What is the benifit of using more than one reistor vs one?

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Discussion Overview

The discussion revolves around the design considerations for an LED array, specifically addressing the use of multiple resistors versus a single resistor in the circuit. Participants explore the implications of resistor placement, LED matching, and current distribution in both series and parallel configurations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the necessity of using multiple resistors for a 6 LED array, suggesting that calculations indicate a single 1.2 Ohm resistor could suffice.
  • Another participant emphasizes that if the LEDs are not well matched, using one resistor per LED may be a better design practice to ensure uniform brightness.
  • Concerns are raised about current hogging in parallel configurations, where one LED could draw more current than others, leading to overheating.
  • A participant notes that using multiple resistors could be beneficial for wattage considerations, as lower wattage resistors may be less expensive than a single high-wattage resistor.
  • There is a discussion about the implications of series versus parallel configurations, with one participant indicating that series connections would require higher voltage than available.
  • Another participant suggests using individual resistors for each LED to achieve better brightness control, while also noting the challenges of driving LEDs from a low voltage battery.
  • One participant expresses a desire for their LEDs to function as brightly as possible without damage, questioning the best configuration and resistor selection without relying on online calculators.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and benefits of using multiple resistors versus a single resistor. There is no consensus on the best approach, as various factors such as LED matching, current distribution, and design considerations are debated.

Contextual Notes

Participants highlight limitations such as the need for well-matched LEDs for parallel configurations and the voltage requirements for series connections. The discussion also reflects uncertainties regarding the optimal resistor values and configurations based on the specific LED characteristics and power supply constraints.

Who May Find This Useful

This discussion may be useful for individuals interested in LED circuit design, particularly those exploring the implications of resistor configurations in LED arrays and the associated electrical characteristics.

Llama77
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I am building a LED array and most of these LED resistor calculators like this one http://led.linear1.org/led.wiz, seem to suggest that I need more than one resistor for the array. From my Calculations I need 1.2 Ohm resistor/s. So my question is given a 6 LED array, why use 6 resistors rather than just one. Also why does the software/website put the resistors on the cathode side of the LED's?
 
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Can you post a schematic of YOUR idea vs. THEIRs?
 
Llama77 said:
I am building a LED array and most of these LED resistor calculators like this one http://led.linear1.org/led.wiz, seem to suggest that I need more than one resistor for the array. From my Calculations I need 1.2 Ohm resistor/s. So my question is given a 6 LED array, why use 6 resistors rather than just one. Also why does the software/website put the resistors on the cathode side of the LED's?

Unless the LEDs are well matched, their light output versus current will not be identical. So why do you think it would be a good design practice to use one resistor per not-so-well-matched LED?

BTW, in some LED lighting applications, the LEDs are well matched, and there is only one current going through a series string of the LEDs...
 
If you have several led's in parallel, one might get hot and begin to hog current. In that case it would get hotter, and the hogging would get worse. That's why high-current devices are needed - if we could safely leave diodes in parallel we would.
 
Here is what I have already

Nk25J.png


Powered by a 4.8V battery pack, using 6 Cree XP-G LED's.
 
The data sheet for these LEDs shows a possible turn-on voltage range of 3.3 to 3.75 volts.

If you did have such a variety, the one with the lowest voltage would turn on first and hold the voltage output of the resistor to about that value. The voltage will rise as that LED will draw a lot of current and this would light up some of the other LEDs but to a much lower brightness. So, you could get a wide variation in brightness.

If you did connect them all in parallel, and they happened to be perfectly matched, they would all get about 200 mA with that 1.2 ohm resistor. Quite safe, but probably not bright enough.

If you want uniform brightness from your LEDs, you could try individual 1.8 ohm, 5 watt resistors with one of these in series with each LED. It doesn't matter which way around the LED and the resistor are, as long as they are in series.

These LEDs are normally driven from a current regulator to get constant current for each LED, but this is difficult in your case because of the low battery voltage. So, the resistors are a bit of a compromise.

Don't charge the battery with the LEDs connected.
 
Well I want the the LED's to function as bright as possible, without damaging them. So would in series be better than in parallel like I have them now? Also how can I find out what type of resistor I need without using these online calculators?


Thank you/
 
Hi Llama77
I think that vk6kro has anticipated your needs and designed the circuit which will give the best sustainable output for your led's.
To learn to make the decisions he has made on your behalf is a matter of some effort, and the labour of the explanation that would required to justify his design is large.
I concur with vk6kro's design and recommend that you adopt it.
 
Resisters also have a wattage rating. So I would think that design considerations might include many resistors in place of one for wattage considerations as well as price. The larger wattage ones might be considerably more expensive than several lower wattage ones in series. I don't know for sure..but wattage rating is something to consider when selecting resistors not just their efficiency(by efficiency I mean within the 5% or 10% or so rated ohms.. the gold or silver stripe if you know what I mean) or Ohms rating.
 
  • #10
Well I want the the LED's to function as bright as possible, without damaging them. So would in series be better than in parallel like I have them now? Also how can I find out what type of resistor I need without using these online calculators?

LEDs have a turn-on voltage which is a minimum voltage that the LED needs before it will turn on. If you stack LEDs in series, the voltages needed to turn them on add up.

So, if you have two LEDs that both need 3.5 volts to turn them on, you will need 7 volts to turn them on in series.

You only have 4.8 volts, so you can't put them in series or none of them will turn on.

Calculation:
As a first step, have your 4.8 volt battery and a 3.3 volt LED. The difference between these voltages will appear across the series resistor. So, it will have 1.5 volts across it. 4.8 - 3.3.

For a current of 0.5 amps, this resistor should have a value of 1.5 volts / 0.5 amps or 3.0 ohms.
At this current, the voltage across the LED will rise to 3.4 volts, so the resistor must have a value of 1.4 volts / 0.5 amps or 2.8 ohms.

Your battery will rise to 1.35 volts per cell or 5.4 volts when fully charged, assuming NiCd or NiMH.
So the resistor would then be (5.4 - 3.4) / 0.5 or 4 ohms.

For a current of 1 amp, the calculation is similar and the voltage across the LED is now 3.5 volts:
So the resistor would then be (5.4 - 3.5) / 1.0 or 1.9 ohms.
1.8 ohms is near enough. Don't go any smaller if you can't find 1.8 ohms.

5 watt resistors are not as cheap as 1/2 watt ones, but much cheaper than high powered LEDs. The power is actually about 2 watts, but it will run hot, so a bigger rating would be better.

I would try one LED with a 3.3 ohm resistor, for a start.
 
  • #11
This answer puts me to shame; and shows the proper spirit.
 

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