Korad KP3305P channel 3 weirdness -- voltage drop under load. Transformer? - Page 1
Forum section: electronics / testgear

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 02:55:52 pm
content: The 3305 has 3 channels: 2 variable, programmable, switchable, etc. etc. No problems there.Channel 3 is supposed to deliver a fixed 5VDC up to 3A when the unit is powered on. Not programmable, not switchable, just always on. And when I got this thing I put a meter to it and found 5VDC, give or take a couple of millivolts. Then I kind of forgot about it and never had a use for it.Until a few months ago, when I tried to power an Arduino with it and found the voltage sagged so severely the Arduino Nano wouldn't boot. Voltage was well under 3v trying to power a device that draws less than 200mA. My other (home built) supply was on the bench and it worked fine. But it bugged me and I finally got around to investigating.Digging around, I found all the ch3 circuitry appears to be contained within the rear main board. The only connection to the front control board is the two output jacks:The transformer has a separate set of 12VAC windings dedicated supply the Ch3 and connect to J7 on the left side. Voltage regulation seems to involve Q9 in the centre. That's a 3 terminal device in a TO-220 case with no heat-sink, but has been sanded so I can only guess what it actually is. There's 3 of them in total, the other two being associated with Ch 1 and Ch 2, respectively. Finally the output is routed through the two right-most pins of J2 on the right to the front panel jacks.Probing the unplugged AC supply to J7 shows some asymmetry on one lead, although summing the two seems to indicate a sine(ish) wave between them:I backprobed the AC supply @ J7 and monitored the output voltage at various constant currents as applied by a Kunkin KP184 electronic load. AC supply quickly drops so that by the time current hits a few hundred mA, transformer supply is down to 11.6VAC while output voltage drops to 2.6VDC. Those numbers are pretty constant from 500mA to 3A. Looking at the board with a Thermal Imaging camera, nothing seems to be getting unusually hot. Backprobing J7 with a scope though shows some strangeness.5VDC circuit only, without any load applied:With 3A load applied:.So now I'm really starting to suspect something is wrong with the transformer. By way of confirmation I decided to try again using the J7 leads to power a separate and independent LM338 based module (https://www.amazon.ca/dp/B09KX3M5WX) set to output 5.000VDC. And I'm seeing similar (but not identical) results. AC voltage supply drops to 11.7VAC and the output of the module drops to 2.6VDC at any current from 500mA to 3A.Unloaded output, AC supply:3A output, AC supply:So what is the verdict? Am I correct in believing that this indicates a problem with the transformer, possibly a shorted winding? Is there anything I can do besides abandoning Ch.3 short of buying a new transformer? (This thing is WAY out of warranty by now!)

author: Martin72
author_rank: Super Contributor
timestamp: May 29, 2025, 03:19:00 pm
content: Hi,QuoteIf the output is supposed to deliver 5V and 3A, that would be 15W, and the part would definitely need a heat sink if it were to have anything to do with that.A 12V transformer voltage may well drop to 11.6V; this is not necessarily an indicator of a problem.Furthermore, I find the images somewhat confusing.Have you simply measured the output resistance between the two connections of the 12V secondary winding?

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 03:44:08 pm
content: 0.21 ohmsScope Ch1 (yellow trace) was connected to one of the transformer secondary leads, Ch2 (purple trace) to the other. White is the math + function using the two as input. Scope lead ground alligators were connected to Korad chassis ground.

author: tautech
author_rank: Super Contributor
timestamp: May 29, 2025, 04:00:47 pm
content: Quote from: torch on May 29, 2025, 03:44:08 pmBut you didn’t. Ch2 is set as the source for both Math channels.I wonder if that winding has a thermal fuse…….

author: Martin72
author_rank: Super Contributor
timestamp: May 29, 2025, 04:09:23 pm
content: Quote from: torch on May 29, 2025, 03:44:08 pmI wouldn't measure it against earth, as you can get results that aren't “true.”The 5V output will be isolated from ground, as will the 12V winding from the transformer.0.21 ohms sounds plausible for a secondary winding. Can you take a picture of the power supply board from a bird's eye view?In short, at the moment I don't think the transformer is defective, but rather that there is a break or a fuse that is defective.

