Shocking News!

[saac]

I've been working on my electrics in the observatory, getting things more permanently wired up now that I've got a main supply installed.  As part of the setup I installed a little panel meter to measure the voltage and current on my dc supply (Maplin XM21X). I gave it a wee test run and to my horror rather than providing a regulated 13.8V it was supplying a regulated (I think) 21.5 V.  I've had this power supply for the past year, bought from Maplin, and it's been powering my AZEQ6 without any problem; until now I've haven't put a voltmeter across it so it may well be just a recent fault (I hope it is at least).  The receipt for it is long gone (habit of mine not keeping receipts), so I'm not really sure what I can do with it now; I'm assuming that there is no internal adjustment to be made. I guess I could drop the voltage externally but it makes me wonder what else may be wrong with it.  I wonder if anyone else has had similar problems with the XM21X, I know it's a popular power supply.

 

Jim

Edited August 13, 2016 by saac

[SilverAstro]

That is a bit shocking ! A cheap low class supply may produce 21v off-load and drop to reg 13v on load. But I would not have expected that of a Maplin one ( or would I ? ! )

Did you measure that while it was supplying your mount, or open-ended ?  Have you something unimportant to hang on it to see ?

[Starwiz]

Hope I'm not stating the obvious, but is the meter switched to read AC not DC?

The reason I'm asking is that low cost digital multi-meters don't read true RMS AC volts, but use a diode to convert the AC voltage to DC then use a multiplication factor to come up with the result.  DC voltage passes quite happily through a diode, so if you had AC volts selected on your meter it could show around double the actual DC voltage, thus fooling the meter.  Swapping the leads over will show zero volts as current can only pass one way through the diode.

Just a thought 

John

Edited August 14, 2016 by Starwiz
Missed word insertion

[Skipper Billy]

Have you tried your meter on your car battery to check it is properly calibrated?

 

[101nut]

There are three types of supply

1) Unregulated - voltage varies and drops with load

2) Regulated on load - voltage can vary but will regulate once a minimum load is reached.

3) Fully regulated - voltage output is independent of load. 

I suspect the Maplin supply will be (2). Check the spec of the supply and see whether it needs a minimum load, stick a resistor, lamp or whatever across the supply to generate that load and measure the voltage again.

AndyG  

[saac]

Funny my initial thought (hope) was that what I was seeing was a no load voltage (supply emf) and that it would drop once connected to a load; what AndyG describes as regulated on load.  But the same voltage was measured under load both on the mount and on bench load.  Not a calibration or meter setting issue , I wish it was a simple mater of fat fingers (has happened before) but not this time.    

Under bench load and while  supplying the mount the voltage read 21.5V and didn't drop to the expected 13.8V.  Now the mount was only loaded lightly with my SW 80mm but even with slewing on both axis the voltage held at 21.5V.  I checked the mount after using a power bank and it all worked well, no harm done.  The mount itself must have an internal voltage regulator.  

Just thinking if the XM21X supply is regulated on load it may well be that a heavier mount load would see the voltage supply regulated to 13.8V.  I suspect that its been operating like this since I got it and the mount clearly hasn't suffered any harm.  Some more investigation needed I think when the mount is properly loaded up.

 

Jim

 

[Gina]

Something definitely not right there.   Glad the mount is alright though

[saac]

Yes it's got to be goosed I'm afraid. A no load supply of 21.5 V against a regulated output of 13.8 V is just a little too high of a margin for comfort.  Time to look for a new psu, I'll consign this one to use in the garage for less sensitive tasks.  Just out of interest, the AZ's manual advice on power supply is copied below. It suggests a max of 16V,  looks like they built in a reasonable margin thankfully.  Off to look for a new psu.

 

Jim

from AZ EQ6 ops manual:

"Output Voltage:  DC 11V (minimum) to DC 16V (maximum). Voltage not in this range might cause permanent damage to the motor controller or the hand controller. • Output Current:  4A for power supply with 11V output voltage, 2.5A for power supply with 16V output voltage. • Do not use an un-regulated AC-to-DC adapter. When choosing an AC adapter, it is recommended to use a switching power supply with 15V output voltage and at least 3A output current. "

[steppenwolf]

I have exactly the same PSU as you and can confirm that offload, the voltage immediately settles at 13.8v and, of course, remains there under load. Sadly this does indicate that your unit is indeed faulty.

