Korad KA6003P power on fault. - Page 1
Forum section: electronics / testgear

author: Marc
author_rank: Contributor
timestamp: August 04, 2013, 03:19:10 pm
content: UPDATE: I have been in contact with SRA-solder and am now being sent the correct (and tested) unit. They have been very helpful. In hindsight I should have stuck with the crappy amazon message system, next time I will go through their website:.http://sra-solder.com/Hey everyone, long time EEVBlog watcher and sometime lurker of the forums here.I ordered myself a KA3005P but was sent the 6003P by mistake.To top it off I was working on a new board, and its input caps unexpectedly exploded when I turned the unit on.This thing intermittently outputs 25 to 30V for a second when it is powered on. This is regardless of the settings.I have my own channel (Dave inspired) and have made a video on it:A heads up, I beeped out some swearing in it, this is also why it is not public yet.Now, returning it is not an option as I live in Ecuador and I used a postal forwarding service to get this here from the states. This has already cost me an extra $100 in taxes and postage.Hopefully it is fixable.I have had the cover off, and have not noticed anything immediately obvious. Although the next step is to completely take it apart and check the soldering on the main power transistors. Hopefully it is as simple as that, but I was wondering if others may have had similar problems or could direct me where to look.Thanks in advance.

author: Bored@Work
author_rank: Super Contributor
timestamp: August 04, 2013, 04:03:19 pm
content: Many cheap power supplies do this overshooting. Thats one of the reasons why people call them crap. Although if you tell people here this cheap stuff is crap you commonly have a bunch of baseball bat swinging idiots chasing you for daring to say the truth.A common reason why cheap crap does this is because the control circuit, analog and/or digital, takes longer to start up than the power stage. I.e. the power stage is essentially unregulated when turning the thing on. And the particular power stage is build in a way that it ramps up when not being regulates, as opposite to ramping down.In your case the output-on button seems to be a soft button. It does not seem as if it directly switches the output on/off, but that it just interacts with the control loop. I.e. as long as the control loop is not fully up, it has no effect.

author: Marc
author_rank: Contributor
timestamp: August 04, 2013, 04:51:52 pm
content: Quote from: Bored@Work on August 04, 2013, 04:03:19 pmWell I think I have proven the fault, the fanbois can get stuffed.Quote from: Bored@Work on August 04, 2013, 04:03:19 pmSo if this is a fault by design a potential solution would be making sure the control circuit is powered on first. Which may mean another transformer. second switch, or relays with some delays.For me something like this is worth doing as any test equipment too expensive or heavy gets a ridiculous tax here in Ecuador. First I would like to know is it a design or manufacturing fault?ie has anyone else seen this problem?, or can test to see if they don't.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 04, 2013, 05:05:31 pm
content: Until proven otherwise I would say it is a design error. Try to find someone with the same model, compare and you will know.The easiest thing to do is not connecting anything to the output until the thing is properly switched on, so just pull the plugs before you switch it off. An easy modification if you want to go that road is put a relay in the output that activates delayed, although I'm not 100% sure if it will not do the same thing when you power it off. In that case: just disconnect your stuff.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 04, 2013, 05:07:08 pm
content: Quote from: Bored@Work on August 04, 2013, 04:03:19 pmHi B@W,Thanks for that explanation.Let me see if I understood it correctly. What it means is that with such a (not so high end) Power Supply that the PS needs some time to stabilize after it is turned on (perhaps a few seconds); after this period of stabilzation the PS Off/On output button (not the main AC Power button) should be used with the device under test.If this is correct, this would mean that the safest approach would be to not use the main AC Power button at all while a device under test is connected, because when you turn the PS on the first time, or any subsequent time, the isolation circuit supposedly invoked by the PS Off/On output button might not really be ready to perform it's function.In summary, the safest approach would be to never power up (or also power down?) the power supply with the main AC Power button while a device under test is connected.Sequence:1. Turn on main AC Power button, let the unit stabilize for (5?) seconds2. Confirm the (Out) put button is off3. Connect the device under test4. Turn on the (Out) put button5. Run test6. Turn off the (Out) put button7. Disconnect the device under test8. Turn off the Main AC Power buttonPlease let me know if this is correct, or not. Thanks! EF