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 06:38:22 pm
content: Quote from: tautech on May 29, 2025, 04:00:47 pmOh shoot, you're right.I'll have to take it apart and try again...

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 06:42:29 pm
content: Quote from: Martin72 on May 29, 2025, 04:09:23 pmYeah, that's why Iintendedto use the math function between the two scope channels, so I would be measuring differentially. Now if only I had actually done that...

author: Martin72
author_rank: Super Contributor
timestamp: May 29, 2025, 07:26:49 pm
content: Measure the 5V output with the scope (+ to -, not ground)....Once idle, once under load – what does it look like?

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 09:38:11 pm
content: Ok, here comes some more pictures:First, the AC from J7 using the correct math parameters.J7 unplugged:J7 plugged in, no load:J7 3A electronic load:And finally, because I think the electronic load was having a hard time applying a constant current, a 2 ohm 20 watt resistor as load:Now for the output. Single channel, positive to negative:No load (note scale changed to 50mV/div)3A electronic load (scale now at 1V/div)And finally, with the 2 ohm resistor as load:

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 09:41:55 pm
content: Requested pictures of the power board:More detail of the left side (note the Ch3 bridge mounted to the heatsink. I forgot to call that out in my first post):And the right side:

author: Martin72
author_rank: Super Contributor
timestamp: May 29, 2025, 09:51:13 pm
content: Quote from: torch on May 29, 2025, 09:38:11 pmI still don't understand why you measure the AC signal from the transformer with 2 channels against ground and then use math.Regarding the measurements at the 5V output:You have set AC coupling, I would like to see theDC signalfrom the output.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 29, 2025, 10:00:21 pm
content: Hi, there are some versions of this model (BTW I think that you are referring to KA3305P) and some of them had an adjustable resistor on back that allows to adjust that "5V" rail voltage - perhaps you made some adjustment (do not remember how much was possible but results were a bit strange if not 5V set)

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 29, 2025, 10:01:32 pm
content: Quote from: Martin72 on May 29, 2025, 09:51:13 pmyep to see what is there

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 11:13:38 pm
content: Quote from: Martin72 on May 29, 2025, 09:51:13 pmAn abundance of caution. I'm dealing with a suspected wonky transformer, I didn't want to risk damaging my scope by shorting something to ground through the scope probe ground. BTDT, have the hat, t-shirt and melted wire.Quote from: Martin72 on May 29, 2025, 09:51:13 pmOk, No-load:Now this one is quite entertaining. The Korad and the Kunkin got into some sort of race condition with the Kunkin trying desperately to compensate and maintain a constant current:A longer view:The 2 ohm resistor as load is far more prosaic:

author: torch
author_rank: Frequent Contributor
timestamp: May 29, 2025, 11:31:30 pm
content: I'm not sure how I missed this before, but this sanded 3 terminal bad boy (IGBT?) mounted to the heatsink next to the Ch3 rectifier is tagged Q6 and is definitely part of the Ch3 circuit. It has a live tab, insulated from the heatsink, which is only live when J7 is plugged in. I'm guessing it's responsible for voltage regulation.

author: Martin72
author_rank: Super Contributor
timestamp: May 29, 2025, 11:45:03 pm
content: Quote from: torch on May 29, 2025, 11:13:38 pmI see.But then I would use either a differential probe or an isolating transformer.If you measure as you are doing now, you are measuring something against the ground from which the secondary winding is decoupled.Another measure would be to simply measure the AC output with a voltmeter; this has no reference to anything.The DC measurements:That's completely off.This can only be explained by a defective regulator or the feed to the regulator.To rule out the transformer, you have to take the hard route.This means disconnecting the plug with the direct secondary outputs of the transformer from the rest, then measuring the output responsible for the 5V; a voltmeter is sufficient for this.Measure this output with a voltmeter unloaded/loaded.Unloaded, the output voltage is higher; loaded, it should be approximately equal to the nominal voltage.

author: torch
author_rank: Frequent Contributor
timestamp: May 30, 2025, 12:22:20 am
content: Quote from: Martin72 on May 29, 2025, 11:45:03 pmI don't have a differential probe. It was my understanding -- based on internet reading, so could be way off -- that using two channels was an alternative method of differential measuring.The ground leads were not really being used in a functional manner. They are already connected to mains ground at the scope, I just connected the alligator clips to the power supply chassis which is also connected to mains ground.The AC output (unloaded) as measured with a meter was a whisker under 12vac.I just tried something completely different. Literally. I disconnected J7 and fed the circuit with a 12v doorbell transformer. No-load: steady 5vdc output. 2 ohm load, not so much:I'm wondering if the bridge rectifier has a blown diode?

