Siglent SPD3303C Floating Outputs or Not - Page 1
Forum section: electronics / testgear

author: Abe Normal
author_rank: Newbie
timestamp: January 23, 2019, 10:22:27 pm
content: Hi All,I just bought a Siglent SPD3303C and have some questions that I hope someone can help me with.From the look of the front panel on the SPD3303C (i.e. it has a separate GND terminal) I assumed that the 3 channels were all floating. When I got the unit, I checked to confirm my assumption. But I found that the common on channel 2 was tied to GND (although channel 1 and 3 common terminals were not tied to GND). Then I tested the power supply in Serial and Parallel modes and in both cases channel 1 common was also tied to GND (when the outputs were turned on) - which is what I suspected would be the case.So this means (if I checked correctly) that channel 1 is floating only when the power supply is operating in "independent" mode, channel 2 is never floating, and channel 3 is always floating.So my questions are:Can anyone else with an SPD3303C confirm that this is standard behaviour?Was I wrong to assume that the presence of a separate GND terminal implied floating outputs? (Maybe a "newb" mistake on my part as I am just getting into electronics.)Thanks for lookin'

author: Abe Normal
author_rank: Newbie
timestamp: January 24, 2019, 05:54:55 pm
content: Did a little more digging and answered it myself.Siglent Americas web site says "Triple Output with2 Floating Outputs":https://www.siglentamerica.com/power-supplies/

author: rf-loop
author_rank: Super Contributor
timestamp: January 24, 2019, 06:15:25 pm
content: Siglent C model data sheet:3 independent controlled and isolated outputI can not test this C model at this time but X and X-E models have all 3 outputs fully floating and this I know sure with my own measurements/tests.

author: Abe Normal
author_rank: Newbie
timestamp: January 24, 2019, 09:23:03 pm
content: Thanks rf-loop. I see what you mean (I have a copy of that data sheet).Seems contradictory to the other statement I found on their web site. I'll see if I can get any clarification from Siglent but I suspect they'll say "working as designed".

author: tautech
author_rank: Super Contributor
timestamp: January 24, 2019, 09:32:48 pm
content: Quote from: Abe Normal on January 24, 2019, 09:23:03 pmIMO the channel isolation status claimed is quite clear but like rf-loop I don't have one in stock so to check 100%.Email sent to Siglent America asking for them to confirm.

author: Unknown
author_rank: Unknown
timestamp: January 24, 2019, 10:12:32 pm
content: PM sent

author: Abe Normal
author_rank: Newbie
timestamp: January 25, 2019, 08:17:07 pm
content: Thanks tautech!I was contacted by a Siglent Americas rep and they confirmed that all 3 outputs should be isolated. I'm returning the unit to Amazon for replacement and, if that doesn't work out, the Siglent rep has suggested another avenue to get a properly functioning unit.Hopefully, 2nd times a charm. I'll post again when I have the new unit in hand.Thanks again for your help tautech and rf-loop.

author: Calvin
author_rank: Regular Contributor
timestamp: January 26, 2019, 07:25:31 am
content: Hi,I checked my SPD3303C and can confirm that all three outputs are isolated from GND and from each other.regardsCalvin

author: Abe Normal
author_rank: Newbie
timestamp: February 04, 2019, 04:30:24 pm
content: Just wanted to close out this thread by reporting that the replacement unit I got from Amazon is working correctly (all 3 outputs are isolated).I also want to say that the rep from Siglent America was very helpfuland gave me confidence in the brand (despite the rough start).

author: slugrustle
author_rank: Frequent Contributor
timestamp: June 18, 2025, 01:18:22 am
content: What is the maximum voltage between the common terminals on channel 1 and channel 2? It is not clear to me from the datasheet, which says:QuoteThat makes me think the terminals of any individual channel can float 500V peak relative to line or earth, but I'm not sure how high the voltage can be between the channels. Is it the same 500V peak?I ask because I am about to test an isolated power converter and want to use one SPD3303C to supply the AUX rails on the primary and secondary sides.

author: thm_w
author_rank: Super Contributor
timestamp: June 18, 2025, 10:19:28 pm
content: Quote from: slugrustle on June 18, 2025, 01:18:22 amIts implied from that spec.If channel 1 is at 500V wrt the case, and channel 2 is earthed, then its going to be 500V from channel 1.

author: slugrustle
author_rank: Frequent Contributor
timestamp: June 19, 2025, 12:05:19 am
content: Channels 1 and 2 can be configured in series mode or parallel mode within the unit. That means there's some kind of switch (relay, MOSFET, etc.) that can connect them. I'm concerned that the switch might have a lower voltage rating than the unit's insulation.Honestly, the spec is pretty sparse and ambiguous.

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2025, 12:20:20 am
content: Quote from: slugrustle on June 19, 2025, 12:05:19 amRelays I think and you should be able to hear them switch when Series mode is selected.

author: slugrustle
author_rank: Frequent Contributor
timestamp: June 19, 2025, 12:47:18 am
content: Thanks. I just asked Siglent support via their web form what maximum voltage can be applied between the terminals of channel 1 and the terminals of channel 2. Hopefully they respond.

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2025, 01:41:11 am
content: Quote from: slugrustle on June 19, 2025, 12:47:18 amYes the spec you need is for channel isolation max V whereas the channel to mains gnd (case, PE) insulation 500V value could be quite different.Insulation value is for when multiple units are placed in seriesandin Series mode where some terminals are at elevated voltages.This mode of operation is accepted by Siglent and they sent my pics of 2 units each supplying 60V and placed in series. I've done it but not been brave enough to even attempt to find the max # of units that can be placed in series.We just have to take the max 500V insulation spec as gospel when using these PSU's in series.....

author: thm_w
author_rank: Super Contributor
timestamp: June 19, 2025, 10:00:46 pm
content: Quote from: slugrustle on June 19, 2025, 12:05:19 amAny sort of general purpose relay is rated for ~500VDC. You can see these SPD3303D teardown photos:https://frank-buss.de/spd3303d/logic-board-right-near-focus.jpgPC817 used is also rated for 5000V.Though the limitation can be their layout, if any mistakes were made there, so I wouldn't immediately trust all supplies to be capable. But it will easily handle your proposed scenario.Also a E36312A teardown here, isolation distance on the PCB from CH1 to earth is the same as CH1 to CH2:https://xdevs.com/review/e36312a/

author: slugrustle
author_rank: Frequent Contributor
timestamp: June 21, 2025, 04:01:16 am
content: I contacted Siglent support, and they replied but misunderstood my question. I have followed up. I might throw together a little boost converter and test this with a current limiting resistance, slowly ramping the voltage.EDIT: I tied CH2 negative to ground and took CH1 negative up to 110.9VAC RMS with a variac through a 10kΩ limiting resistance. Current draw was 4.15mA RMS, consistent with the ~100nF from each channel negative to earth. I measure ~50nF between channel negatives so I'm assuming the capacitance is each channel separately to earth.That's enough capacitance to trip a GFCI (assuming 6mA threshold) at 159VAC RMS 60Hz or 191VAC RMS 50Hz... I was hoping to use this supply to run an AUX rail for a power converter that's referenced to input bridge rectifier negative. Should work if I'm running from a programmable AC source (floating output) but won't work in a test at 240VAC input from the line if the lab has GFCI.