[sloz1664]

I agree with Steve. Fully regulated fixed 13.8 volts dc supply from Maplins, I have two and they have never faltered. Although I do have voltmeters indicating the relevant voltage sources.

 

Steve 

 

[saac]

Thanks Steve and Steve, good to have positive confirmation on the psu's normal output characteristics.  In one respect, it looks like I've been lucky that the mount hasn't suffered, I suspect it has been operating like that for some time.  Also ironic that the work I've been doing on the observatory was to install a control panel - few switches for the mount and ancillaries and a panel meter to monitor the supply to the mount.  I should have known better but that is the first time I have checked the supply output. That little panel meter could well have just saved my mount from being fried! It may be cloudy tonight but I'm happy

 

Jim

[steppenwolf]

A lucky escape but I'm very pleased for you! I have a nifty little LCD voltage reading module connected to my PSU and it is very reassuring to see its blue light telling me that all is well.

[Gina]

I too have one of those Maplin PSUs and yes it gives 13.8v including off load.

[saac]

Thanks Gina. There is a Maplin store in Dundee, I'll take a drive in tomorrow after work and pick up a replacement.  This time I'll make sure to do two things - measure the output voltage and keep the receipt!

Jim

[The storm fisherman]

Hi Jim

i was just reading your thread contrary to popular myth you do not require a receipt as an ex Maplin manager a proof of purchase is all that is required  IE a bank statement and also i can confirm that there is a fault with regulator unless they have change the regulation circuit there is an internal variable resistor to adjust the voltage i had great success in turning one of these into a variable voltage psu by mounting a pot variable resistor out to the front panel.

[saac]

Picked up a new psu all is well, looking like clear skys tonight as well - one happy astronomer

 

Jim

[saac]

10 hours ago, The storm fisherman said:

Hi Jim

i was just reading your thread contrary to popular myth you do not require a receipt as an ex Maplin manager a proof of purchase is all that is required  IE a bank statement and also i can confirm that there is a fault with regulator unless they have change the regulation circuit there is an internal variable resistor to adjust the voltage i had great success in turning one of these into a variable voltage psu by mounting a pot variable resistor out to the front panel.

Thanks for that, I must admit I've always had good a experience with Maplin, they do tend to be very reasonable.  I've been a bit silly though - I opened the faulty psu to have a look inside.  The variable resistor you described was  there (2 x) one I believe to control the regulated output the other not sure - part of the regulation circuit.  Could not see anything obvious - capacitors all looked good as was the external fuse.  On the back panel it says there is also a internal fuse but I couldn't spot it - maybe a thermal fuse on the transformer - something was wrapped in heat-shrink coming off the transformer. I may yet have a word with Maplin to see if there is anything they can do but I'd need to let them know I've opened it - I think that will the killer - ach well lessons learned.  

 

Jim

[The storm fisherman]

1 hour ago, saac said:

Thanks for that, I must admit I've always had good a experience with Maplin, they do tend to be very reasonable.  I've been a bit silly though - I opened the faulty psu to have a look inside.  The variable resistor you described was  there (2 x) one I believe to control the regulated output the other not sure - part of the regulation circuit.  Could not see anything obvious - capacitors all looked good as was the external fuse.  On the back panel it says there is also a internal fuse but I couldn't spot it - maybe a thermal fuse on the transformer - something was wrapped in heat-shrink coming off the transformer. I may yet have a word with Maplin to see if there is anything they can do but I'd need to let them know I've opened it - I think that will the killer - ach well lessons learned.  

 

Jim

Hi Jim

Unfortunately if there signs of tampering they will not touch with a barge pole. I started as a Technical sales assistant when Maplins  were renowned for their expertise and i made the lofty heights of   shop manger 9 years later.  

  But all is not lost any good electronics enthusiast could probably fix it  but need to put a multi meter and a basic scope to find out where the fault is but my best guess it is still the regulator because you still a have dc voltage and 21 volts output and plenty of current, The transformer is good and  is big enough to cope it may be  a 24v dual wound transformer. so it has to be on the output side beyond the bridge rectifier stage as you have dc voltage just hence no regulation only full output. I do not suspect any fuses at this time. capacitors  will sometimes have bulge on top but not always there may be a transistor or diode as well. The fact there are two variable resistors one is voltage and the other is for current possibly connected to the big metal transistor on the heat sink on the rear, looking at a LM723 voltage regulator IC circuit pins 4 and sometimes Pin3 along with pin 13 is volt set pins this is as an example but this getting into technical territory.

