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Old 11-12-2012, 09:25 AM   #1
CBBOO
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Default Two resistors in paralell?

Hi!
So I f.ed up. Bought some cheapish resistors for my Melotti indicators for my Brut. I have only one pair left and right, from my experience that will do it, no need for two pairs.

But this did not work. I am reluctant to place a pair in the front so now I am wondering, can I install another pair in paralell in the rear. Reason beeing the cables from the indicators in the rear are now to short to eliminate the first pair...

Cheers!

/M
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Old 11-12-2012, 11:12 AM   #2
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Resisters in parallel are not the way to go. Electricity will seek the path of least resistance, literally.
To slow your blink rate, add resistance in series with the light.
You should be able to calculate how much resistance you need based on the power (wattage) consumption of the original lights. Those resistors should be heat sink resistors or they will fail in time.
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Old 11-12-2012, 01:03 PM   #3
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Actually I think you'll find that the resistors do need to be placed in parallel with each indicator. Electricity doesn't take the path of least resistance it's just that more electricity will flow through the lesser resistance than the higher resistance. If you had two drain plugs in your bath one twice the size as the other you would still get some water flowing out of the smaller drain, current flow in an electrical circuit is just the same. The thing about fitting LED indicators is that they require less current than a filament indicator this then changes the flash rate. A correctly sized parallel resistor allows the correct current for the LED and the flash relay. Probably best to buy from Melotti as they will be sized for their indicators.

Here's a typical wiring diagram
http://www.matstrad.com/resistors_tech.php

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Old 11-12-2012, 01:17 PM   #4
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Depends on what you are trying to accomplish and how big or small you want the resistive load to be. In series it is additive. In parallel, it is inversely additive.

http://www.allaboutcircuits.com/vol_1/chpt_5/3.html

http://www.bbc.co.uk/bitesize/higher...rs/revision/1/
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Old 11-12-2012, 01:24 PM   #5
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Chuck.

Brian is correct.. I've been through this issue on my Ducati and found out the hard way..

Here is a picture of a resistor I had made, but in the end didn't need to use it.

The late Ducati's ECU actually works out the correct voltage adjustment for LED lights.

In the picture I've stripped back the heat shrink protection to show the resistor wired in parallel.

.
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File Type: pdf Resistor Wiring Diagram.pdf (174.4 KB, 41 views)
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Old 11-12-2012, 01:59 PM   #6
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John, I don't disagree with Brian. But, it does depend on what you are trying to accomplish. A 5 ohm resister in parallel has a very different effect from a 5 ohm resistor in series. i.e., for the same values of resistance and voltage, the parallel circuit will have a much higher current flow.
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Old 11-12-2012, 02:39 PM   #7
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What we are trying to achieve is the same resistive load as the filament lamps as that is what's required to keep the flash relay happy. So for example if the original filament lamp has a resistance of 5 ohms that's what we have to end up with for the LED lamp. Typically the LED indicator has a higher resistance due to the fact it has internal resistors to drop the voltage to the level required for the LED's. So if you'll allow me to make up some figures for ease of demonstration we'll say that the LED indicator has a resistance of 10 ohms, this would mean that it would require a parallel resistor of 10 ohms which would result in the circuit resistance being 5 ohms. I agree that if you installed raw LED's then you would probably require series resistance but I think that the reality is that LED indicators are manufactured with internal resistors.

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Old 11-12-2012, 02:44 PM   #8
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Sorry about this guys. But I dont think I really explained what I mean.

I have already a paralell resistor but that did not do the trick. To correct this "quick and dirty" I just want to add another one in paralell without removing the first one.

Its not that I want to keep the useless resistor, It will just be a whole lot simpler.

Thanks!

/M
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Old 11-12-2012, 03:06 PM   #9
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Quote:
Originally Posted by CBBOO View Post
Sorry about this guys. But I dont think I really explained what I mean.

I have already a paralell resistor but that did not do the trick. To correct this "quick and dirty" I just want to add another one in paralell without removing the first one.

Its not that I want to keep the useless resistor, It will just be a whole lot simpler.

Thanks!

/M
Two thoughts You can buy a solid-state flasher relay which is happy with any combination of incandescent or LED loads....look on here, I posted a part number, if you cant find, PM... or get a variable resistor (potentiometer) and temporarily wire it in your setup, see what position gets you your desired flash rate, measure that resistance and get a fixed value resistor. Usually resistors are wired in series with an LED to limit current, if you want the relay to see a higher load, then yes, wire the resistor in parallel.
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Old 03-01-2013, 06:26 AM   #10
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Damn, going back and re-reading this, Brian and John are correct and my brain was off that day.
The resistive load needs to be in parallel with the LED light. To raise resitive load, the resitors should be in series with each other, but in parallel with the bulb.
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