author: Marc
author_rank: Contributor
timestamp: August 04, 2013, 05:25:34 pm
content: Quote from: Electro Fan on August 04, 2013, 05:07:08 pmI have been using it since in this manner, also I have seen it spike to 15V when turning off the main AC although this is more difficult to re-produce.Another relay on the output is a good idea, a delay when turning on and no delay when turning off.I am thinking two relays in series one directly connected and one with a cap and transistor for the delay.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: August 04, 2013, 05:38:31 pm
content: Let me clutch at a straw.If you enable Over Voltage Protection and save the configuration, would it kick in early enough to stop the voltage ramping to 30V when the PSU is switched on?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 04, 2013, 05:38:34 pm
content: Quote from: Marc on August 04, 2013, 05:25:34 pmI'm not sure that the one without the delay will turn off before the output goes crazy, so you will need a lot of testing. The best thing to do would be to power it on its own, more or less directly from the switched AC. I assume that the control circuit of the psu will continue to function a little bit longer due to the caps in the thing.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 04, 2013, 05:39:30 pm
content: Quote from: MasterOfNone on August 04, 2013, 05:38:31 pmIt will probably not, because the delayed power up from the control stuff is the reason this happens in the first place.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 04, 2013, 06:01:24 pm
content: Ok EEVers, Let me guess - one of these might solve the problem:http://www.home.agilent.com/en/pd-1401523-pn-U8001A/dc-power-supply-30v-3ahttp://www.home.agilent.com/en/pd-1401542-pn-U8002A/dc-power-supply-30v-5a?nid=-35673.783965.00&cc=US&lc=enghttp://www.home.agilent.com/en/pd-836877-pn-E3644A/80w-power-supply-8v-8a-or-20v-4a?nid=-35718.384191.00&cc=US&lc=enghttp://www.home.agilent.com/en/pd-836877-pn-E3644A/80w-power-supply-8v-8a-or-20v-4a?nid=-35718.384191.00&cc=US&lc=enghttp://www.home.agilent.com/en/pd-835964-pn-E3632A/120w-power-supply-15v-7a-or-30v-4a?cc=US&lc=engSeems like every time you think you have solved some problem with test equipment you discover there are some more that could be solved and the answer is always the same - just some more time or moneySeriously, would the U8000 series or the 3600 series fully overcome whatever the Korad is experiencing? How/Why? Thx

author: mariush
author_rank: Super Contributor
timestamp: August 04, 2013, 07:19:35 pm
content: It would seem that the cheapest solution would be to add a relay that's by default off. Then you could use a one shot timer (for example a 555 chip) to energize the relay 5-10 seconds after you push the power button (you could power it directly from the transformer winding that's always used by default (if the psu uses a large transformer)

author: Electro Fan
author_rank: Super Contributor
timestamp: August 04, 2013, 08:35:06 pm
content: Quote from: mariush on August 04, 2013, 07:19:35 pmThat's a cool idea - maybe Korad should consider it.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 04, 2013, 09:21:09 pm
content: Another question for the guys who design/build power supplies - is there any possiblity that the control loop circuit as Bored@Work calls it is more likely to be agitated out of it's preferred state (or inhibited from getting to it's preferred state) by rapidly or semi-rapidly cycling the main power supply AC button on and off? Based on Marc's video and the much earlier video of the failed Korad it's now pretty clear that the Korad (and perhaps/probably other power supplies) can get into a state where they deliver unexpectedly high voltage; I'm just wondering if that is more likely to happen when a user is fairly rapidly turning the PS on and off (as would be the case when searching for an intermittent problem). (Or is it completely random?) At the end of the day, like all test equipment, I think a PS should be reliable and dependable, but maybe we can learn to help entry level power supplies stay in their preferred operating zone while we look for other solutions. Thanks

author: Marc
author_rank: Contributor
timestamp: August 04, 2013, 11:33:23 pm
content: Quote from: mariush on August 04, 2013, 07:19:35 pmPerfect I have a 555 and a relay on hand, if do not find an obvious fault then I will do this. Thanks.

author: Marc
author_rank: Contributor
timestamp: August 05, 2013, 03:22:06 pm
content: An update. I have been in contact with SRA-solder and am now being sent the correct (and tested) unit. They have been very helpful. In hindsight I should have stuck with the crappy amazon message system, next time I will go through their website:.http://sra-solder.com/