Queston of overvolted bulb

Stefan

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Oct 4, 2002
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Alberta, Canada
I can't figure something out here. If I originally have a 6 volt 5 watt light bulb, and apply 7.2 volts to it, will it still be a 5 watt bulb? For example, a 5 watt system at 6 volts is 0.83 amps. Will it be the same power consumption on a 7.2 volt system, or do I need to break out the multimeter to find out?

Any suggestions? /ubbthreads/images/graemlins/help.gif
 

PaulW

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Mar 23, 2003
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Laurel, Maryland
Stefan,

If you increase the voltage by 20%, the current will also increase (nonlinearly) by about 5%. So, the power consummed would rise to about P = V * A = 7.2 * 0.87 = 6.3 Watts. All of this assumes, of course, that you don't incinerate the filament. Depending on the manufacturer, bulbs start flashing at voltage overdrive levels of perhaps 10 to 30 per cent.

The power actually consumed by a bulb rated at 5 Watts could vary a lot, depending on how the applied voltage changes. Thus, as batteries run down, the power to the bulb decreases.

Paul
 

Ginseng

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Feb 27, 2003
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A 6V 5W bulb pulls 0.83A. If you drive it to 7.2V, your current will likely go up (as the Welch Allyn bulb rerating formulas indicate) so your total power will go up to at least 6W and possibly as much as 6.5W.

Wilkey
 

Stefan

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Oct 4, 2002
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309
Location
Alberta, Canada
Here's a headsup FYI: I'm using a 6 volt 5 watt MR11-C bike light, on a 7.2 volt RC battery pack. I'm not sure what the capacity of the battery pack is. I did a runtime test, and running the 5 watt bulb I get 1 hour 15 minutes until the bulb changes color from a bright white to a sickly orange. Totally drains the battery 1 hour 20 minutes. Would this roughly mean my battery pack is a 1200 mAh battery pack?
 

PaulW

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Stefan,

Let's assume the average current during the run is 700 milliamps. If that is true, you have gotten 1.33 hours * 700 mA = 931 mAh. That means that the battery pack is rated for at least 931mAh, because you have not sucked out every bit of juice available on two counts:

1. The rating is valid usually for low current draw, one that would give 10 hours of run time. You get fewer mAh under higher current conditions.

2. You have probably not completely exhausted your battery, and wisely so. It's not a kindly thing to completely run down rechargeables.

Oh yeah, I have assumed that you're talking about rechargeable batteries. But if not, the reasoning still applies, somewhat.

Paul
 
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