ONT power usage

[StuartHK]

Hi all,  Wondering if anyone can help me with a bit of information?  Namely, what’s the power requirements of the Openreach ONT?

I’ve seen a number of posts on this already but they all miss the point, unfortunately.  A UPS will need to power the PSU for the ONT at 230V so the significant question is the INPUT to the PSU not the output or low voltage input to the ONT, especially as transformers are so inefficient.  That is, the real question is the ? in 230V ?A  I suspect this will be a bit higher than absolutely necessary as it is the max input, but I also suspect they don’t over spec the PSU by much as the ONT is a one trick pony and so the load should be pretty constant.

Anyone any ideas?  Thanks in advance.

[Paul608085]

@StuartHK 

I asked a similar question a while back. I don't have the documentation to hand, but IIRC the ONT requres something like 7.5 watts.

So @ 230volts, that means around 0.033 Amps.

As you say, transformers are fairly inefficient - but for as long as I can remember now, just about all mains powered electronic equipment uses Switched Mode Power Supplies, which are far more efficient.

[StuartHK]

@Paul608085  Thanks for that.

I agree with your calculation but the point I’m making is that, without an actual figure for the efficiency, it’s largely a waste of time.  While I know mode switching is more efficient, I don’t know how much more.  Can you cite a source for the figures behind that?

[licquorice]

I'm bemused to know why you need that level of accuracy for the power consumption, surely an order of magnitude is sufficient.

[Paul608085]

@StuartHK 

"Can you cite a source for the figures behind that?"

No, as I said in my first reply I don't have the documentation to hand.

If it is important that you know the exact mains power consumption, why don't you buy a plug-in power meter and measure it?

 

[StuartHK]

@Paul608085 Now that's a good idea.

[Paul608085]

@StuartHK 

One thing to note is that Ohm's law can only be relied upon with DC, (Direct Current). It is only correct for AC, (Alternating Current) when the voltage and current are in phase - which only happens with a purely resistive load.

Old style transformer power supplies have an inductive load, whilst I think that Switched Mode Power Supplies present a capacitive load. The consequence of this is that in both cases the voltage and current peaks are out of phase with each other, (ie in one case voltage leads current and in the other it lags it - I can't remember which way around it is?).

The point that I am trying to make is that if you do buy a power meter, the ONT may well show a different reading that the one I calculated above.

If you do buy one, it will be interesting to see your findings - so please post back!

[Paul608085]

@StuartHK

You may find this document of use:

https://www.openreach.co.uk/cpportal/content/dam/cpportal/public/images-and-documents/home/help-and-...

Section 2.3.8 on page 27 has some power supply information.

[StuartHK]

@Paul608085 

Yes, that is useful.  Thanks very much.