Cheap (Crappy?) bench PSU found on eBay UK  - Page 1
Forum section: electronics / general

author: mianchen
author_rank: Regular Contributor
timestamp: November 12, 2012, 12:11:08 am
content: There were some of theseand some Mastech ones for sale as 'with faults or faulty blah blah' on eBay recently for < £50. I bought a 5A 3 output one myself. It was brand new and perfectly working when it arrived. I haven't got time to open it up yet, but it seems be working fine and does delivery 5A on both channels...Now the seller has some more for sale again, I'm not sure they are good or just a piece of crap, but for £50 delivered I think it's ok isn't it?Anyway, I'm not trying to do advert for the seller, you can search eBay yourself if you are interested. Just trying to let ppl know that there are some cheap PSU on eBay UK that MIGHT be a bargain (or CRAP - in that case don't blame me)...

author: RJSC
author_rank: Regular Contributor
timestamp: November 12, 2012, 12:28:04 am
content: Damn...Seller only ships within the UK...QuotePeople outside the UK, don't even need to bother...

author: mariush
author_rank: Super Contributor
timestamp: November 12, 2012, 03:39:46 am
content: The one in the pictures is a Caltek :http://uk.farnell.com/caltek/psm2-5a/psu-laboratory/dp/1270465Open it up and inspect the solder joints and everything else, I've seen repair threads about Caltek threads that were saying quality control at Caltek was almost non-existent

author: Monkeh
author_rank: Super Contributor
timestamp: November 12, 2012, 04:50:44 pm
content: Well, I've just bought one..

author: DaveW
author_rank: Frequent Contributor
timestamp: November 12, 2012, 09:09:26 pm
content: In terms of QC they're a mixed bag, but in terms of power and capability per buck, they're pretty damm good. I've had one for several years and it's been solid, a fairly quiet output, pretty accurate readout. The major downside is that you have to turn the output on to set the output voltage, and short the terminals to set the current limitIf you want to have a play, this might be useful,http://www.rcgroups.com/forums/showthread.php?t=1030827

author: alanb
author_rank: Frequent Contributor
timestamp: November 13, 2012, 10:28:52 am
content: I managed to obtain a Mastech 30v 3a PSU described as faulty for £29 inc postage. My intention was to use the case and hopefully the mains transformer as the basis of a home made PSU. When it arrived the only fault was a damaged case.

author: Monkeh
author_rank: Super Contributor
timestamp: November 15, 2012, 01:44:16 pm
content: Well, it's arrived. I haven't done anything but look at it yet, but I bought the one which said 'rattling noise'.It's totally new in box, never even been plugged in. The rattling noise is a 35V 2200uF cap which broke off at the solder pads. Oh so terribly hard to repair.E: Cap isn't even used, there's no wiring to that section of the power board, the bridge rectifier isn't there, etc. Looks as if it's for the fourth output (this is a 2/2A unit).

author: shebu18
author_rank: Frequent Contributor
timestamp: November 15, 2012, 08:18:48 pm
content: Please put a link of the ebay product.

author: Monkeh
author_rank: Super Contributor
timestamp: November 15, 2012, 08:46:27 pm
content: Quote from: shebu18 on November 15, 2012, 08:18:48 pmLong gone now anyway, and they don't ship outside the UK.

author: tom66
author_rank: Super Contributor
timestamp: November 17, 2012, 09:40:54 am
content: We use these in our microcontrollers lab (the 30V/3A plus 5V/1A model) and they have a fairly major flaw. They blow fuses all the time! Turn them on, then "pop" fuse goes, have to replace it (in the rear of the unit), then it works OK.

author: Monkeh
author_rank: Super Contributor
timestamp: November 17, 2012, 12:23:42 pm
content: Quote from: tom66 on November 17, 2012, 09:40:54 amI'm guessing you have the ones with toroidal transformers? Apparently they do eat fuses.Easily fixed.

author: tom66
author_rank: Super Contributor
timestamp: November 17, 2012, 01:11:14 pm
content: Quote from: Monkeh on November 17, 2012, 12:23:42 pmAbout 5-10% of the time after turning it on it'll blow the fuse. Teacher thinks that students keep shorting them out... I didn't feel like pointing out that they are CC/CV.How do you fix it? Bigger fuse? May be OK for the average user but I think the health and safety people would get upset as it is at a university.