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 30, 2025, 01:07:04 am
content: you can always probe from rectifier's output...

author: tautech
author_rank: Super Contributor
timestamp: May 30, 2025, 04:52:56 am
content: Quote from: torch on May 30, 2025, 12:22:20 amI just tried something completely different. Literally. I disconnected J7 and fed the circuit with a 12v doorbell transformer. No-load: steady 5vdc output. 2 ohm load, not so much:I'm wondering if the bridge rectifier has a blown diode?Yep, just catching up and screenshots indicate a blown bridge IMO.Think about one diode leg open while trying to charge the smoothing cap and then apply a load, it’s gunna fall on its face.

author: torch
author_rank: Frequent Contributor
timestamp: May 30, 2025, 05:31:59 am
content: The dang bridge seems OK diode testing in circuit. But there's definitely asymmetry between the two AC pins and the - pin when powered. For fun, I jumpered a 3300uf cap to the + and - pins on the bridge. Voltage drops to 4.9VDC @ 1.6A current via the electronic load.Then either I blew out the power grid or a tree fell on the line.Estimated time of power restoration is 03:45. I'm going to bed.

author: EvgenyG
author_rank: Regular Contributor
timestamp: May 30, 2025, 05:47:30 am
content: I've got a 3303 (Well, the element14 rebrand "Tenma") and I am also quite disappointed with the dedicated 5v regulation. Looking at your photos I can see some capacitors very close to heat sinks, must be to keep them warm during winter monthsSo, just a capacitor replacement solved it?

author: Martin72
author_rank: Super Contributor
timestamp: May 30, 2025, 10:49:11 am
content: Quote from: torch on May 30, 2025, 12:22:20 amCan the “bell transformer” even supply that much power?If so, then at least you know that it can't be the transformer on the Korad.Then the fault lies in the rectifier/regulator circuit.Testing the rectifier bridge has already been mentioned here, so I assume you know how to do that.Then maybe check the electrolytic capacitors directlyafter the rectifier(andbefore the regulator), and finally the regulator itself.

author: torch
author_rank: Frequent Contributor
timestamp: May 30, 2025, 12:26:57 pm
content: Quote from: Martin72 on May 30, 2025, 10:49:11 amThe doorbell transformer has a stated rating of 4200mA @ 12VAC.Quote from: Martin72 on May 30, 2025, 10:49:11 amDangerous assumption!Powered off but still in circuit using the diode check function and allowing for the inevitable delay as connected capacitors adjust themselves to the meter's own voltage, I'm seeing a forward voltage drop of about 0.4 - 0.5 in all the right places and 3.3V when leads are reversed. Live with no load, I'm seeing 11.9VAC at the AC pins and a little over 10VDC from + to -. After connecting - to mains ground at the front panel connectors, I probed the AC and + pins using a single scope channel (no math) on DC. + shows the 120Hz half-wave similar to above pictures. One AC pin shows a somewhat normal 60Hz 1/2 wave, the other shows a severely mal-formed 60Hz wave that never drops to 0.My hypothesis is one of the diodes in that half of the bridge breaks down at a voltage higher than the meter's test voltage. It is equally possible that I'm doing this wrong; I am not now, nor will I ever be, an EE. I'm more like walking proof that a little bit of knowledge is a dangerous thing. ;-)Quote from: EvgenyG on May 30, 2025, 05:47:30 amOh heck no! Not yet, anyway. I took a loose 3300uF electrolytic and used two alligator clip jumper leads to connect it across the bridge + and -. My working assumption is that the 10,000uF cap in the top right corner is also connected across the bridge, but I haven't yet removed the power board to see the traces on the underside.What adding the capacitancediddo is vastly improve the performance. Ch3 is supposed to deliver up to 3A @ 5VDC but the voltage plunges below 3VDC with only a few hundred milliamps draw. With the added capacitance, the output voltage at the front panel only sags to 4.9VDC @ 1.6A. It starts dropping off more rapidly as current draw is increased from there, but is still above 4VDC @ 3A. Probing the output shows a 120Hz "ripple" (more of an inverted triangle really) of 500mA initially. The ripple increases (and measured voltage decreases) slowly over time, suggesting performance is degrading as something gets warmer.EDIT: added screenshot of DC output waveform with added capacitance across bridge rectifier:By this time, voltage was down to 4.2VDC and the "ripple"/dip was over 1V p-p.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 30, 2025, 02:17:37 pm
content: Quote from: torch on May 30, 2025, 12:26:57 pmI meant probing with scopeto test it properly you need to take out the board, put some screwdriver or metal part on big cap legs to make certain it is discharged but I think considering that particular parts price point it is faster to just swap it with new one + those few caps there. It may be a fix or then will remain clear situation what is not broken. They do not use particularly super quality caps there so replacing them is always a good idea

