Variable switching power supply - Page 1
Forum section: electronics / testgear

author: harrimansat
author_rank: Regular Contributor
timestamp: May 02, 2015, 09:10:16 pm
content: 0-30V, 0-5A, multiturn pots:

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2015, 09:20:23 pm
content: Quote from: harrimansat on May 02, 2015, 09:10:16 pmShhhhI'd had hoped these were my little secret.Great little PSU's, beaten them up heaps, 100% reliable.Smart fan cooledRipple ~30 mV5 turn pots. CV or CC

author: harrimansat
author_rank: Regular Contributor
timestamp: May 02, 2015, 09:31:44 pm
content: Quote from: tautech on May 02, 2015, 09:20:23 pmCheap deleted

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2015, 09:49:08 pm
content: Quote from: harrimansat on May 02, 2015, 09:31:44 pmCall them cheap if you wish or "well priced".The only shortcoming IMO is lack of binding posts instead of 4mm banana sockets.They available on Aliexpress, but of course better pricing in bulk from the manufacturer.Several very similar units around, not sure but they might be rebrands.

author: rdl
author_rank: Super Contributor
timestamp: May 02, 2015, 11:02:34 pm
content: There are a few that look a lot like that one on ebay. Some versions have the regular style binding posts. Price runs around $80 US.http://www.ebay.com/sch/i.html?&_nkw=power+supply+variable+switching&_sacat=0Why do they insist on putting ground in the middle? And use yellow instead of green?

author: atferrari
author_rank: Frequent Contributor
timestamp: May 03, 2015, 01:16:47 am
content: Quote from: rdl on May 02, 2015, 11:02:34 pmWhy not?In a scale, ground IS in the middle.

author: rdl
author_rank: Super Contributor
timestamp: May 03, 2015, 03:23:01 am
content: Because ground is rarely used but positive and negative are often used together.Picture shows jacks, but plugs are also very common.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 03:42:35 am
content: Don't have a clue why they need (think they need?) all those lytics, but design looks pretty solid and safe. Look at those slots!Tim

author: dom0
author_rank: Super Contributor
timestamp: May 03, 2015, 07:27:17 am
content: Quote from: rdl on May 02, 2015, 11:02:34 pmIt's not yellow. The socket is green/yellow, the inner sleeve is greenAnd why in the middle... no idea. The old ones got it right. +,-,PE.

author: dadler
author_rank: Unknown
timestamp: May 03, 2015, 08:55:55 am
content: Hmm, these all seem to be 220V. Can't seem to find one with the same essential internals that will work in the USA.

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2015, 10:14:18 am
content: Quote from: dadler on May 03, 2015, 08:55:55 amPulled one from stock to check, yes the manual and the rear of the PSU says......230V.However the label next to the IEC mains socket has a 110 V option that is blank.That is, only the 230 V is marked on my units.This indicates there might be a 110 V version available.

author: Rerouter
author_rank: Super Contributor
timestamp: May 03, 2015, 10:29:26 am
content: for a free forum, eevblog sure is expensive to my bank account....

author: nowlan
author_rank: Frequent Contributor
timestamp: May 03, 2015, 10:49:45 am
content: Might want to put the model in the title.Needs better lighting, suggest outdoors.Also what is the output like during switch on? Any screen shot?

author: rdl
author_rank: Super Contributor
timestamp: May 03, 2015, 11:13:40 am
content: Quote from: dom0 on May 03, 2015, 07:27:17 amYes, the socket in the photo in the op is green/yellow - and it actually looks pretty nice. Looking at the results from the ebay search I posted the ones made with regular binding posts all seem to use solid yellow. For some reason the Asian manufacturers just don't seem to care about the details. I have even seen pink binding posts on some power supplies, but hey, they're cheap.

