Power Supply Issue

[Shibby]

Hi,

Last night I set up an imaging run and found that, around 30 minutes after I went to bed, my power supply had shut down. I have a 30A regulated bench supply, originally from Maplin, but I believe it is this (an EP-925): https://www.manson.com.hk/product/ep-925/

The warning light had tripped and the supply turned off. This has happened twice now, but the first time was quite a long time ago, so it may be very infrequent. I had to turn it on and off several times before the power was restored.

At my mount, I have my own DC distribution board with each device fused separately, so an overload seems unlikely.

Could the supply be faulty? My cabling to the mount? I'm not sure how to diagnose the problem - any ideas?

Thanks!

[Dr_Ju_ju]

I have exactly the same power supply from Maplin, which is now a good many years old, and I use it for a 'Lab' workshop supply, but I've never had a shut-down....   

As these supplies are now old, initially I would give it a good clean out, especially if there is an internal build up of dust that could act as an insulator, on the power transistors, that not only may erroneously 'fool' the temperature trip, but also act as 'short' circuit across the terminals/current-voltage sensors.

But also being old, and I take it, lots of 'wear', then components do sometimes fail with age, so you may need to replace some parts...

 

  

[kbrown]

It's quite often the electrolytic capacitors that "wear out" first. Especially if they're frequently subjected to high temperatures.

Not related but I recently had a PC power supply go bang with some smelly smoke. Turned out a lot of the electrolytic caps had just had enough. Didn't even bother fixing it any more as the shorts the caps caused probably caused a lot of other components to fail as well...

You might want to look inside your PSU to see if you can spot any leaks or bulging of the el-caps. Maybe just replace them as a precaution, if you can access them easily (careful with the polarities!)...

Edited September 1, 2022 by kbrown

[Shibby]

Thanks for the tips @Dr_Ju_ju @kbrown

I've just found some time to open it up and have a look. Very little dust in there and no sign of bulging from the capacitors. I do notice some sort of leakage around the coils which is odd but I suspect this might just be glue or could it actually be a sign of overheating? See attached.

 

[Dr_Ju_ju]

That looks like an overspill of the epoxy varnish used in the manufacture of the transformer, and nothing to be worried about.... 

[Mandy D]

2 hours ago, Shibby said:

Thanks for the tips @Dr_Ju_ju @kbrown

I've just found some time to open it up and have a look. Very little dust in there and no sign of bulging from the capacitors. I do notice some sort of leakage around the coils which is odd but I suspect this might just be glue or could it actually be a sign of overheating? See attached.

 

If a transformer has overheated to the point of insulation breakdown or conductor failure, it will smell pretty awful. There would be no mistaking that kind of failure. A more likely failure mode for a small transformer like this would be a thermal fuse embedded in the primary windings. Check the windings for continuity with a multimeter on the ohmss range. Expect a few ohms to tens of ohms for a 230 volt primary and a fraction of an ohm or two for a 15 volt secondary.

I really would not worry about the goop on top of the core. It is not charred so all is well there.

[Dr_Ju_ju]

There is no transformer thermal fuse, according to a circuit diagram I've dug up https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925eng.htm  the site even offers some mods for better performance/safety...

[Mandy D]

8 hours ago, Dr_Ju_ju said:

There is no transformer thermal fuse, according to a circuit diagram I've dug up https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925eng.htm  the site even offers some mods for better performance/safety...

Don't count on it. Because this is an integral part of the transformer, it might not be shown on a schematic.

[Carbon Brush]

I'm with @kbrown on electrolytic capacitors.

These are a 'cost saving' component with known limited life.
By limited, I mean hundreds of hours at 60C for example.
Inside the box, the (another cost cut) transformer will chuck out lots of heat and 'cook' the electrolytics.

Just look at the big electrolytics that are in there and buy new ones with a known brand.
Not the cheapest 'Nippinoono' or whatever.
See what the likes of RS components sell and use that as a guide.

This costs you a few ££. It is a likely fix for an intermittent fault that will ruin many sessions.

HTH, David.

 

[Len1257]

The overload LED gets its signal from the measured current flow in one of the current share resistors. Could well be as a result of problems with the regulator circuitry but the most obvious cause of an overload is the external gubbins. You need to check rigorously the external cables / distribution units etc.  In particular if you are using ordinary PVC cables they can crack if moving under freezing conditions.  

[wulfrun]

1 hour ago, Mandy D said:

Don't count on it. Because this is an integral part of the transformer, it might not be shown on a schematic.

Unlikely in a transformer of that size. Besides, if it were there and it tripped it'd be a dead transformer now. They are put into small transformers because it's not feasible to protect them with a normal current-fuse. A large transformer can and should be protected with a normal fuse.

[Mandy D]

23 minutes ago, wulfrun said:

Unlikely in a transformer of that size. Besides, if it were there and it tripped it'd be a dead transformer now. They are put into small transformers because it's not feasible to protect them with a normal current-fuse. A large transformer can and should be protected with a normal fuse.