author: EvgenyG
author_rank: Regular Contributor
timestamp: May 30, 2025, 02:33:49 pm
content: Quote from: torch on May 30, 2025, 12:26:57 pmWell, adding that 3300uF cap clearly makes things better. Check that 10,000uF cap, could be just a dry joint, or dried up. I doubt they put Chemicons or anything else decent there.

author: Martin72
author_rank: Super Contributor
timestamp: May 30, 2025, 03:34:24 pm
content: Quote from: kmoonwalker on May 30, 2025, 02:17:37 pmAnd here we have instructions on how to destroy electrolytic capacitors....Always discharge via a low-resistance resistor.Depending on the state of charge, currents of several amps may flow briefly during “hard discharge.” Electrolytic capacitors don't like this, and you don't know the current state of charge.So always use a resistor.And then, of course, don't touch it directly with your hands, but use test leads.

author: torch
author_rank: Frequent Contributor
timestamp: May 30, 2025, 03:41:56 pm
content: Quote from: EvgenyG on May 30, 2025, 02:33:49 pmYou question the longevity of components within this fine example of building down to a price point? Blasphemy! I'm certain they used nothing but the highest quality Chinesium in the manufacture of those caps.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 30, 2025, 04:55:34 pm
content: Quote from: torch on May 30, 2025, 03:41:56 pmOr counterfeit Chinesium

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 02:55:52 am
content: I may have found the problem. The Chinesium leaked out and now the cap needs to be refilled:My $20 LCR ESR Transistor checker (I can't believe that thread is still going!) reports "No, Unknown or Damaged Part".The bridge rectifier, however, appears unscathed. Using my other power supply the forward voltage drops are all less than 1VDC and reversed shows no current flow at 35VDC (the limits of that supply). BTW: it's a KBU 610. The part number was not removed, just coated in thermal paste and stuck to the heat sink.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 31, 2025, 03:31:14 am
content: Quote from: torch on May 31, 2025, 02:55:52 am...replaced...Considering how those "legs" are looking no wonder there is no contact hence no diagnosis from LCRKBU and GBU are so typical and widely used that it would take a really "special" individual to scratch marking from them (but I seen so) ^^

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 03:45:43 am
content: Just for fun I connected the bridge to the doorbell transformer.Ch1 is rectified output, 2 and 3 are the AC pins.Bridge only:With 3300uF cap across output:With 2 ohm load added to the above:Looks good to me.

author: Martin72
author_rank: Super Contributor
timestamp: May 31, 2025, 08:18:45 am
content: Quote from: torch on May 31, 2025, 03:45:43 amIs that all measured against the ground again?

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 10:40:49 am
content: Quote from: Martin72 on May 31, 2025, 08:18:45 amYes. I'm not aware of any other way to show both the AC inputs and the rectified output at the same time (short of using a differential probe, which I don't have). All 3 probe grounds have to be at the same potential.