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2015, 07:48:34 pm
content: Quote from: nowlan on May 03, 2015, 10:49:45 amSorry haven't got a saved waveform to post.I'll try and get one up later today.NOovershoot.

author: rob77
author_rank: Super Contributor
timestamp: May 03, 2015, 08:36:00 pm
content: the board layout and how it's built.... looks like my Manson bench supply inside.and actually manson has a very similar model...http://www.manson.com.hk/products/detail/28

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2015, 07:24:38 am
content: Quote from: tautech on May 03, 2015, 07:48:34 pmK305D power on waveformNO LOADPSU setting 10 VDSO settings:10:1 probe, 2v/div, Trigger @ 2V, Single triggerTimebase set at 20ms/div to more clearly show a small glitch in the start of the voltage ramp up @ ~3.5V.NoteFurther test with a 12 V bulb at the same 10 V setting displayed no discernable difference in the power up waveform. PSU displayed a 420 mA load.

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2015, 07:49:52 am
content: This seems like a nice little extra PSU for the bench. Would it be difficult to add an output enable switch?

author: Rerouter
author_rank: Super Contributor
timestamp: May 04, 2015, 07:58:44 am
content: tautech, could you re-try that at something like 3.3V, the little bump at the start has me curious,

author: dadler
author_rank: Unknown
timestamp: May 04, 2015, 08:14:37 am
content: I found this one which is 110v/220v, but I am not sure if the internals/pots are (mostly) the same:http://www.ebay.com/itm/281663049598

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2015, 08:24:54 am
content: Quote from: nctnico on May 04, 2015, 07:49:52 amWhy would you bother if it doesn't overshoot?The only space available on the front panel is in close proximity to the mains switch.Mains and LVQuote from: Rerouter on May 04, 2015, 07:58:44 amK305D 3.3 V 250 mA Resistive (bulb) load power upVoltage when Glitch occurs is reduced with a reduction in output setting. Now ~ 2 V.DSO settings:10:1 probe, 500 mV/div, Trigger 500 mV, Single shot

author: Rerouter
author_rank: Super Contributor
timestamp: May 04, 2015, 08:30:36 am
content: Thank you very much, i just went ahead and ordered one last night anyway, glad to see everything is in order

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2015, 08:46:45 am
content: Quote from: Rerouter on May 04, 2015, 08:30:36 amFirst one I got, we tried to bust and couldn't.2 x 10A stud diodes in series, forward biased for ~1.4 V across the outputs.....no problem, it just cycled the fan when needed.Want to charge batteries, LA, 24 V (28.4 V @ 5A) ALL weekend....no problem.They self Current limit @ just over 5A.Yet to find an application where the fan runs continuously.

author: opty
author_rank: Regular Contributor
timestamp: May 04, 2015, 08:58:47 am
content: Quote from: Rerouter on May 04, 2015, 07:58:44 amcould you please show shut down traces as well?

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2015, 09:08:52 am
content: Quote from: opty on May 04, 2015, 08:58:47 amThey will depend on the load. Internal capacitance with no load can take as long as 5 seconds to discharge.3.3 V:It is a CLEAN curved ramp downwards, steep at first, just a reverse of the 3.3 V rising waveform without any glitch.

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2015, 09:23:23 am
content: Quote from: tautech on May 04, 2015, 08:24:54 amSwitching it on/off using an output enable switch often leads to a more controlled ramp, doesn't wear out the mains switch and doesn't cause nasty mains-switching spikes which may upset circuity connected with longer wires (programming tools for example).BTW: does it retain it's settings between power cycles?

author: Rerouter
author_rank: Super Contributor
timestamp: May 04, 2015, 09:42:34 am
content: It would be easy enough to add a shutdown to most things, the controller they use could well have an enable pin, i cant really forsee myself needing it, but at the minimum i'll atleast be pulling it down to give the soldering a look over,Really glad to hear about being able to charge batteries with it,

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2015, 10:04:10 am
content: Quote from: nctnico on May 04, 2015, 09:23:23 amYep, 5 turn WW pots.I had wondered on "upgrading" my K305D bench unit with 10 turn pots, but that defeats the purpose of a very handy portable PSU. It's my "grab first" unit because its small, light and unbreakable.Only 100 mV resolution on the display, but 10 mA on the current display.Units I have checked, the displays are reasonably accurate, not precise, but fine for the price.Quote from: Rerouter on May 04, 2015, 09:42:34 amI gave the first one I got aprivate teardown, undocumented, to check build quality.IIRC it was quite reasonable, but not quite TE quality.Done a few SLA's with the K305D too.Nice to be able to set it correctly for a cyclic charge and watch the current slowly fall away.Real handy for those odd 6V jobbies that you sometimes come across and don't have a charger for.

