersupplys show a slight drop in voltage if you get really close to the limit (e.g. a few mV at 99% of set current limit).

author: Alfons
author_rank: Regular Contributor
timestamp: December 30, 2015, 07:39:32 pm
content: In parallel mode, the device should have enough reserves. My old Digimess PN300 also has to power audio circuits no problems to keep the voltage constant, not even close to the set current limit. The Rigol really seems a bit underpowered, or the regulation does not work well.

author: Silveruser
author_rank: Regular Contributor
timestamp: December 30, 2015, 09:22:07 pm
content: I just did a quick test with a big resister as a load. With the same settings I can get 3.2 amps per channel but the voltage drops to about 7.8. Not much better.

author: dadler
author_rank: Unknown
timestamp: December 30, 2015, 09:43:30 pm
content: Hmm, at max voltage Dave seems to be able to pull max rated current (seek to 35:30)Also note that the DP832 is only rated to 3A.

author: sync
author_rank: Frequent Contributor
timestamp: December 30, 2015, 09:46:56 pm
content: The display shows "On CC" (= Constant Current mode) on both channels. Likely the current peaks of your amplifier are reaching 3.2A. Then the power supply switches to constant current mode and reduces the voltage. This happens at the top and bottom of the sine wave. It will be clipped. The power supply only shows the averaged voltage and current.

author: ConKbot
author_rank: Super Contributor
timestamp: December 30, 2015, 10:24:28 pm
content: Quote from: sync on December 30, 2015, 09:46:56 pm+1 Current limiting is fast, much faster than the display, and if you have a rapidly changing load you can get all sorts of strange results, or at least strange according to the display. I.e. constant current mode but apparently drawing less than the limit.

author: MattFL
author_rank: Contributor
timestamp: January 12, 2016, 06:19:33 pm
content: Thanks everyone for the replies. I think you guys are correct; with the 35Hz output from the amplifier I'm sure the input current is not steady, and maybe the display on the power supply just doesn't refresh fast enough to show the peaks, but the current limiting is fast enough to kick in. I don't have this same issue when working with smaller more linear loads.So far this has been a really nice power supply to work with. I'm not a fan of the circular keyboard and the fan is a tad noisy, but otherwise I've been very happy with it.