Paul 

Edited August 15, 2016 by The storm fisherman

[saac]

Paul, yep I don't think I'd embarrass myself now by taking it into Maplin, I should have done that in the first instance - I'm too impatient at times.  I may ask the technicians at school to take a quick look at it, if there is no easy fix then I'll chalk this one down to experience. I'm just so relieved that the mount wasn't damaged.  Overall what is your experience with this line of psu regarding reliability? I was happy to go for a like replacement given the comments from other SGL users (above) and I must admit, I do like the build quality of it.

 

Jim

[Gina]

I think you were just unlucky in getting a bad one - as has been said, Maplin stuff is usually very good.

[The storm fisherman]

Hi Jim 

For electronics like this a 2-5% failure rate is to be expected as to a new peice of tech I think it was upto nearer 30-40% rate .

That unit has been sold for over 30 years which I think is testament to its reliability over the years and the simple component design . The 3amp little brother that I modded  is still going strong I spoke to its owner earlier this eve and must be at least 20 years old  I my self have A 30 amp mason psu with twin meters as I am also a licensed radio operator 

Paul

[saac]

Gina, Paul, thanks for the advice, appreciate it.  You know in the classroom I often remind pupils to confirm the output from the bench supply with a voltmeter and not assume it will kick out what the dial is set to. I should have taken my own advice.  I need to be more respectful with this stuff or I'm going to lose some expensive kit. At the end of the day  I think you are right in that I was just unlucky with this one, it wasn't meant to be my "forever" psu as Kirstie Allsop would say  The new one is humming gently in the background and the my little panel meter shows a steady at 13.8V.  I'm happy, and I've learnt something new about power supplies.

Jim

[saac]

Hopefully the last update on this story. Despite the experience from the first Maplin bench supply unit I bought a second one about a year ago .  I tested it on getting it home and all was ok; I didn't need it in the obsy straight away so it has been sitting in its box for the best part of the past year. Guess what I found when I went to install it - yep the voltage regulator had failed just like the first unit and was defaulting to an output of 21V instead of the stated 13.8V.  What is it with me and these bench supplies   Our technician at school confirmed that it was the voltage regulator that was goosed.  With Maplin having gone into receivership I had no return possibility and I was reluctant to scrap it.   So I've installed a little  programmable constant voltage/current programmable controller.  Installation was relatively straightforward and all seems to be working well so far - still need to test it under load.  So looking for the silver lining I now have a variable power supply  - fingers crossed  

I noticed that there is a another version of this unit styled  " with communication module" - If  I understand it correctly, it looks to be a usb module that would allow for remote monitoring of the output. I may consider doing a similar mod for the bench supply I have supplying the mount - could be useful for remote monitoring.  I wonder if anybody has any experience with such a setup or similar?

 

Programmable Voltage Controller

Programmable Voltage Controller with communication module

 

Jim 

 

 

Edited October 12, 2018 by saac

[JamesF]

I had one of these supplies fail a while back, only in my case it was giving out almost no volts at all.  I bought two at the same time however, so switched it out for the second.  I shall have to keep an eye on the new one to make sure it behaves itself.

James

[saac]

James, I was really disappointed when the first one failed as otherwise they are generally of a good build quality.  The replacement (which drives my mount) has been working fine now for the best part of a year although I have just noticed the output voltage has just recently crept up to 14 V (used to be steady at 13.8 V).  I got the second unit, again because I thought the general build quality was good, and was annoyed that it failed in the same way having only ever been in dry warm storage for a year.  Anyway, I think there must be a faulty component somewhere in the supply chain - interesting that the unit was discontinued when I had picked up the second one  (I got it at a discount).  If you haven't already done it then I would attach a voltmeter  to the output permanently just to give you confidence. I'm seriously thinking about replacing my simple panel meter on the first with a constant voltage meter like I did on the second unit.  

 

Jim 

Edited October 15, 2018 by saac