author: bookaboo
author_rank: Frequent Contributor
timestamp: May 04, 2015, 11:48:51 am
content: I have the MCH-K302D, identical unit with the exception that it's 0-2A. Very happy with it in all respectsAlso I have the MCH-K1204D which gives 120V at 0-4A variable which I've been using 24/7 for long term testing since January, again for the price it's superb.

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2015, 01:13:50 pm
content: I received mine today (30V / 5A) and opened it up for inspection. There are a few safety issues with it:1) The heatsinks for the primary switching mosfets are hot (=connected to mains). The PSU uses a diode as a temperature sensor with some thermal paste to goo things together. I'm not convincent this meets the requirements for a proper isolation barrier. Also -unlike what you'd expect- thermal paste isn't a very good conductor of heat.2) The terminals of the mains switch are not isolated. In my one wires from the secondary circuit where touching the connections of the mains switch.3) There is no wire from the mains entry ground terminal to the case.4) The wires to the mains entry are not going through the holes of the terminals.Besides that I'm also missing a power factor correction circuit. For sale in the EU that would be required. But then again there is no CE marking on it so at least it doesn't pretend to be any better than it is.I'm going to test it and afterwards I'm going to make the following modifications to increase safety:- Redo the wiring to the mains entry and add heat shrink tubing for isolation and mechanical strength- Add a ground lead from the casing to the ground terminal of the mains entry- Put heatshrink tubing around the mains switch- Glue the mains leads to the boardAnd while I'm at it I'll also add a output enable/disable switch. The Chinglish manual says 'please do not load to start the power switch'. I think master Yoda means the PSU should not be switched on with a load connectedI think I'll also upgrade the output terminals to better ones. I have a bunch of those anyway.

author: slurry
author_rank: Regular Contributor
timestamp: May 18, 2015, 05:50:23 pm
content: Quote from: rob77 on May 03, 2015, 08:36:00 pmi have one of those Manson supplies, happy with it, actually i will take a photo so we can compare..BRBedit: now i'm backthe psu does not look the same inside, not at all.My psu was actually not a Manson, it is a PeakTech but it looks the same, maybe it look different inside?I did some measuring at 3V, with no load and with 1A load, the ac ripple is low, around or less than 2mV regardless high or low voltage and with or withoaut load.I checked the output when i turn the output on 3V, no load...and 3V with 1A load.

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2015, 06:17:18 pm
content: I found some bad soldering in the unit I got. I could pull one of the wires from the power switch and one of the wires to an LED also came off by itself. It seems this things need restoring straight out of the box.

author: tautech
author_rank: Super Contributor
timestamp: May 19, 2015, 08:15:04 am
content: Quote from: nctnico on May 18, 2015, 06:17:18 pmGuess it would be that horrible lead free muck, for PCB production it might be fine, but for terminals and off board connections. What were they thinking?I've already pointed the manufacturer to this thread, I'd better do it again.

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2015, 08:23:22 am
content: It has nothing to do with lead free soldering; I get my lead free joints just as nice as with leaded solder. It just takes better soldering skills and properly adjusted equipment (right temperature and tip).

author: tautech
author_rank: Super Contributor
timestamp: May 19, 2015, 08:54:35 am
content: Quote from: nctnico on May 18, 2015, 01:13:50 pmThanks for the teardown analysis.Did you mean the terminals on the mains switch were notinsulated?As I stated none I've sold have had any issues, including a class set of 15 that have been in daily use for ~2 years.

