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Author Topic: Big Screen Efficiency  (Read 6284 times)

Offline pirunner

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Big Screen Efficiency
« on: 04/02/2008 04:00:27 »
My dad and I have been arguing over this question for quite a while now. We really need an answer and so good data to go with it.

We own a fairly new big screen TV (not a plasma or LCD, but the old electron gun kind). I know that it takes a comparatively large amount of energy to initially power up the TV when you first turn it on. This is where the problem comes in. My dad says that when he stops watching the TV for a relatively short amount of time (20-30 minutes)it takes less energy to run the TV than to run it off and restart it after the lapsed time. I could see this fact being true, up to a certain point.

What is the rough point at which the power to run a TV exceeds the power to turn it on?
Any numbers would be greatly appreciated!
Thanks!


 

lyner

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Big Screen Efficiency
« Reply #1 on: 04/02/2008 13:33:43 »
Where would this 'extra power' be going, when the set is switched on? I can't think of any extra load except, possibly, the de-gaussing coil and that doesn't operate from 'standby'. In some equipment there is a is temperature controlled oven for an accurate frequency reference. This would operate at switch on.
There is nothing like that, afaik, in a normal TV.
Of course, large CRTs are very power hungry.
I would be interested to find out.
 

another_someone

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Big Screen Efficiency
« Reply #2 on: 04/02/2008 13:54:24 »
The question is, where is the power usage in a CRT.

The coils would consume a fair amount of power, but they are constantly fluctuating, so there is no different whether the fluctuation is just after the on switch has been pressed, or because they are starting a new frame immediately after the previous frame has finished.

Where I could imagine there might be a surge is in filament in the the electron guns warming up, but what percentage of the current goes here, and what percentage goes to drive the coils, or other components, I have no idea.
 

Offline Pumblechook

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Big Screen Efficiency
« Reply #3 on: 04/02/2008 15:35:33 »
CRT is producing a lot of light...obviously that needs energy use.  I think the peak beam current is only 1 mA but the EHT is 24 kV so that is 24 Watts with a bright scene.  Magnetic deflection will use some power.   

As regards switch-on the de-gaussing coil and the tube filiaments (3 of them) will take a bit of extra energy.    Capacitors in the PSU will charge up. Inrush current will be quite a bit higher than the running current but it will settle down within less than a second.

I would have thought the 'extra' energy would be pretty small as the time is small.   

Say it was 200 Watts for 2 secs.  It would use only 0.00011 kWh.... about 0.0012 pence worth of electricity.   

A TV (70 Watts say) for 20 minutes would use 0.023 kWh....about 0.25 pence worth.
« Last Edit: 04/02/2008 15:46:46 by Pumblechook »
 

lyner

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Big Screen Efficiency
« Reply #4 on: 04/02/2008 21:41:48 »
The scan coils are the major power consumers in a conventional telly. The circuits are necessarily quite 'lossy' in order to provide a linear scan (spot must travel at constant speed across the face of the tube and vertically, too)  and, cleverly, the EHT for the electron beam energy is provided from the 16kHz 'line scanning' circuit during the fast 'flyback' pulses.
All colour CRTs are inefficient at producing the light from the electron beam energy because there is a grid / grille / mask which absorbs a lot of the beam current in order to prevent electrons from each gun reaching the wrong coloured phosphors.
 

Offline techmind

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Big Screen Efficiency
« Reply #5 on: 04/02/2008 23:11:10 »
We own a fairly new big screen TV (not a plasma or LCD, but the old electron gun kind). I know that it takes a comparatively large amount of energy to initially power up the TV when you first turn it on. This is where the problem comes in. My dad says that when he stops watching the TV for a relatively short amount of time (20-30 minutes)it takes less energy to run the TV than to run it off and restart it after the lapsed time. I could see this fact being true, up to a certain point.

What is the rough point at which the power to run a TV exceeds the power to turn it on?
Any numbers would be greatly appreciated!
Thanks!

To summarise the others: as far as electricity usage is concerned, any switch-on surge will be so small and short-lived as to be irrelevant. It can't really be equivalent to any more than a couple of seconds of normal usage.

Some might argue that there's some additional "wear and tear" on the set through switching it on and off. But again I doubt this will be significant unless perhaps you are really switching it on and off every five minutes!
 

Offline Pumblechook

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« Reply #6 on: 04/02/2008 23:23:09 »
Sets do use Voltage Dependent Resistors and other means to soften the start up.

Line output (15,625 Hz for scanning and as by-product generates the EHT) transistors or valves are fairly high power devices.  So there is quite a lot of power there. 

 

lyner

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Big Screen Efficiency
« Reply #7 on: 05/02/2008 10:48:25 »
Quote
So there is quite a lot of power there.
Up to a point. We are only talking of a few watts. In a small TV the beam currents are only in the order of a few 100 microamps and the EHT IS ABOUT 15KV. That involves a few watts total. Some of the components  have to be able to handle these voltages and some spikes of, perhaps, a couple of kV.
 

Offline that mad man

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Big Screen Efficiency
« Reply #8 on: 05/02/2008 17:23:13 »
You will always get an initial short lived surge when turning on any inductive device as things take time to charge and with a CRT tube you also have a warm-up time. This is the main reason why CRT TV's don't come on straight away. During this warm-up time, just a few seconds now compared to several seconds on older sets, more current will be used.

How much, I'm not sure.

The current will also lag the voltage so what's important is to have the correct Power Factor Correction capacitors installed in the circuit to help correct this lag. If there is no PF correction then the circuit could be unbalanced resulting in more current drawn than needed = higher electricity bills. Cheaper sets may also have cheaper lower tolerance components resulting in more current drain and a shorter lifespan.
 

lyner

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Big Screen Efficiency
« Reply #9 on: 07/02/2008 10:48:37 »
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If there is no PF correction then the circuit could be unbalanced resulting in more current drawn than needed = higher electricity bills.
The bills would not be higher. The conventional 'rotating disc' energy meter  derives the force for turning the disc from the Vector PRODUCT V X I, i.e. VI cos (phase angle). The  the recent electronic equivalents do the same. This is affected by the power factor and only costs you for the actual energy consumed. The lost energy due to extra current in supply cables to your house is paid for by the generating company. `That is why they stipulate that the PF of heavy equipment must be a reasonable amount (better than 90%).
 

Offline that mad man

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Big Screen Efficiency
« Reply #10 on: 07/02/2008 15:52:34 »
Thanks sophiecentaur.

I forgot that domestic meters take this into account so what I said mainly applies to industrial installations.
 

lyner

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Big Screen Efficiency
« Reply #11 on: 07/02/2008 19:20:57 »
If I remember correctly, there are some swingeing penalties for some big users for over-current and, also, over-power conditions. It is very much in the interest of operators of heavy three phase motors to sort out their power factor and to predict their maximum load over the next billing period.
 

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Big Screen Efficiency
« Reply #11 on: 07/02/2008 19:20:57 »

 

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