Am I overloading this adapter?

[bungeejumper]

I am a confirmed ignoramus in physics-related matters, having been cruelly excluded from school science lessons 55 years ago after an unfortunate little experiment in class with a lead acid battery. Which is why I hope that somebody more clued-up can set me straight?

We have two identical Hozelock pond pumps, which each run on a 24 volt AC power supply from a plug-in adapter that's plugged into a socket in the garden shed. Each of these two pumps is rated at 15 watts, so that's a total of, errrm, 30 watts. (I'm quick like that. ) The Hozelock adapter units are each rated as 24V AC, 1,000 milliamps, or 24 watts - exactly the same info on the label as this one (which is another brand):

So far, so good. But it occurred to me recently that perhaps I should try running both pumps off just the one adapter, and that that might save on running costs? So I did, and it all works perfectly. I fancy, however, that the adapter is getting warmer than some other adapters I have known in the past. It's about as hot as you'd want your bathwater to be.

Would that be because I'm trying to impose a higher load than the adapter's maximum wattage? (Is that even possible? Or would it just say to the two pumps "sorry, lads, 24 watts is all you're getting, share it out between yourselves as best you can"?)

Or am I imagining the whole thing, and AC adapters just get to those temperatures all the time?

If it helps (and it might not.....), I note that this adapter's label says "T = 130 degrees C", and it certainly isn't as hot as that. Not even halfway!

The adapter doesn't seem to be in any distress - no scorching, no smells. Am I running it beyond its limits?

BJ

[Itsallaguess]

Am I running it beyond its limits?

Yes, and I'd recommend that you stop doing so at your earliest opportunity...

Cheers,

Itsallaguess

[kiloran]

You're trying to get 30W out of a 24W (rated) adapter. It'll work, as you have found, but it will be more stressed. A professional-grade piece of kit would probably be OK, but a consumer-grade adapter like this will be designed to lower specs. The adapter will not limit the output to 24W, the actual output will be somewhere between the rated 24W and the demanded 30W.

It may work for a very long time indeed if it was designed and built well, it might fail quite quickly if it's a cheap bit of kit. How will it fail? Maybe just abruptly cease to work (probable), maybe set on fire/melt (less likely but possible).

I would make sure that the position of the adapter does not make it a fire hazard, or create a risk of burns (especially if a child might touch it)

--kiloran
edit.... another potential failure mode..... the 240V input might get through to the 24V output. Unlikely, but extremely undesirable. Are you feeling lucky?

[Urbandreamer]

OK first let us assume that there is some margin and leave until later the question of if a 15W pump consumes 15W of power.

Instead let us consider the case of taking two instead of one 15W pumps and supplying them using the same cable and adaptor (yes I know you are probably using seperate cables).

The cable and adaptor will need to provide double the current (amps).

Let's ignore the adaptor and just consider the cable.
It has a resistance, just like the wire that provides the eliment in an electric fire or oven.

The heating effect formula is I2Rt where I is the current and the 2 should be superscript meaning I squared. By doubling the current you have increased the heating effect by a factor of four. If you increased the current by 4 the heating effect would be 16 times as much.

Another simplified formula is P=IV. Hence 30W at 24V is 1.25A or 1250mA.

Ok now let's consider the adaptor. It's got wires in (probably a transformer). So four times the heating effect in the addaptor too from when you just run one pump.

Now let us consider WHY the addaptor has a limit at all.

Well the temperature caused by the heating effect can "burn out" wires or tracks on a PCB. Transistors (if part of the adaptor) also have upper current limits.

IF, the pumps are both consuming 15W then you risk burning out the adaptor. Usually there is a "fuse" or other safety device that should cause the adaptor to stop working before the heat can cause an actual fire. Cheap knock off's sumetimes skip that safety device.

I would not advise running two 15W pumps off that adaptor. It certainly won't reduce running costs.

Finally the pumps. Don't go down that rabbit hole please. The power a pump draws depends upon it's design and the pipe work/ obstuctions, flowrate. Central heating pumps have computers built in to regulate and economise how much power they draw.

[bungeejumper]

Thank you all, that's really helpful. This isn't a sophisticated set-up (the two pumps and their adapters were only thirty quid apiece), so I'd be a fool to expect super-tolerant technical standards. But you've answered my main query about whether too much load could over-stretch the power supply, as distinct from the power supply saying "stuff it, that's all there is and there ain't no more".

Back to the drawing board, methinks. Darn it, it looks like my daily running costs (12 hour day) have just doubled from 6p to 12p

BJ

[GrahamPlatt]

Your costs won’t have changed. It’s the power the pumps are consuming that’s the cost. Indeed, your cost may go down with adding an extra adapter, as you’ll be losing less to heat (possibly). Whatever, your risk of losing the unit will diminish considerably.

[Itsallaguess]

Your costs won’t have changed.

They would have done eventually.

Just not in the way he was hoping for...