author: nctnico
author_rank: Super Contributor
timestamp: May 20, 2015, 02:04:02 pm
content: Meanwhile I did some more testing. One of the problems I found was an excessive amount of noise on the output (about 1.5V). I added an extra output filter but that did absolutely nothing. It turns out the design has some EMC flaws which cause excessive current into the ground wire. I added 2 10nF 2kV capacitors in series between the minus of the PSU and the minus of primary section. This reduced the levels to around 150mV.Another problem is that the line regulation is not that good at full load (30V / 5A). I've noticed a 0.5V 50Hz ripple at the output. Perhaps the capacitors on the primary side are too small.I attached some photo's to show the goo on the heatsinks, output enable switch, better binding posts, a ground strap to the case, isolating the mains switch and fixating the mains wires.My version is slightly different than the one from the topic started. Mine has several through hole components.

author: nctnico
author_rank: Super Contributor
timestamp: May 20, 2015, 02:05:06 pm
content: And some scope images showing the noise reduction by adding the capacitors and the output ripple at full load.

author: slurry
author_rank: Regular Contributor
timestamp: May 20, 2015, 02:27:03 pm
content: Certainly built to a price Point..

author: OldDogSleeping
author_rank: Contributor
timestamp: May 20, 2015, 07:32:04 pm
content: I have this same LW-K305D version as nctnico, and also seemed to get a lot of noise ( over 1v ) on the output, but when I checked at the output point on the board using a short GND wire, most of the noise disappeared and dropped bellow 50mV. So being a newbie, I assumed the noise was down to my poor measurement technique at the binding posts. Is it likely to be real, if it isn't present at the output of the board ?Also I do agree with nctnico's observations on the poor earthing, and connections to the switches etc, the general workmanship is very poor.

author: nctnico
author_rank: Super Contributor
timestamp: May 20, 2015, 08:11:42 pm
content: The problem isn't noise between the output binding posts. The problem is that the power supply is pushing HF noise from the switching stage into outputs if one of the outputs is connected to ground.Edit: What puzzles me is how Tautech was able to get such a clean trace. I wonder whether his PSU and scope are grounded.

author: nctnico
author_rank: Super Contributor
timestamp: May 27, 2015, 12:33:15 am
content: I have to admit this PSU somehow got me obsessed with fixing the EMC problem it has. It turns out the switching MOSFETs create a lot of junk around 80MHz (see purple FFT trace). I added 3 ferrite beads (Wurth 742792515 with a high impedance at 60MHz; blue circles) to contain the junk and added some snubbers (220pf + 560 Ohm; red circles) between the source and drain of the MOSFETs, added 150 Ohm series resistors to the gate of the MOSFETs (purple circles) to slow the edges down and put a 10nf 2kV capacitor (yellow circle) between the + and - power supply rails of the MOSFETs for extra decoupling. With these modifications the amount of noise it produces with the scope attached gets reduced to 200mVpp (with peak detect enabled!). The earlier screendumps where with the 20MHz bandwidth on (I overlooked that).Now the manual says there is also a 30V 10A version and I already spotted that one on Ebay.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2015, 03:42:16 am
content: Ehhhhhhh... please don't increase the gate resistors. They're probably marginal as-is, and with poorly chosen, antiquated transistors, the rise time was probably pretty nasty to begin with.That said, it doesn't look like much heatsinking, so maybe the switching design wasn't too terrible. Which makes worsening it all the more dangerous. If nothing else, don't call it "done" until you've done a full load test (elevated ambient temperature, various load conditions including full rated current at low voltage, half and full voltage). Keep spare transistors (and whatever's in the gate drive circuit) handy.You should be able to identify what loops are ringing. This is the kind of thing you can track down piece by piece: squash one squiggle, find the next largest one, and so on. A near-field probe (E or H) is good for this. H probe can be a small coil of wire, insulated: wave it around the circuit and see where signals peak, and rotate its axis to test directionality (it's sensitive to currents flowing along the turns in the coil, which is the same as saying, magnetic fields parallel to the coil's axis, because currents produce perpendicular fields). Fields are gently curved and fade gradually, so you can track sources by the relative intensities and wave shapes near different components.Ferrite beads may not do very much, because they saturate quite easily, and can actually make other things worse (increasing overshoot or reducing damping in the power supply network). Ditto bypass caps (or both). It's worth some back-of-the-envelope numbers to determine whether what you're adding will affect these or not... or do anything at all.That said, it still seems likely that adding a ferrite bead to the offending node (I would guess it was diode reverse recovery..?) would still help, because of two effects: the remaining impedance (even when saturated), which won't be too bad at 80MHz (~10s of ohms, probably comparable to the circuit's impedance there, thus damping it, or shifting it down at least), and using the saturation as a feature rather than a bug: when a ferrite bead is saturated, it acts like a hunk of wire, but as it comes out of saturation, it absorbs a big gulp of flux (volts * time), which softens the blow to the diode and makes reverse recovery softer. Once the diode is off, the ferrite bead isn't carrying current either, and it resumes its ferrite-beaded glory, dramatically increasing the inductance (and loss) in the diode's loop, damping oscillation versus the diode's capacitance.Ferrite beads are also commonly seen on transistors, usually the input (base/gate) or common (emitter/source) terminals, or both. I wouldn't recommend the latter, because again, it's probably not going to do much (that's where load current goes). But there can be value in the former, especially if there is pressure to keep gate resistance minimal (for fastest switching).What happens there is, if there's a big beefy gate driver chip, it can deliver such a jolt to the gate that it momentarily saturates the series ferrite bead, so after a short delay (less than 10ns?), the gate swings as quickly as ever, and as gate voltage comes to a rest (damped by circuit resistance), it comes out of saturation and resumes being a higher impedance, damping possible gate oscillations.Don't forget to check common mode as well as differential. If your probed measurement changes whether there's a ferrite bead on the probe cable or not, suspect everything!Good luck,Tim

