# 15 LED parallel circuit

#### 1ntegr0

Joined Dec 29, 2011
2
Probably a stupid question, but I'm not sure so I have to ask

I have 15 LEDs (TSAL6400 TR Diode @ 100 mA) in parallel, divided into 2 groups. First group is 4 LEDs, second group is 11 LEDs. The first group is always on, the second group can be switched on and off by a switch. Wiring is all done, no problems there.

The problem is my power source. Previously, I used a special device that allowed me to send exactly 100 mA through the entire system, so no resistors were needed. Now, I can no longer access that device so I need to use a 9V battery (the square one). This also means that I have to use resistors now.

I read somewhere that it is necessary for parallel circuits to use a different resistor for EACH LED (is this correct?). I also read that for parallel circuits Req = 1/R1 + 1/R2 ... 1/Rn but I'm not sure how to interpret this.

By doing R = U / I I find that R = 90 Ω. Does this mean that I need to give every LED a 90 Ω resistor, or do I have to devide 90 by 15? If so, wouldn't that give problems with my switch? Because if the switch is off, only 4 LEDs are powered so I would have to divide 90 by 4 instead of 15..

Joined Dec 26, 2010
2,148
If you read up on basic series and parallel circuits this should all become obvious to you. It isn't rocket science, as they say.

Until then, here are a few pointers.

1. A separate resistor for each LED is very desirable. It is possible to use a single resistor for a group, but the brightness of the LEDs may be less consistent, and the reliability will be poorer.
2. The resistor for each LED or group of LEDs must be calculated for the current and voltage concerned. Why in the name of... would anyone imagine that a single LED would require the same series resistor as a group of several in parallel?
3. If you have two parallel groups switched separately, the minimum requirement would be a separate series resistor for each group. This will avoid a current change when the second set turns on.
4. The resistor value needed for a single LED or parallel group is (Vs-Vf)/I, where Vs is the supply voltage, Vf is the LED forward voltage, and I is the total current for the number of LEDs concerned. Therefore, if a single LED requires R ohms, a group of ten LEDs in parallel would require R/10 ohms.
5. Running large parallel groups of a little 9V battery is a very bad plan. You won't get 100mA for long at all - maybe a few hours. Better results might be obtained with the LEDs rearranged in series pairs [with less series resistance: R=(Vs-2Vf)/I ].
6. Better still, use a battery made out of AA cells - depending on the LED voltage you may be able to use three or four cells to run LEDS - not in series but singly.

Joined Dec 26, 2010
2,148
Good grief, these things are rated at 100mA (max) each ! http://www.vishay.com/docs/81011/tsal6400.pdf

There is no way that you could run 15 of these contraptions off a small 9V battery. At full power, a parallel group of 15 would draw 1.5 amps. Even putting them into series strings of two or three would still need more juice than a battery like that could provide.

Edit: You might well choose to run each LED at less than 100mA, especially if you use the riskier option of not having individual resistors, but this still looks way beyond what this sort of battery can handle. http://en.wikipedia.org/wiki/Nine-volt_battery

What sort of battery did you have in mind, and how long do you need this light to run for?

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