Cheers,

Itsallaguess

[Urbandreamer]

I'm not sure what you are trying to achieve or what your system is.

However remember what I said about pump power consumption being dependent upon the system.

Obviously you can't fit 1.5 pumps. However if you regulate one pump so that it pumps half the volume of the other, then assuming that the pump is a centrifugal pump, you may have reduced the running costs by 20% or so.

Like I say, it's a rabbit hole.
https://www.inspection-for-industry.com ... -test.html

Note that link presumes the common type of pump used for pumping fluids. It doesn't apply for constant volume (piston or screw) pumps.

I have assumed that two pumps are used to fill the same tank.

Then again you might have two pumps in series.
https://blog.craneengineering.net/benef ... ugal-pumps

Or you could be using two filtration circuits. The filters may have different requirements. Possibly one circuit could be on a timer, reducing running costs.

What is not a good idea is trying to run 1.25A through a circuit intended for a max of 1A.

[swill453]

Back to the drawing board, methinks. Darn it, it looks like my daily running costs (12 hour day) have just doubled from 6p to 12p

Wow, how much do you pay for electricity?

Scott.

[jfgw]

This power supply uses a transformer. If you double the load on a transformer (by running two pumps instead of one), the transformer will draw double the current from the mains (approximately). Two tranformers running one pump each will use about the same amount of electricity as one transformer running two pumps. You will not save electricity by running both pumps off of one transformer.

The "T=130°" is above a symbol for a fuse. There is (or should be) a thermal fuse as part of the transformer itself. If the transformer winding gets too hot, this fuse blows. It cannot be reset so the whole power supply will need to be replaced.

A cheap, knock-off power supply may still bear this symbol but may not have this safety feature. The case may also be made of a plastic which is not flame retardant.

The power supply in the photo shows "24VA". With a resistive load such as a filament lamp, VA and watts are the same value. With an inductive load such as an electric motor, VA will be greater than watts. A 15W motor will be more than 15VA. Two 15W motors will be more than 30VA.

I would expect a transformer to fail relatively much more quickly if you run two pumps from it. If it is a cheap illegally imported one, your shed might burn down.


Julian F. G. W.

[bungeejumper]

Brilliant, thank you jfgw. Just the kind of detailed response I was hoping for, really helpful. And thanks also for confirming what others have said about how using one adaptor instead of two conveys no advantage whatsoever, because it pulls a variable amount of current that''s determined by the load, not a fixed amount. That's the kind of thing that my schoolteachers would have taught me if only they hadn't been so keen to get rid of me.

Right then, so where did I put that second adapter? It must be around here somewhere. Come out, come out, wherever you are.......

BJ

[PrincessB]

I did reply earlier, but for some reason my posts go missing once in a while.

I did wonder whether a laptop charger might be an option to run both pumps?

You'd want 24volts and as I understand these things (shorthand for I might be wrong) a laptop charger would use a swich mode system which could do the conversion from mains to 24V more efficiently.

I also understand that power supplies of this ilk run most efficiently at about 50% load.

So a 24V 60watt laptop charger and a bit of tinkering with the wires and you have a solution.

B.

I've just had a quick root around in the PC bits cupboard and found a 19V 65watt unit (which would probably work), so something suitable must exist.

[Urbandreamer]

I confess that I do think an adaptor per pump is the sensible choice.

It makes the wiring easier and doesn't increase the running costs.

A laptop supply is unlikely to work in this case as they almost universally provide DC to the laptop while BJ's original adaptor produces AC.

I did a quick ebay search and found a couple of people selling Hozelock 24V 60VA (60/24 = 2.5A) pump transformers if there really is a desperate desire to just use one "adaptor".

[bungeejumper]

Thanks yet again to everyone for all the useful information. And thanks also for not taking the pee over my misunderstanding about how the maximum ratings on these adapters work. (I'd honestly assumed that the 24 watt output was the max that it could deliver, not the max load that it ought to be asked for!) Very gentlemanly of you to let me off so lightly, under the circumstances.

Right, the search continues for the second adapter, which is still lurking around the premises somewhere. On a slightly brighter note, I checked up on the spec for my pumps yesterday and it seems that they're rated at 11 watts each, not 15 watts as I'd thought. Now, 22 watts isn't as bad as 30 watts where the adapter is rated at 24 watts, but it's still a bit too close to the sensible line. I shall give the matter my most earnest attention.

Thanks also to those who reminded me that Ebay is full of cheapo adapters which might be falsely claiming to attain the necessary technical standards. In this particular case I think it's unlikely that a prominent brand like Hozelock would take that kind of a shortcut (think of the negative publicity), but the warning is well taken anyway. You can't be too careful.

Right, back to the chaos that passes for my lockdown workshop. The search continues. I may be gone some time.

BJ

[csearle]

Haven't read it all, sorry, but good practice is not to exceed 80% of an item's rated power capability, even if it works. Function is one thing safety is another.

Chris