author: nctnico
author_rank: Super Contributor
timestamp: May 27, 2015, 02:30:36 pm
content: Quote from: T3sl4co1l on May 27, 2015, 03:42:16 amI didn't look up the transistors' specs but they don't get very warm to begin with even at full load. It is mostly the rectifier diode which gets hot. With low switching frequencies (this PSU runs at a mere 30kHz) you can use quite high values for the gate resistors.I just checked: the 60N10 MOSFETs are rated for 600V 9.5A which could qualify as overkill.I did use a H field probe to find the part of the circuit which is most problematic. IMHO the circuit board layout isn't optimal as well so there is another limit to what can be achieved. I tried several different component variations and what I have now seems to give the best performance versus the amount of hacking.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2015, 01:43:52 am
content: Another recently discovered thread on these same PSUs:https://www.eevblog.com/forum/testgear/mch-k305d-cheap-switching-lab-power-supply/

author: nctnico
author_rank: Super Contributor
timestamp: August 07, 2015, 04:00:08 pm
content: After waiting for more than two months I finally received the 30V 10A version. The circuit board looks exactly the same but the MOSFETs and rectifier diode have bigger heatsinks. It also has the same safety concerns I noted earlier so it will need a partial rebuild before use.

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2017, 01:14:33 am
content: So the unit I've had for a few years developed a fault.....the amps display shows the mark of the devil: 666Display driver IC is of course suspected and confirmed with coms to the manufacturer and also a mate that has had the same problem with his. The IC is a SOP-24 GC7137AP seven segment driver. They can be found on Aliexpress for ~$1 ea.https://www.aliexpress.com/item/GC7137AP/32810722187.html?spm=2114.40010308.4.24.8fLoLjAs yet I've not found a datasheet in anything but Chinese however it's a pdf that can be copied and pasted into a translator bit by bit.http://www.sungine.com.cn/static/upfile/xzzx/201606131526533225.pdfMy buddy had found K305D schematics and I share them below.

author: RetroDan™
author_rank: Contributor
timestamp: July 26, 2017, 12:10:35 pm
content: I bought this from Amazon, and I can adjust the Voltage, but I get nothing on Amperage. If I turn the V all the way up and THEN muck about with A, I see a single 0.05A spike on the display that quickly returns to 0.00. I'm super pissed off that I wasted my time on this piece of dreck.

