# Adding up Electrical usege ?



## vicdicdoc (May 14, 2005)

Electrical law :? [such as ohms / watts etc] is not my strong point, but I've changed halogen bulbs [both 10w & 20W ones] for LED's . . . usually we have 2 x 12v 10w halogen lights on evening times and sometimes the 12v 11w fluorescent strip on as well - what is the amperage of this 'usual' useage against having 6 of these [150mA LED's ? http://www.atenlighting.co.uk/product.php?productid=16145&cat=106&page=1


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## Vennwood (Feb 4, 2007)

With all 4 lamps on you will be using 3.5 amps by switching to LED's this should drop to less than 0.8 of an amp

Pete

Edit: Sorry I misread the post you say there is usually 3 (not 4) standard bulbs - this would be approx 2.6 amps

Pete


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## Rapide561 (Oct 1, 2005)

*Lights*

Hi

Do fluoescent (mispelt) lights use as much as halogen spots? Example is a 20 watt halogen the same as a 20 watt tube light?

Russell


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## Vennwood (Feb 4, 2007)

Hi Russell,

I always believed that flourescent lighting gave a more diffused, therefore better spread light whereas spots gave a more directional light and less of a spread. Both probably give the same brightness overall

Pete


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## DiscoDave (Aug 25, 2008)

a halogen filament vs a fluorescent tube of the same wattage should in theory use the same ammount of power, however; the control gear needed to light the tube can consume an enormous ammount of power - the older they are the more power the will use, the newer they are the more efficient they tend to be!

as for working out the current used by a particular item of equipment can be roughly calculated by dividing the the wattage (W) by the voltage (V)

so where a 12w lamp is running from a 12v battery the current used would be 1 amp

12/12 = 1A


suffice to say leds use considerably less power and therefore less current,


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## sallytrafic (Jan 17, 2006)

DiscoDave said:


> a halogen filament vs a fluorescent tube of the same wattage should in theory use the same ammount of power, however; the control gear needed to light the tube can consume an enormous ammount of power - the older they are the more power the will use, the newer they are the more efficient they tend to be!
> 
> as for working out the current used by a particular item of current using equipment is calculated by dividing the the wattage (W) by the voltage (V)
> 
> ...


You have made the assumption incorrectly in my view that the wattage would be constant. It is more likely that the resistance will be constant so an increase in Voltage would give a corresponding increase in Current not a decrease. Now constant resistance is not a given, in a halogen lamp it will alter from cold to hot but I doubt that it changes much at working temperature.


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## safariboy (May 1, 2005)

The lamps will certainly not be constant power or constant resistance. Both will go up but not quite in proportion to the voltage. So the current will be higher. There is an equation but is complicated and not very accurate.


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## DiscoDave (Aug 25, 2008)

i've edited the my above post - if it's wrong it doesn't need to be here, can you explain why the current goes up with voltage? or show the formula?

cheers


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## sallytrafic (Jan 17, 2006)

Ok lets assume for the moment that the resistance will be constant (it won't bebut we can get to that later).

Then by Ohms law R=V/I and using your figures the resistance will be 12 Ohms

Increasing the voltage to 14V and using I=V/R gives 14/12 or 1.17A


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## Rapide561 (Oct 1, 2005)

*Electric*

Hi

My bottom dollar question is does a 20watt tube/strip light - call it what you will use the same amount of battery power as the 20 watt halogen?

R


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## sallytrafic (Jan 17, 2006)

The resistance of a halogen lamp is non linear so it doesn't obey ohms law. It rises very quickly as the temperature goes from cold to red hot. Thereafter it continues to increase but at a lesser almost linear rate the attached graph from OSRAM shows that if you increase the voltage by 50% the current rises by around 25% and and that is because the resistance has risen also somewhat as the temperature has increased. The wattage should have risen by around 87.5% but thats off the chart.

The halogen lamp should never be under run the halogen cycle needs temperatures of several thousand kelvin at the filament and perhaps as much as 1000 kelvin at the envelope to operate effectively . Hence the envelope is small and made of quartz. Under run a conventional lamp and it will run practically 'for ever' but with a poor light output under run a halogen and it will fail prematurely.


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## sallytrafic (Jan 17, 2006)

*Re: Electric*



Rapide561 said:


> Hi
> 
> My bottom dollar question is does a 20watt tube/strip light - call it what you will use the same amount of battery power as the 20 watt halogen?
> 
> R


By and large yes that's why they quote the wattage. However the lumens per watt is not the same.


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## twinky (Aug 11, 2008)

A 20w lamp of the same voltage consumes the same power whether its a fluorescent or a filament or anything for that matter. You pay for the current you use either in terms of money (for your household bill) or reduced battery time in the case of a 12v dc system - so reducing your current consumption is whats needed. 
Ohms law Volts= Current X Resistance - Volts remain roughly constant either 12V dc or 230Vac so if you increase the resistance the current MUST drop proportionately. This is exactly what happens in your dimmer switches at home - you are increasing resistance and reducing current to the lamps.
But Power (watts) = Volts X Current so assuming the volts remain constant again a reduction in Watts MUST show a proportional drop in current - so lower the wattage = longer battery life.

Where LED's come into their own is that they give more luminance for their current than traditional lamps so can be relatively low wattage.