OK, fair enough. If the transformer is working, then clearly it cannot be what I suggested.

BTW, that is a small transformer. 😁

Here is a link to the thermal fuses we use inside small transformers like the one in that power supply:

https://uk.rs-online.com/web/p/thermal-fuses/7976030/

They are 15 A rated.

[Shibby]

1 hour ago, Len1257 said:

the most obvious cause of an overload is the external gubbins. You need to check rigorously the external cables / distribution units etc.  In particular if you are using ordinary PVC cables they can crack if moving under freezing conditions.

This is what I'm worried about! The main cable runs through guttering underground so I'll have to pull it up to check. If I can't find anything, perhaps I'll try @Carbon Brush's suggestion of new capacitors.

[Dr_Ju_ju]

7 minutes ago, Shibby said:

This is what I'm worried about! The main cable runs through guttering underground

How long is this cable run, and what type of cable are you using ??  Personally, and what I use for star camps etc., are cables rated for power and temperature extremes e.g. https://www.ebay.co.uk/itm/322123764915?hash=item4b001294b3

they are not cheap, but peace of mind....  btw, I also use Neutrik NL2FX Speakon Connectors, which are self locking/wiping on connect.....

[Shibby]

15 minutes ago, Dr_Ju_ju said:

How long is this cable run, and what type of cable are you using ??

I had to look through my order history... Looks like I opted for a massively overrated cable! The run is only about 4m.

https://www.12voltplanet.co.uk/P00831.html

[Dr_Ju_ju]

....  according to their published specs., they should be ok, but as we've been experiencing some weird temperature extremes, I'd check them out, especially the sheathing 

• Nominal current rating: 110A
• No./size of conductors: 206/0.30mm
• Conductor cross section: 16mm²
• Maximum overall cable diameter: 8.3mm
• Resistance per m at 20°C: 0.00126 Ω
• Conductor material: Plain copper
• Insulation material: PVC
• Working temperature range: -30°C to +70°C
• Chemical resistance: Petrol, lubricating oils and acids

Part Number:  PVC16R.100

[wulfrun]

1 hour ago, Mandy D said:

BTW, that is a small transformer. 😁

It's a case of context, no? It's tiny compared to (say) a power substation transformer. In a domestic appliance setting, it's getting pretty big (probably 400VA ish?). Small, in terms of a thermal fuse being the only option for safety, is somewhere up to around 15-20VA. A properly designed setting using a 400VA transformer shouldn't require a thermal fuse and I'd be mightily miffed if the designer cheaped-out by using one (of the buried-in-windings, non-replaceable type).

[Mandy D]

4 minutes ago, wulfrun said:

It's a case of context, no? It's tiny compared to (say) a power substation transformer. In a domestic appliance setting, it's getting pretty big (probably 400VA ish?). Small, in terms of a thermal fuse being the only option for safety, is somewhere up to around 15-20VA. A properly designed setting using a 400VA transformer shouldn't require a thermal fuse and I'd be mightily miffed if the designer cheaped-out by using one (of the buried-in-windings, non-replaceable type).

Yes, I agree it does look like the designer might be "cheaping out", but very often it is the customer that is forcing such a decision on the designer. The domestic microwave oven is the best example I can think of where such cost-cutting is taken to the extreme and transformers of 1500 VA rating are not uncommon! These are produced on a production line with one transformer hitting completion every 30 seconds as part of the "cheaping out" process! Thermal fuses are used in these and once the transformer has been running hot enough and long enough for that fuse to fail, the insulation will be in bad shape and no way should you risk putting it back in service.

If that PSU is designed for 13.8 V, 25 A output, as I think I have read, then a properly rated transformer feeding a rectifier with capacitor filter and having sufficient overhead for the regulator will need to be closer to 600 VA for continuous use, or is it assumed that the supply will only be used intermittently?

Anyway, I guess this does not help resolve the OP's problem, so I'll leave it at that.

[DaveL59]

7 hours ago, Shibby said:

This is what I'm worried about! The main cable runs through guttering underground so I'll have to pull it up to check. If I can't find anything, perhaps I'll try @Carbon Brush's suggestion of new capacitors.

Any chance a rodent has decided to visit that ducting/guttering your cable is run in?

They are notorious for gnawing at insulation and causing shorts/failures. Any moisture build up in the duct would then cause issues and mice to pee a lot which would make a good conduction medium as well as unpleasant to clean out.

[Shibby]

Update

I eliminated all other causes by making up a new, temporary main power cable. The power supply still entered "warning" mode on the next 2 nights. Once in warning mode, I tried restarting it with each piece of kit disconnected in turn. Eventually, with everything disconnect the supply still remained in warning mode!

So (and somewhat lazily) I decided to just pick up a new power supply. Our second baby is nearly 2 weeks overdue so I don't anticipate much free time in the near future!

I now have a Tenma 72-13350 and it worked perfectly all night last night. I like that it has an on/off button for the output.

If anyone wants the old supply and is willing to (preferably) collect from Peterborough or pay for the postage, they're welcome to it.