author: Monkeh
author_rank: Super Contributor
timestamp: November 17, 2012, 01:14:09 pm
content: Quote from: tom66 on November 17, 2012, 01:11:14 pmInrush current weakening the fuse.QuoteBigger fuse works, yes. Mine has a 1.6A for example, and the first time it blows it's getting a 3A ceramic fitted. Still quite capable of dealing with an actual fault.Alternatively, limit the inrush current.

author: SeanB
author_rank: Super Contributor
timestamp: November 17, 2012, 01:29:46 pm
content: Slow blow fuses? Alternatively add a NTC inrush limiter to the power supply input side. 5R units will work, and are either cheap or free from an old PC power supply unit.

author: tom66
author_rank: Super Contributor
timestamp: November 17, 2012, 02:31:52 pm
content: I suspected inrush current from the main filter caps, but it could be the transformer too. I have a 3000VA variac which will trip the main breaker almost every time it is plugged in.

author: T4P
author_rank: Super Contributor
timestamp: November 17, 2012, 05:09:38 pm
content: Definitely due to inrush ... Huge (>300W) toroidals have massive inrush current and a variac is in the shape of a toroidhttps://www.eevblog.com/forum/product-reviews-photos-and-discussion/vantek-dps-3305p-32v5a160w-programmable-dc-power-supply/msg97135/#msg97135

author: tom66
author_rank: Super Contributor
timestamp: November 18, 2012, 07:02:26 pm
content: Oddly, I have found my variac will only trip the mains about 5% of the time if it's set to 0% output first. If it's set to 50~115%, it's more like 50% of the time. I wonder why this could be? I think I need to exchange the breaker for a less sensitive one, but I wouldn't know what to use. Currently type "B" 32A 250V.

author: Monkeh
author_rank: Super Contributor
timestamp: November 18, 2012, 07:05:44 pm
content: Quote from: tom66 on November 18, 2012, 07:02:26 pmIt trips the MCB? Impressive.I've never seen a type A before. They start at B (or older types, 1) to avoid confusing with the other A.Changing the breaker will require testing to ensure the replacement will trip appropriately. You may be better off just adding some inrush limiting to the variac.

author: tom66
author_rank: Super Contributor
timestamp: November 18, 2012, 07:08:48 pm
content: Quote from: Monkeh on November 18, 2012, 07:05:44 pmWell it is 3,600VA or so, 15A at 240VAC. I scored two of them on eBay for £52, which has to be my best find so far. (I sold one, simply by offering a courier, instead of pick up only, for £150.)It is type B, I was mistaken in saying type A. Rather dodgy, perhaps, but the shed circuit runs off the utility room ring main through some overhead T&E. (I didn't install it.) So the 32A rating presumably has to include the fridge, freezer (x2) and tumble dryer. And the T&E is rated for only 32A, as is the ring main wiring. So I'd probably be best sticking with a 32A breaker.

author: Monkeh
author_rank: Super Contributor
timestamp: November 18, 2012, 07:14:11 pm
content: Quote from: tom66 on November 18, 2012, 07:08:48 pmWell, inrush current limiting for that would be a task, but not too expensive..QuoteWhereas fixing that 'installation' (I use the term very loosely) would probably cost a few hundred quid.

author: tom66
author_rank: Super Contributor
timestamp: November 18, 2012, 07:17:31 pm
content: Quote from: Monkeh on November 18, 2012, 07:14:11 pmI considered using something like this. I also built a relay inrush limit circuit (which tied three 680ohm 25W resistors in parallel with variac, then used a relay to short them out after approx 1 second), but I never got around to using it as I'm stuck doing a MEng now and priorities...Quote from: Monkeh on November 18, 2012, 07:14:11 pmIt's a matter of convincing parents to change it, so I doubt I have a chance.

author: SeanB
author_rank: Super Contributor
timestamp: November 18, 2012, 07:22:24 pm
content: Inrush limiting is easy, use a contactor with pneumatic delay ( on eBay cheap) to bypass a parallelled trio of 10R 20W wirewound resistors. I did the same with my variac, though I used a 48V relay and drove it from a capacitive dropper with around 470uF of capacitance to give a delay on power on. Not much delay is needed, under 1 second is fine. Thermal fuse on the resistors ( in case the relay dies) and away you go.