Author Topic: nobody talking about switching PS wasting power on input filter caps?  (Read 10761 times)

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Offline stefonTopic starter

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They say like PS are 80-90% efficient, but they all have filter capacitors on input. So if I calculate correctly 5W cellphone charger (230V/50Hz) with 220nf X2 filter cap, just plugged to wall will waste 3.5W of power...
Some power supplies have like mains filter with dual 0.47uf cap... so just this fitler is wasting 16W???
But also this cap is not getting hot at all... so I'm calculating something wrong? But I measured current and it matches calculations....
 

Offline BravoV

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #1 on: August 22, 2019, 12:10:47 pm »
.. so I'm calculating something wrong?

How do we know if its wrong or not ? Show us the calculation.

Offline capt bullshot

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #2 on: August 22, 2019, 12:54:04 pm »
This is called reactive power. The current is 90° phase shifted to the voltage, energy flows forward and back with each cycle of the mains frequency.
You can measure this current, and it will match your calculation, but you won't get billed for it ;)

No one talks about it in public, but yes, it's there and it loads the utilities and creates losses along the distribution network.
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Offline ArthurDent

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #3 on: August 22, 2019, 01:28:55 pm »
I believe you're wrong blaming the capacitors. If you check RC charging circuits and formula you will see that the current drops to zero when the capacitor is fully charged. For capacitors in a SMPS to dissipate 3.5 watts they would have to be defective. The voltage across those filter capacitors is still connected to the inverter circuit that could draw some current but with no output load, that is close to zero in small SMPSs. I Just checked the supply for my laptop and my wattmeter registered 0 watts or voltamps.

I think the efficiency you mention is generally for a fully loaded supply and there are lots of components in a SMPS that contribute to losses under full load.
 

Online themadhippy

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #4 on: August 22, 2019, 02:00:13 pm »
Quote
You can measure this current, and it will match your calculation, but you won't get billed for it ;)
yet,once there's a  100% enforcement  take up of smart meters the energy suppliers will have the  ability to start charging for it ,as they already do with large power users.

 

Offline Ice-Tea

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #5 on: August 22, 2019, 02:12:19 pm »
I believe you're wrong blaming the capacitors. If you check RC charging circuits and formula you will see that the current drops to zero when the capacitor is fully charged.

This is true after a rectifier bridge, but there's also mains filtering which puts a cap directly on L and N. But as others have stated: this is reactive power.
 

Offline james_s

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #6 on: August 22, 2019, 03:34:40 pm »
Look up the difference between true power, apparent power and that will lead into power factor.

While it does result in losses in the wiring, something that helps is the fact that inductive loads lag and capacitive loads lead so when you have a mix of these they tend to cancel out. With the demise of iron transformer power supplies there are a lot fewer inductive loads in the typical house though.
 

Offline GerryR

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #7 on: August 23, 2019, 12:36:06 am »
In most industrial environments where there are many motors, they have huge capacitor banks, which switch automatically for pf correction, depending on the different loads throughout the day.  Capacitors, for the most part, are considered "lossless."
Still learning; good judgment comes from experience, which comes from bad judgment!!
 

Offline Ysjoelfir

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #8 on: August 23, 2019, 06:02:04 am »
You can measure this current, and it will match your calculation, but you won't get billed for it...
... yet. But as themadhippi already wrote, smart meters can measure reactive power. I am working at a manufacturer for precision power analyzers, so we have rather good connections to many many energy providers. I assure you that they would absolutely LOVE to enforce the useage of smart meters in every house in germany, not for the added features like peak power reduction in combination with flexible plans or automatic transmission of readings, nooo, they would be happy to bill you all that reactive power as they can measure it then precisely.
Greetings, Kai \ Ysjoelfir
 

Offline Ice-Tea

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #9 on: August 23, 2019, 07:56:11 am »
I am working at a manufacturer for precision power analyzers...

Yoko?
 

Offline Ysjoelfir

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Greetings, Kai \ Ysjoelfir
 

Offline radiolistener

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #11 on: August 23, 2019, 10:24:02 am »
it costs less, but it doesn't means that it is more efficient from point of view of power consumption  :D
 

Offline dmills

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #12 on: August 23, 2019, 10:48:26 am »
But it IS more efficient!
The reactive power flow only causes losses in the distribution cabling (I^2 R), And the power company will correct large aggregate PFC issues) while real power flow is total loss.

The better sort of small switcher these days has a PFC input stage anyway, but as ever that is not itself 100% efficient (NOTHING is), so from a power efficiency perspective PFC on a small DC supply may or may not be a win (From a power quality perspective it is very much a win, mainly for reduced flat topping).

Regards, Dan.
 

Offline radiolistener

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #13 on: August 23, 2019, 02:28:22 pm »
The reactive power flow only causes losses in the distribution cabling (I^2 R), And the power company will correct large aggregate PFC issues) while real power flow is total loss.

but you will pay real money for reactive power. So this is expenses for your wallet.  See conversation above...  ;)
 

Offline james_s

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #14 on: August 23, 2019, 04:16:56 pm »
In the USA residential consumers do not pay for reactive power, I can't speak for elsewhere.

