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Old 04-30-2012, 02:39 PM   #1
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"Final" bulb in parallel circuit brighter than rest. Why?


In a previous thread (Wire suitable for underwater usage), I enquired about what cord types would be suitable for underwater usage because I was building a submersible fishing light to attract baitfish. Well, the light is done, and the bulb "farthest away" from the battery is by far the brightest in the chain (I hesitate to say chain since they are wired in parallel).

Attached image "photo 1" is a picture of the light I made. Notice the bulb all the way to the left is far brighter than the rest.

All bulbs used are the same spec, and look like this:

To wire the bulbs, I cut the flange off with a Dremel tool so that they would fit in the tube. Two pair of them have their terminals soldered together (neg to neg, pos to pos...that's the "gap" you see in the lighting), and the first bulb in line is upside down, with it's terminals facing the incoming wiring. The first bulb receives the wiring from the main line, then hookup wire is used to jump from those terminals to the next set, etc.

There IS one anomaly with the final/far left bulb. When I was cutting off the flange, I accidentally cut too far on that piece, basically severing the terminals from the bulb. So, when wiring it into the circuit, I had to improvise and soldered the hookup wire directly into the solder points located on the bulb itself (instead of the underside). Hopefully you can see what I'm talking about in attached "photo 2."

I have no idea why this anomaly would make the bulb brighter though. Can anyone think of any other reason the bulb would be brighter? Unfortunately, I cannot do any readings on the individual bulbs because I did not notice this until everything was soldered into place and the lamp sealed/waterproofed).
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"Final" bulb in parallel circuit brighter than rest.  Why?-photo-1.jpg   "Final" bulb in parallel circuit brighter than rest.  Why?-photo-2.jpg  

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Old 04-30-2012, 03:03 PM   #2
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"Final" bulb in parallel circuit brighter than rest. Why?


Without a wiring schematic or some inside knowledge about the lamps, it's impossible to say what's going on. Logic says it has something to do with the fact you lost the wiring terminal and improvised. My experience tells me whenever a lamp suddenly goes very bright, it's about to fail.

I would go to the vendor you bought the LED lamps from and ask them.

Lesson for today: Never permanently seal anything before testing.

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Old 04-30-2012, 03:11 PM   #3
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"Final" bulb in parallel circuit brighter than rest. Why?


Just a guess, without knowing what you did with the terminals.

Did you bypass a resistor by going directly to the PCB?
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Old 04-30-2012, 03:14 PM   #4
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"Final" bulb in parallel circuit brighter than rest. Why?


Thanks. I tested everything before sealing to see that all the bulbs were lighting up before sealing. I just didn't notice that brighter bulb until after I sealed the tube. It's not that it can't be undone; it's just that it would be a PITA.

Anyway, obviously my knowledge of electronics is very basic, but, to me, the fact that I could touch the pos wire on any positive point on the bulb and get the whole thing to light up indicates that it really shouldn't matter which point it is. And, if doing that IS what caused the bulb to get brighter, well I would have done that to ALL of them!

But seriously, let's pretend I soldered to the terminals as with the rest of the bulbs, and this extra brightness was still the case. If the bulbs were all wired in parallel, would this mean the brighter bulb is simply out of spec with the others? Or could there be another cause? My understanding is that in a parallel circuit, all the bulbs should be receiving the same voltage.
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Old 04-30-2012, 03:23 PM   #5
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"Final" bulb in parallel circuit brighter than rest. Why?


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Originally Posted by AandPDan View Post
Just a guess, without knowing what you did with the terminals.

Did you bypass a resistor by going directly to the PCB?
That seems logical! Thank you.
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Old 04-30-2012, 03:34 PM   #6
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"Final" bulb in parallel circuit brighter than rest. Why?


Yes you appear to have wired directly into the LED array but probably bypassed the current limiting circuit which is minimally a resistor(s) but maybe more than that.
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Old 04-30-2012, 04:00 PM   #7
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"Final" bulb in parallel circuit brighter than rest. Why?


Quote:
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Yes you appear to have wired directly into the LED array but probably bypassed the current limiting circuit which is minimally a resistor(s) but maybe more than that.
No harm in doing this, I imagine?
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Old 04-30-2012, 04:26 PM   #8
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"Final" bulb in parallel circuit brighter than rest. Why?


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Originally Posted by 7.62 View Post
No harm in doing this, I imagine?
Definite harm in doing this. The current limiting circuit (which could just be a resistor, or may involve active electronics) is what keeps the LEDs from being driven beyond their ratings. By the looks of it, you're running way higher current through that last device than normal. It is likely to fail quickly.
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Old 04-30-2012, 04:31 PM   #9
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"Final" bulb in parallel circuit brighter than rest. Why?


Like running a 3 volt flashlight bulb on a 6 volt lantern battery, it'll be nice and bright - for a while, then it's all done.
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Old 04-30-2012, 04:33 PM   #10
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"Final" bulb in parallel circuit brighter than rest. Why?


I guess it's a good way to experiment, then. This is just a prototype, and if the bulb fails, it fails, and I'll know why (and the rest will still function). I've left it on for several hours so far, and had no issues yet. These bulbs are designed to be used as vehicle headlamp bulbs. Could it be the resistor is simply there to keep the bulb from being too bright and not necessarily from being driven beyond the LED's rating?
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Old 04-30-2012, 05:04 PM   #11
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"Final" bulb in parallel circuit brighter than rest. Why?


Usually a resistor is used to limit the current through the LED. That's the limiting factor in led design.

One thing to consider, in an automobile it's not uncommon for the voltage to be as high as 14.7 or so volts during normal operation. The lights are likely designed to account for this.

Time will tell.
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Old 04-30-2012, 05:13 PM   #12
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"Final" bulb in parallel circuit brighter than rest. Why?


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