author: nctnico
author_rank: Super Contributor
timestamp: July 26, 2017, 12:45:36 pm
content: Quote from: RetroDan™ on July 26, 2017, 12:10:35 pmDid you connect a load? Without a load there will be no current flow and hence the display will show zero. I guess what you are missing is the current limit setpoint. This is often not shown on power supplies.

author: Dave
author_rank: Super Contributor
timestamp: July 26, 2017, 02:54:38 pm
content: Quote from: nctnico on July 26, 2017, 12:45:36 pmCome on, he can't possibly be that thick...... can he?

author: Enoch
author_rank: Newbie
timestamp: July 27, 2017, 02:51:58 am
content: This should be from China's design

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2017, 09:43:31 am
content: Quote from: rdl on May 02, 2015, 11:02:34 pmIs that center terminal ground or common? I assume it is ground since that is what it says so the output is floating but that just means a standard 3/4" banana plug cannot be used so I agree, the design is poor.And since they did not use the binding post style of banana jack, you cannot easily bridge ground to one side. Or maybe the output actually is not floating but then they did not bother indicating that?I do not need to evaluate anything more to know that the designers did not know what they were doing. This thing is trash.The best terminal arrangement I have used places the binding posts for negative, positive, and ground in a 3/4" equilateral triangle. For a tracking supply, this requires 5 posts in a W configuration with two ground posts or two common posts.Quote from: T3sl4co1l on May 03, 2015, 03:42:35 amThe first thing I thought upon seeing all of those small aluminum electrolytic capacitors is, "There is a wearout mechanism and reliability problem waiting to happen."

author: RetroDan™
author_rank: Contributor
timestamp: July 27, 2017, 02:21:01 pm
content: Quote from: Dave on July 26, 2017, 02:54:38 pm*rolls eyes* Yes, I connected a load. I didn't think that I needed to explain things down to that level. I also inhaled and exhaled in order to keep from passing out, but I didn't feel the need to have to explain that, either.I'll just buy the parts to build my own - I was just trying to do things the easy way, and got the short straw.Oh, by the way? Super thrilled at your commentary, Dave - always nice to have someone assume I'm a complete ignoramus.

author: Rerouter
author_rank: Super Contributor
timestamp: July 27, 2017, 02:28:53 pm
content: dan, try lifting the lid and make sure there is nothing loose, there is a plug for the current set pot, equally does it sit on the CV light or CC, if you have a load and it remains at CV, then your from terminals may not be making contact,The voltage is measured pretty close to the output, and the current over a shunt near to the output,

author: RetroDan™
author_rank: Contributor
timestamp: July 27, 2017, 02:32:32 pm
content: Quote from: Rerouter on July 27, 2017, 02:28:53 pmAlready checked the connections for continuity. I'm not sure where the problem is, but it's a moot point now. I've already sent it back; as mentioned in my previous post, I'll just build my own.

author: nctnico
author_rank: Super Contributor
timestamp: July 28, 2017, 10:22:17 am
content: Quote from: RetroDan™ on July 27, 2017, 02:21:01 pmOfcourse you have to explain that. Or do you have a working crystal bowl? You'd be the only one! There are lots of elementary things that can go wrong. For example: did you measure the voltage across the load with a multimeter? Perhaps you did but it is impossible to judge your fault finding skills from behind a screen so you have to tell us what you did to diagnose the problem yourself in great detail to get answers which are usefull to you.

author: RetroDan™
author_rank: Contributor
timestamp: July 28, 2017, 01:12:43 pm
content: Quote from: nctnico on July 28, 2017, 10:22:17 amFair enough; from now on, I'll err on the side of loquacity. Still think Dave's response was unneccesary.

author: Dave
author_rank: Super Contributor
timestamp: August 10, 2017, 12:07:12 pm
content: There is a fair number of complete beginners rolling through the forums and for many some things as trivial as these might not be trivial to them. So the default assumption when a newbie calls a piece of test equipment shit is to assume that they are not using it correctly. We can't possibly know your level of experience to rule out the trivial stuff by default.That being said, I apologize for being condescending and hope you managed to sort the problems out with the power supply.Side note: I'm aware that 2 weeks have passed since the last message, I only now stumbled upon the thread again.

