Voltage/current for noods

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robibobbie
 
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Voltage/current for noods

Post by robibobbie »

If I put two 30 cm noods in serial the voltage should be 2x3V.
What current should I use?
2x50mA or just 50mA?

And if I put two noods in parallel should I just use 3V?
And how about the current in this case?

Thansk!

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adafruit_support_bill
 
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Re: Voltage/current for noods

Post by adafruit_support_bill »

Wired in series, you will need twice the voltage, but still the same current.
Wired in parallel, you would need twice the current, but the same voltage.

However, when wiring in parallel, it is best to use a separate resistor for each nood, since natural manufacturing variations can result in more current flowing through one nood than the other.

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robibobbie
 
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Re: Voltage/current for noods

Post by robibobbie »

Great!

So should the current be limited to 50mA for one nood?

That confuses me...
I'm used to that the voltage is important, too little will not work and too much will burn down the house.
And the current could be anything just its enough. The device will use what it needs.

Thanks!

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adafruit_support_bill
 
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Re: Voltage/current for noods

Post by adafruit_support_bill »

You want to limit the current to 50mA per nood in all cases.

When wired in series, there is only one path through the circuit so the same current needs to pass through both noods, so you limit the total current to 50mA.

When wired in parallel, there are two possible paths and you want to limit the current through each path to 50mA.

Power is voltage multiplied by current. So in both cases the power is the same:
  • 6v x 50mA for series
  • 3v * (50mA + 50mA) in parallel

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robibobbie
 
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Re: Voltage/current for noods

Post by robibobbie »

Excuse a newbie :-)

Why do I need to limit the current?
For other devices current can be more than specified, the device just takes what it needs.

Thanks!

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Re: Voltage/current for noods

Post by adafruit_support_bill »

Why do I need to limit the current?
Because you don't want to burn out the noods.
For other devices current can be more than specified, the device just takes what it needs.
That is true of some devices under some conditions. Most devices will draw more current as the voltage increases. If the input voltage is well regulated, the current drawn by the device will be fairly stable as well.

Diodes (including LEDs) in particular have a non-linear response to the applied voltage. Above a certain level (which happens to be the level at which LEDs begin emitting light), the current begins to increase rapidly. So even small variations in the supply voltage - or small manufacturing variations in the characteristics of the individual LEDs can result in dangerous current levels.

https://www.waveformlighting.com/pcb-de ... -resistors
Image

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robibobbie
 
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Re: Voltage/current for noods

Post by robibobbie »

My head hurts :-)

How to limit the current?
Its mentioned to use a resistor of 50Ω at 3V.
Is that regardless what current you use as long as its more than 50mA?

I = V/R = 3 volt/50 ohm
= 0.06 ampere (A) = 60 mA

So for 6V do I need to double the resistor to 100Ω?

So do I put the resistor in serie with the serial noods?
Is there a image of this?

Thanks!

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Re: Voltage/current for noods

Post by adafruit_support_bill »

As the graph shows, the current through a LED will increase non-linearly with increased voltage. In theory, you can limit the current by limiting the voltage. But as the graph shows, even very small variations in the supply voltage - or of the LED itself, the current can go dangerously high very quickly.

The noods have a nominal forward voltage of 3v and are rated for operation with a forward current of 50mA. You can omit the resistor if:
a) you have a power supply that will absolutely never ever ever deliver more than 50mA.
- or -
b) your particular nood has an actual forward voltage of 3v or more and your power supply will never ever ever deliver more than 3v.

Not many power supplies have active current limiting, so case (a) is typically limited to specialized LED drivers.

And it is difficult to guarantee that both conditions of case (b) will always be true at the same time.

So we add a resistor in series with the nood - just as we would with any other LED - to absorb any excess power that might flow through the circuit.
https://learn.adafruit.com/all-about-le ... ge-and-kvl

The suggested resistor values sacrifice a little bit of absolute maximum brightness for a bit of safety and a longer lifetime of the LED.

50 Ohms should be sufficient if your power source is close to the sum of forward voltages of the noods that you have in series. i.e. 3v for 1 nood. 6v for 2 noods in series.

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