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Offline Timbox

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So who is the electrics guru?
« on: 06 November, 2018, 06:55:13 PM »
Ive been doing a bit of testing of batteries etc, and wanted to set up a drain on a battery and monitor it. So I wired up a 55w H7 bulb and connected it in series with my ammeter to the battery. Now, the watts/amp calcs say that a 55W draw on a 12v source pulls 4.58 amps    (Amps= Watts / Volts)  So why does my ammeter only show  2.56amps?? 

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Online Yellow Dog

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Re: So who is the electrics guru?
« Reply #1 on: 06 November, 2018, 07:52:00 PM »
That sounds like way to much draw for a bulb.

You know your ammeter is spot on aye?

Mine was giving funny numbers, but when I changed its 9V battery, all was good again.
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Offline Timbox

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Re: So who is the electrics guru?
« Reply #2 on: 06 November, 2018, 08:34:02 PM »
*Originally Posted by Yellow Dog [+]
That sounds like way to much draw for a bulb.

You know your ammeter is spot on aye?

Mine was giving funny numbers, but when I changed its 9V battery, all was good again.

Funny thing is, ive just tried with another bulb, also rated at 55w, and this time it is showing 4.5amps, duff bulb?
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Offline seangee

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Re: So who is the electrics guru?
« Reply #3 on: 06 November, 2018, 08:54:48 PM »
Bulbs do go off over time and should be replaced regularly even if they appear good. Sure one of those electrics gurus has an explanation.
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Offline Timbox

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Re: So who is the electrics guru?
« Reply #4 on: 06 November, 2018, 09:08:43 PM »
*Originally Posted by seangee [+]
Bulbs do go off over time and should be replaced regularly even if they appear good. Sure one of those electrics gurus has an explanation.

Both bulbs Im testing are brand new, but the one pulling the greater amps is significantly brighter
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Offline Paul2bikes

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Re: So who is the electrics guru?
« Reply #5 on: 06 November, 2018, 09:22:43 PM »
Is it just a poor connection.
The greatest deception men suffer is from their own opinions.
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Offline Paullie

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Re: So who is the electrics guru?
« Reply #6 on: 06 November, 2018, 10:40:51 PM »
What is the make and source of the bulbs? Seems pretty clear that one has a problem, or the connection on that one is going down.

Seems to me you don't need any gurus - doing a pretty good job yerself! Now for your next test, if you want to pull off a starter motor .........  :015:

Offline ZuluTiger

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Re: So who is the electrics guru?
« Reply #7 on: 07 November, 2018, 10:50:56 AM »
Okay, let's try for a for a more definitive answer:

Firstly, the watt rating you refer to is based on rated values. In other words, it is a projection of performance under perfect, constant conditions. The manufacturer is telling you what the bulb will do at one fixed point. That point should be 12V but is more likely the charge point of 13.6 or thereabouts. More later.

To perform your test, the first thing to understand is that the bulb is actually not a 'wattage' device, but rather a fixed resistance. Therefore, as voltage decreases, so will the current. Applying a declining voltage to a fixed resistance will result in a declining current.

So, step one is to determine the resistance of the bulb. For an incandescent, it's simply a matter of substituting in the power formula and solving for R. P is power in Watts. P=VI and R=V/I. Therefore, R =V^2/P. For a 12V system, and a 55W load (at 12V), the result is 144/55 or 2.61 ohms. Now put your meter in resistance (ohms) mode and measure the resistance of the bulb to see how close it is. Bear in mind that this changes with temperature as the bulb gets hot, typically increasing the resistance and lowering current draw.

Now, as you do your test, constantly perform the calculation. If your bulb measures 2.6 ohms, then:

At 12V, you should draw 12/2.6 = 4.61A
At 10V, you should draw 10/2.6 = 3.84A

If your bulb was a 3 ohm load, it would result in 4A and 3.33A in the two examples above. Hopefully you have the picture now and can validate your current draw! Nothing is static and you have to understand the conditions at the precise time you take your reading.

Finally, no two bulbs are identical, nor will they behave exactly alike. The all-important resistance and temperature point will result in the output. At any given point in your test, you can record your voltage and current to determine the power (watts) being consumed by your bulb, which will NOT be 55W. At 10V and 3A, it would be 30W of actual load, for example. Hope this helps!

« Last Edit: 07 November, 2018, 11:00:11 AM by ZuluTiger »
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Offline Paul2bikes

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Re: So who is the electrics guru?
« Reply #8 on: 07 November, 2018, 10:56:57 AM »
Doesn't explain why one bulb 2.56 & tutha 4.5?
The greatest deception men suffer is from their own opinions.
Leonardo da Vinci

Offline ZuluTiger

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Re: So who is the electrics guru?
« Reply #9 on: 07 November, 2018, 11:01:03 AM »
Yes it does. See last part of my comment.
When folks ask me how I am, I reply "I'm here". Seems odd until you consider the alternative...