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2017, 11:18:59 pm
content: Quote from: tautech on June 24, 2017, 01:14:33 amAnd a month or two later the volts display went to the other side too.Sourced from the link in reply #45 were 5 pcs GC7137AP, the 24 pin SOP display driver.They're mounted on the rear of the display PCB and reasonably accessible after the power and data leads for the display are unplugged (mark them in some way) BUT it's a lot easier then just to remove the 3 screws holding the display PCB to the front panel and work on it on the bench. Mentioned is a trap....the power and data leads for the display share the SAME plug and the lengths of their leads enable them to be swapped/reversed.This pic shows the two leads to the display removed and the display still mounted to the front panel.The two driver IC's are clearly visible.I removed the old IC's with a hot air handpiece equipped with a 4mm nozzle and tweezers, then diluted the Pb free solder with leaded solder, chucked some flux down and tacked the replacement IC's into their positions. Note: the silkscreen pin 1 identifier for the IC is under the ICso care need be taken to ensure you get them the right way around. Of some little help is they share the same orientation so if you get one down and then the other it'll be hard to stuff it up.There's not much room to drag solder so with a 1 mm pointed tip I finished soldering each pin one at a time.Success.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2020, 12:10:34 am
content: Got a customers K305D back that was dead !Really instead it was a dud IEC mains lead and not the one it was supplied with.Easy fix.However after assigning it to SLA charging duties as a check that it was working OK the Amps display showed considerable offset, wouldn't zero with zero load and readings were unstable.Anyways, after the obligatory inspection for dud caps, poor solder joints and reseating cables from the current shunt to the display circuitry the offset is gone and and the PSU is working perfectly again.Bothand

author: mava70
author_rank: Newbie
timestamp: February 22, 2021, 08:14:54 am
content: this is mine after a kamikaze mods on the gate of mosfets that killed both, now I have 22ohm resistor on the gate and 100mV of noise on the output, I ordered also 2 capacitor for finishing the mods on the mosfet as found on a youtube video, I let you know as soon as they arrives