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 11:03:02 am
content: Quote from: kmoonwalker on May 31, 2025, 03:31:14 amYeah even I know you can't buy a refill kit. I was being a little bit cheeky there. Besides, I've misplaced my really tiny funnel...Quote from: kmoonwalker on May 31, 2025, 03:31:14 amHere's a more serious question: What exactly is the difference between the KBU and GBU families? They seem to parallel each other in terms of the basic voltage and current ratings available within each family. Is it just the flat vs round pins and case dimensions? Is there an advantage to choosing one over the other? If I needed to replace a bad KBU but all I had on hand is a GBU can I just stuff it in there and call it good or is there a fundamental difference?

author: Martin72
author_rank: Super Contributor
timestamp: May 31, 2025, 12:13:39 pm
content: One last time, then you're off the hook...Without knowing the exact circuit diagram, here is a rough overview of the fixed 5V output:This is an output isolated from earth.If you then measure the output of the rectifier/output of the regulator from earth, you will get a “floating” measurement with an AC component.Better to measure it like this:At the moment, MP2 against 0V and MP3 against 0V are of interest (not the transformer).

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 02:37:34 pm
content: Quote from: Martin72 on May 31, 2025, 12:13:39 pmI'd say that's a pretty good guess. The regulator is a little more complicated than a single 3 terminal device but as a block diagram that pretty much covers it. C1 is the bad 10,000uF cap, C2 is actually on the front panel board, right at the output jacks.If you are interested in the details, the regulator circuit actually incorporates a 1.25V "AZ431 adjustable precision shunt regulator" (U3 ), a trim pot and some other discrete components to control the output of Q6 (IGBT?). Top of the board:Underside of the board:I thought this was interesting. Hand soldered to the underside of R21 (1K, 5%) is a second resistor (1K, 1%) in parallel. Apparently some custom fine tuning was required:Unfortunately some of the traces disappear under the heat sink and I still have no idea what the mysterious three terminal device tagged Q9 is or what it does. But it's definitely associated with Ch3. Monitoring current to control the cooling fan on the heatsink? Overload protection?Quote from: Martin72 on May 31, 2025, 12:13:39 pmI agree, it appears the transformer is not at fault and I understand that the outputs are normally floating.I am not questioning your expertise, but just trying to wrap my head around this. If I understand correctly, to connect a scope at MP1 I have 5 choices:1. Use a differential probe. This will keep the scope and DUT chassis grounded for safety while keeping the outputs floating. Cons: None really, I just don't have one due to the expense.2. Use 2 channels of the scope to perform a differential voltage measurement. Connect one probe to one AC leg and the second probe to the other, then the math function to measure one channel against the other instead of against ground. Scope and chassis remain grounded and transformer output remains floating. Cons: I'm still having trouble understanding how the scope and chassis ground come into play here?3. Connect a single channel probe to one leg and the probe's ground to the other. Cons: mains ground is now connected to the circuit internally in the scope, so it's no longer floating.4. Use an isolation transformer on the scope to isolate it from mains ground. I actually have one of those, but understand that it's use constitutes a Bad Idea due to safety concerns since the exposed bits of the scope's chassis would now be floating.5. Use a battery powered scope. I actually have one of these -- a relatively cheap single channel unit I got for automotive use. Con: small screen, crappy resolution and no recording capability.The choices at MP2 and MP3 appear similar, but I think I understand now why I can't attempt to measure all 3 at once with 3 scope channels. Connecting AC2 to the probe ground while 0V (green) output is connected to probe ground(s) effectively shorts the AC2 side of the bridge to the 0V output, right? I would need at least one differential probe to keep the AC floating or a scope that could still display 2 channels for the DC while the other two are compared mathematically for the AC? (I'm not sure if that's even possible with my Siglent SDS 1104X-E. It has a lot of features I haven't figured out yet.)I will follow this up and get the MP2 and MP3 measurements independently -- once Amazon delivers the replacement cap and I reassemble things!