Also as I mentioned earlier, if you have any inductive loads they will cancel out the the capacitive loads.
 

Offline Ice-Tea

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #15 on: August 23, 2019, 04:47:59 pm »
Haven't heard of any place that charges residential customers for reactive power, but as some others have pointed out the advent of 'smart' meters may lead to that.
 

Offline soldar

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #16 on: August 23, 2019, 04:50:57 pm »
Let us not confuse the speed with the bacon.

Reactive power is one thing. Power factor is a different thing. They are loosely related.

A capacitor or an inductance across the line consume reactive power. This means they take power, store it and return it to the network. This means a power factor of less than unity.

A rectifier bridge followed by a capacitor does not consume any reactive power. At all. None. It keeps everything it gets and returns nothing. And yet has a bad power factor.

All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Ice-Tea

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #17 on: August 23, 2019, 06:30:10 pm »
Reactive power is one thing. Power factor is a different thing. They are loosely related.

Loosely?  :o
 

Offline stefonTopic starter

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #18 on: August 23, 2019, 07:27:08 pm »
> A capacitor or an inductance across the line consume reactive power. This means they take power, store it and return it to the network. This means a power factor of less than unity.

I don't get it, it is AC, so current flows thru cap, so it is wasting power, because it is shorting L to N.

> A rectifier bridge followed by a capacitor does not consume any reactive power. At all. None. It keeps everything it gets and returns nothing. And yet has a bad power factor.

i don't care about caps after bridge.
 

Offline soldar

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #19 on: August 23, 2019, 07:41:05 pm »
Loosely?  :o

In the sense that there is not a biunivocal relation. Reactive power lowers power factor. So do non linear loads. The problem is not reactive loads. The problem is low power factor, whatever the cause.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline james_s

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #20 on: August 24, 2019, 01:16:00 am »
> A capacitor or an inductance across the line consume reactive power. This means they take power, store it and return it to the network. This means a power factor of less than unity.

I don't get it, it is AC, so current flows thru cap, so it is wasting power, because it is shorting L to N.

> A rectifier bridge followed by a capacitor does not consume any reactive power. At all. None. It keeps everything it gets and returns nothing. And yet has a bad power factor.

i don't care about caps after bridge.

No, you don't get it. It's not shorting anything and it's not wasting power.

Current flows yes, but no power is consumed. Ok in reality nothing is ever 100% but for the sake of simplicity we can call it none. A reactive load draws energy during part of the cycle and dumps it back into the line on another part of the cycle. It is power that is just sort of bouncing around, it is passing through the wires but then getting returned to the source without being consumed.
 

Offline soldar

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #21 on: August 24, 2019, 07:03:07 am »
I don't get it, it is AC, so current flows thru cap, so it is wasting power, because it is shorting L to N.

Current flows through capacitor. Correct. Charging it and storing energy in the capacitor. This energy is returned back to the grid, it is not lost.

Think of an elevator counterweight. As it is raised it consumes (and stores) energy. This energy is returned to the system the next time the elevator is raised and the weight falls.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline radiolistener

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #22 on: August 24, 2019, 11:02:36 am »
Current flows through capacitor. Correct. Charging it and storing energy in the capacitor. This energy is returned back to the grid, it is not lost.

yes, but this "returned back" energy creates impedance change for power source. So, a power source (power plant) needs to spend more energy in order to fight with this "returned back" energy. Because this "returned back" energy will works against power source. Isn't it?

Also, there is additional power loss in the capacitor and power loss in the wire, due to higher current. All this energy is lost and cannot be consumed. Yes, not all this loss happens inside capacitor, but this energy is wasted due to capacitor :)
« Last Edit: August 24, 2019, 11:15:55 am by radiolistener »
 

Offline Ice-Tea

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #23 on: August 24, 2019, 11:30:24 am »
yes, but this "returned back" energy creates impedance change for power source. So, a power source (power plant) needs to spend more energy in order to fight with this "returned back" energy. Because this "returned back" energy will works against power source. Isn't it?

No, there is no net effect. The generator needs to make an extra effort to charge the cap, that effort is returned a little bit after that.
 

Offline radiolistener

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Re: nobody talking about switching PS wasting power on input filter caps?
« Reply #24 on: August 24, 2019, 11:50:19 am »
No, there is no net effect. The generator needs to make an extra effort to charge the cap, that effort is returned a little bit after that.

"little bit after" amplitude in the mains grid will be changed, because this is AC. So, returned energy will work against power source. This energy will works to distort sine waveform. For power source it will looks the same as additional load. So, electricity generator will needs to spend more power in order to keep the same voltage on the grid.

The difference with usual load is that this energy will be returned back and wasted on AC electricity generator side instead of capactior. But anyway it will be wasted.

First generator needs to make an extra effort to charge the cap, and then generator needs to make an extra effort to discharge the cap. This is worse than just waste that energy on dummy load. If you spend that energy on dummy load, there is no need for generator to spend extra effort to discharge the cap.
« Last Edit: August 24, 2019, 12:00:33 pm by radiolistener »
 


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