The issue with fluorescents is normally associated with when you have a large bank of them on a 3 phase industrial type supply. They cause a phase shift in the phase they are connected to and a reduction in efficiency - a big problem in 3 phase but not a problem with domestic single phase.

I've just read this back and confused myself


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## CliveMott (Mar 10, 2008)

*KEEP IT SIMPLE*

LED lamps and Flourescents are very similar in efficiency. Both consume about one fifth of the power of filiament bulbs. i.e. 20%.

The current (Amps) flowing in a circuit is directly proportional to the applied voltage (volts) and inversely proportional to the circuit resistance (Ohms)

The POWER (watts) is the circuit voltage multiplied by the circuit current (amps).

From these statements all formulae may be derrived.

C.


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## Rapide561 (Oct 1, 2005)

*Electrics*

Thanks Clive.

I have three tube light things down the back end of the van and I leave them on all the time because they look nice when lit up! If they were drawing the same power from the battery as a halogen, I think I would have had a problem by now!

Russell


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## Sargent (Feb 1, 2008)

Hi Vicdicdoc,

I think to give a basic answer to the original question running 6 x 150mA LED lights costs less than running one of your original lights. Normally LED lighting offers around a 70% saving. In this case more.

Indeed with temperature resistance does change but with up to date manufacturing techniques and the gases used this won't make a large difference to the current being used by any given lamp or Tube, as already mentioned tubes and halogens offer around the same costing and are rated at normal operating temperatures. 

Best Regards

Karl Hulse
Sargent Electrical Services


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## DABurleigh (May 9, 2005)

"From these statements all formulae may be derived. "

I can trump that. If you add in special relativity, and allow magnetic monopoles to exist (damned irritating that they don't seem to), then ALL of electromagnetism reduces to a single, simple, symmetric equation. Magic.

Dave


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## framptoncottrell (Jan 6, 2006)

I hope I'm not just repeating what has been said above.

A lamp that is rated at a given wattage will consume that wattage whatever type of lamp it is. A fluorescent lamp of 20 watts* will consume the same wattage as an incandescent lamp (tungsten or halogen) of 20 watts. It will, however, produce about three times as much light once it has warmed up, which can take anything from pretty instantaneous to thirty seconds. Incandescent lights are pretty well instantaneous. The lost wattage from an incandescent lamp goes in heat. In this respect a fluorescent lamp, while still getting warm, is much less wasteful of input energy.

The resistance of the filament of an incandescent bulb is virtually nil at room temperature but rises as its temperature rises after being switched on. A 60 watt domestic bulb has a resistance of close to 0 ohms when cold and about 1000 ohms when the filament is glowing at full output. That's why normally when a bulb blows, it happens when you turn it on, because the instantaneous current is very high until the filament has warmed up.

LEDs are different - they are very efficient at producing light and little heat. That is why the makers of these - and energy saving fluorescents - often quote their output with reference to equivalent incandescent lights. (When you buy a domestic 20 watt energy saving light bulb you will find the makers claim it is the equivalent of a 60 watt incandescent bulb, for example.) LEDs are also very directional, so to produce a spread of light they have to have clusters of LEDs each pointing in a different direction. This may or may not be aesthetically pleasing, or convenient.

Incandescent bulbs have a limited life, unless you run them at well below their rated voltage, when they are dim but have a very long life. The output of incandescents does drop over time, but not by very much. Fluorescent bulbs last longer but have a clearly deteriorating output over their lifetime as the inner glass coating - the bit that fluoresces - ages. LEDs seem to have an unlimited and non-dimming life, unless abused with, for example, AC current or over-voltage without protection.

Dr (musical, not medical) Roy

* divide the wattage by the voltage to get the current: 20 watts in a 12 volt vehicle will use 1.67 amps. 20 watts in a 240 volt house will use .083 amps.


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## eddied (May 9, 2005)

*Black magic*

 Ciao Russell,
like wots been said before - it's all black magic. Be like me, and just switch 'em on. When they start going dim, switch 'em off and check the battery.
saluti,
eddied


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## sallytrafic (Jan 17, 2006)

My earlier posts about change in resistance were only to refute the idea that if you increased the voltage to a conventional lamp the current would fall. A remark that has since been withdrawn by the poster.

edit inserted the word 'conventional' by which I also include halogen


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## vicdicdoc (May 14, 2005)

Sargent said:


> I think to give a basic answer to the original question running 6 x 150mA LED lights costs less than running one of your original lights. Normally LED lighting offers around a 70% saving. In this case more.
> 
> Indeed with temperature resistance does change but with up to date manufacturing techniques and the gases used this won't make a large difference to the current being used by any given lamp or Tube, as already mentioned tubes and halogens offer around the same costing and are rated at normal operating temperatures.


. . . :? so to recap, LED's will save me a lot of battery power against the same number of halogen / fluorescent lights. That's good enough for me !
- As DAB says "magic". [now to find out about perpetual motion machines and how to make one in the shed].
Thanks everybody for your replies.


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## CliveMott (Mar 10, 2008)

Keep one eye out for side by side comparisons of various lights in MMM early next year. OK! Don,t forget the standard Candle Dave (DAB).

C.


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## SidT (May 9, 2005)

Hi Vic. I bet you regret asking the question :lol: :roll: . As Eddied says, if they go dim switch them off and charge the battery.
Cheers Sid


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