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 31, 2025, 04:42:01 pm
content: I really think you need to watch this video (turn on subtitles)

author: Martin72
author_rank: Super Contributor
timestamp: May 31, 2025, 05:15:09 pm
content: @torch:The scope should not be connected to an isolating transformer; instead, connect the object to be measured, in this case the power supply.*)Then you can also measure MP1 with one channel.If you want to measure all MP1 and MP2/MP3 simultaneously, you must use a differential probe (for MP1).However, only MP2 and MP3 are of interest here, and these can be measured simultaneously because they both have the same potential (0V(5V)).The faulty electrolytic capacitor will have been the cause of the error: the DC voltage supplied to the control circuit must be buffered, otherwise the controller cannot work properly.*)This is something I always tell my employees in the test lab: please don't connect the scope to the isolating transformer.Background:When the scope is galvanically isolated from the mains, it also has no connection to the protective conductor (earth).This means that the BNC connectors on the scope, which are otherwise connected to earth, are potential-free andcantherefore conduct dangerous foreign potentials from the test object.If the isolation transformer does not provide sufficient power, differential probes are used.The worst combination is to use an oscilloscope on the isolation transformer and then also use differential probes, as these do not work properly without the ground.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: May 31, 2025, 05:35:29 pm
content: @torchmain problem with people using either:- separation transformer- power cable with cutted PE wire(same result)is that they forget about that during next measurementhere in this circuit you have most likely some low voltages but then imagine connecting probe to 100Vpp or even more and touching with bare hand anywhere grounded on scope (usually during connecting or disconnecting one of probes to use different oneBTW what is not in the movie but also an error that some tend to do since all BNC connectors are on PE wire:- they take one more scope to measure that different potentialLemmi tell you it is a "special feeling" when you would have few fancy scopes on desk and one of your friends during testing a prototype decided to let em go in smoke via such error...In case you dunno what voltage distance you have between parts of circuits masses you always use that cheap or not cheap battery powered scopeWe also use sometimes a real sine output PSU charged and disconnected from mains to power normal scope during such measurements and few of my friends have normal battery powered scopes for example Owon and Rigol have such and they are really safer option (still one needs to obey rules between channels)And always in case of not knowing how real Vpp may loook in circuit during diagnostic (DMM always integrates value you see so you may not see fast large spikes) it is best to sacrifice that cheap battery powered one for preliminary overview or use like 1,5kV passive or some active probe to be certain and not damage input of scope (seen that a lot)

author: torch
author_rank: Frequent Contributor
timestamp: May 31, 2025, 10:56:09 pm
content: Epilogue:It's fixed!MP2 on Ch1, MP3 on Ch2No load:1A load:2A load:3A load:Thanks to all who shared their experience, I learned almost as much as I should have.Note to those cautioning against plugging a scope into an isolation transformer, that was my understanding as well, but thanks for confirming it.Interestingly, I acquired the isolation transformer because it was bolted to the scope cart of the Tek 475 that was auctioned as surplus by Ontario Hydro (the provincial electric utility). You'd think they'd know better!One question for kmoonwalker though: you mention using a true sinewave PSU on battery instead of an isolation transformer, but is that really any safer? The same bits connected to the scope chassis (BNC shields, etc.) that can become energized with the mains ground disconnected can still become energized running off the PSU, no?