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 27, 2021, 05:21:40 pm
content: @mava70I guess you're referring to this YT video :-I tried this mod just this week on the four digit display version I'd purchased from BG some 16 months ago (virtually identical to the main board in the three digit version shown - three 330R 5W instead of just two). However, I was a little leery about emulating the ultimate 33R with 47nF caps mod so compromised with 33R and 22nF. Using these values the input power for 320W output went up from 394W to 397W and the IR thermometer readings of the power fets showed just a modest ~4 or 5 deg C rise.A week or so later, I decided to test the 33R 47nF combination (I couldn't find any suitable 33nF in my collection of salvage). For exactly the same output power (32V@10.08A), the input power rose to 404W and the IR thermometer readings on the power fet heatsinks went up another 7 to 10 degrees. After 40 minutes, I finally got to prove the efficacy of the overheat protection which simply disables the output until the cooling fan has taken the temperature back down below the cut-in temperature point.This gave me the motivation to pick through my collection of circuit boards in my basement which had been awaiting the Round Tuit needed to recover another batch of 'salvage' from which I managed to find just three suitable 33nF 100v mylar capacitors. Swapping the 47nF caps for these 33nF caps reduced the input power to just below the 400W mark, allowing it to run flat out for over 45 minutes without cutting out. In fact I simply reduced it to quarter power (16V 5A) at the 45 minute mark which allowed the fan to shut off after another 5 minutes before cycling to keep the temperature in the safe zone over the next 15 to 20 minutes before I resumed the full loading state.This result answers (to some extent) nctnico's then untested speculation of the 10A version being less reliable than the 5A version he'd been playing with way back in 2015. Considering the abuse I'd applied to this 4 digit version, the reliability (so far!) seems to be holding up. Mind you, I entirely agreewith his thoughts and observations about the safety and EMI/RFI issues (but this is par for the course with most cheap Chinese kit - opening them up to defuse the various time bombs contained within is SOP for any seasoned 'cheapskate' collector of such "Sows's Ears").In this case, turning it into a "Rayon Purse" will be difficult enough and you'd best give up all hope of ever converting it into some facsimile of a "Silk Purse" - it is what it is, a cheap 'n' cheerful bench supply to experiment with less critically sensitive projects or testing the immunity to noisy DC power of kit designed to be powered via a DC input jack.As for my own efforts, as well as optimising TheStuffMade gate drive mod, I also threaded about 40uH's worth of small ferrite rings over both output wires between the main board and the banana binding posts for common mode filtering, along with a 470nF 250vac mylar cap at the board end and a couple of 2.2uF 100v mylar caps across the banana binding posts plus about 10uH's worth of ferrite rings over each individual wire to provide additional differential mode filtering.I also replaced the rather weedy earthing link wire connection to the C14 PE tag with a toroidal choke to reduce pollution of the mains wiring with common mode RFI and tied the negative to the case ground with a 3k3 1W resistor to suppress the 25vrms 50Hz imposed by the various Y caps linking to the half mains live voltage from the HT rectifier pack down to half a volt (also neatly providing a relatively lo-Z 'static drain' connection). That choice of resistor still permits it to be safely stacked in series with another psu up to 50v (60v at a push).These measures reduced the switching spike noise from its original 1.5v ppk down to a "mere"50 to 60mV at the full 680MHz BW of my 'scope (the 20MHz BW setting reduced this by about another 12 dB), leaving the 'ripple' unaltered (varying between 2 and 14mV ppk depending on voltage and loading). It's still far from being a 'stellar result', more a case of being just "meh" rather than "absolutely horrendous".I did try nctnico's Y cap mod to link the output switching noise back to its source in the HT rectifier pack but I couldn't detect any improvement in spite of using a couple of 8nF caps in parallel for "maximum smoke"... er benefit. I think the rather spread out circuit layout, in spite of my linking the centre tap of the secondary to the nearby negative rail of the HT connection to the low side FET's source to minimise unwanted inductance, rather defeated this attempt at reducing this capacitively coupled switching noise between the primary and secondary sides of the transformer.I'm beginning to think this switching noise is being magnetically radiated directly from the transformer rather than via just the usual conductive pathways. Fitting a copper shield around the whole transformer to short out leakage flux might provide a significant level of mitigation in this case.I might try making up such a copper 'shield' to fit over the transformer to test this hypothesis when I'm next in the mood for yet another modding session. Anyway, I thought it was worth mentioning just in case anyone else fancied trying out this possible solution.Incidentally, for anyone looking for a cheap smpsu based bench supply where the cheapest or cheaper option is "The wrong mains voltage type for their region", the conversion from 220 to 110 v mains is merely a matter of linking the junction of the two smoothing caps to the neutral side AC connection on the bridge rectifier or removal of this link to convert a 110v only unit into a 220v unit. Adding this link converts the full bridge rectifier into a half bridge full wave voltage doubler (+/- 170vdc wrt neutral/capacitor junction - a total of 340vdc from the 110vac supply). Removing this link on a 110vac unit converts it back into a full bridge circuit giving the same 340vdc total off a 220vac supply. This is the standard way to add a dual mains voltage selector switch to smpsus which came into general use with the advent of the IBM desktop PC (and clones) some 40 years ago (and it wasn't a new idea even then!).One of the very first things I did with this bench psu was to disconnect the voltage selector from the main board to eliminate the risk of the selector switch being accidentally (or deliberately) moved to the 110v position since I have absolutely no plans whatsoever to operate it outside of the UK.I did mark the original connection points to cover the extremely unlikely need to operate it from a 110v supply at a later date. I'd had it in mind that if I didn't, Sod's Law would guarantee that such a need would arise no matter how extremely unlikely this seems (second guessing the Lords Sod and Murphy is a losers' game). The 'time bomb' in this case being the power fet's heatsinks' tending to lean towards each other to varying degrees of (undesirable) 'intimacy', some to the point of touching each other at the slightest hint of vibration (as a few unfortunates discovered to their cost). All those YT tear down videos provide a valuable service in forewarning the prospective purchaser over what needs sorting out to avoid the "Tears Before Bedtime" effect.