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: June 01, 2025, 12:12:11 am
content: Quote from: torch on May 31, 2025, 10:56:09 pmQuotewell there are always risks attached with this job - after all everything runs on electricity so farThing is that same like with battery powered scope when using UPS disconnected from mains you cut the connection between GND on scope and PE cable in socket so you avoid the possibility that current will go through the path of lowest impedance being a scope not inside the circuit under test and that is the first thing we wish for then second is to know what we are doingAlso sometimes we need to probe without connecting the GND clip though it comes with a certain cost and as you learned here also improper grounding comes with that.There are some times when we need to do something with what we have or for example normal "ordinary" oscilloscope has other functions we need to use - your case was very simple here - imagine this was a complex board with complex logic and you would want to have fast diagnose but this requires more than one scope and then comes GND to PE problem and usually a digital zero V is not real 0V or lovest potential on board. Some also shift logic levels via few to more volts to have a lower signal crosstalk and other reasons cause for that separate logic part of circuit and logic elements there it is irrelevant how much is real voltage - they will have 5V from their reference level totally OK but connecting the scope with GND will immediately cause damage cause it can be 5-10-20 or sometimes more and it is not GND-PE level or zero in mass of other channels point. Btw I seen some "geniuses" that used additional scope connected to other mains phase ie. one was on L1 and other on L2 and results were even more "spectacular" in smoke/sparkles generation terms so pls avoid that - all phases inside building usually share the same PE cable.If you would make a wire between one special PE in the distance to another between buildings you can have a classic light bulb showing faint light (there is voltage), which is also a bad idea.This problem is nothing newThere are special scopes like this one in movie:https://www.rohde-schwarz.com/au/knowledge-center/videos/the-advantage-of-isolated-channels-video-detailpage_251220-584899.htmlPlease look how BNC connectors are made - there is no contact voltage when you touch those with finger as he does - note how recessed is metal part insideMy battery powered HDS2202S is made the same way in terms of connectors so it provides that finger contact security but it sadly does not have isolated channels nor even the generator thus I usually use it as a single channel scope. So always check that if you would want to get one with a multimeter - seen ones wired to "overcome" opto isolation as brain dead as it may sound.lot of scopes ie. for Automotive industry is made this way (with channels isolation) so that not very trained in Oscilloscope basics user can use them safely for him and device (thou not always they have some nice trigger options if only predefined mode for testing spark plugs and other elements)With such properly isolated channels scope (but pricey with functions or cheaper very simple wave shower) you could see everything from mains on one channel to transformer output on another then those two points after rectifier Martin mentioned and for such scope it is perfectly normalSo that is perhaps sad but real about reality as it getsDifferential probes are usual way and very nice to have but expensive and most of amateurs does not have a probe with cost of simple scope or more (talking about simple and not bad one that you can watch mains AC sine for example here that has Vpp= 230 * 1,41 * 2 V AC +/- mains variation tolerance and they have freq limits higher the more you pay.

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2025, 12:17:08 am
content: While it may be fixed the screenshots are unconvincing and maybe confusing to some, trigger level settings are in the 12V region on a 5V supply.This might imply probe and scope input attenuation settings don’t match.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: June 01, 2025, 12:26:48 am
content: I think that yellow is rectifiers output

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: June 01, 2025, 12:46:08 am
content: @torch

author: torch
author_rank: Frequent Contributor
timestamp: June 01, 2025, 01:44:27 am
content: Quote from: kmoonwalker on June 01, 2025, 12:26:48 amCorrect. Easier to trigger on the rectifier's ripple than on the regulator's DC.Nothing to do with any of the above, but to answer my own question: Yes my scope can still display all four channels while applying a math function to two of them:

author: torch
author_rank: Frequent Contributor
timestamp: June 01, 2025, 02:27:24 am
content: Quote from: kmoonwalker on June 01, 2025, 12:46:08 amInteresting that he shows using the UPS to power the DUT, not the scope.His part 2 of that is here:It shows using the math function and two channels to measure differential voltage. He also demonstrates a 4 channel scope measuring input and output of a bridge rectifier this way. I should have watched that two nights ago I guess!

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: June 01, 2025, 03:17:44 am
content: Quote from: torch on June 01, 2025, 02:27:24 amas it was stated in first vid I posted for you - you need to isolate either DUT or the scope - whatever works for you and is safer/easier in given situation (one always needs a bit of planning and imagination what if)Worry not - most of folks do not know much about how to use them till fist scope is burnt - after they know what to watch for (I always recommend some cheap scope for newbie as he in most cases will forget something at some point and "bzz paf")Usually users of fancy ones know due to the fact that accidentally damaging a scope worth a good car or more is a disaster everyone wants to avoidboth autors I shared have a lot different movies on youtube I think worth of your attention (even if one I shared first is only with subtitles but he makes more about metrology side which is also a good thing to know a bit about)

author: torch
author_rank: Frequent Contributor
timestamp: June 01, 2025, 01:17:01 pm
content: Oh great: Now I've fallen into yet another YouTube rabbit hole.This video gives the clearest explanation I've come across of why it is preferred to power the DUT rather than the scope using an isolation transformer. The first half is mostly background, the best part starts about 13:30.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: June 01, 2025, 03:43:56 pm
content: he makes a real good examples basing on real experiences - some as in this movie are from basic electrical training some are a bit modified still very good for you to know how to not get zapped or not to zapp something elselike in 16:39 you always need to know if that isolation transformer has PE wire continuity or notsame like with scopes claimed as isolated - simply take a DMM and check when you first use something new - also good practice to check in case of complex installations just to be on safe side