Splitting DC power supply risk?

Question

I am looking into taking the power supply 12V output of my PrintrBot and plugging a very simple parallel circuit with some LEDs.

I know I can connect a parallel circuit similar to this Splitting a power supply between two circuits. And it would be particularly easy if they use the same voltage.

I also know that I could put 4 3V LEDs in series without a resistor (or add a resistor if the LEDs are rated for less voltage), and they should work properly with a 12V input... and I believe my power supply has enough juice for this (it is a 6A power supply).

My question is:

How to minimize the risks to my printrbot circuit? (I guess there is always a risk that an issue in my LED circuit would affect my printrbot circuit).

While a 4 LEDs in series circuit sounds like the most simple circuit in the world, I know little about electronics (and my experience is very limited). For instance, I am concerned that things like a short in my LED circuit would ruin my PrintrBot circuit or that a LED getting burnt while the printrbot is printing would cause an issue.

Note: I know some people powers the LEDs out of the BED port in the board, but that's too risky for me.

UPDATE after getting answers (2014-09-28).

As explained in the 3 answers so far, some of the asumptions related to the LEDs made by me in the question above are plain wrong. Please read the answers below and do not let my asumptions mislead you.

Answer 1

You need resistors to limit the current. This circuit should meet your needs:

^{simulate this circuit – Schematic created using CircuitLab}

The resistors limit the current through each pair of LEDs to about 15 mA. The total current draw can be easily handled by your 6A supply. Provided you construct the circuit correctly, there will be no risk to your printrbot circuit.

Answer 2

Firstly, no you can NOT just throw 4 LEDs in series that happen to add up to the supply voltage and expect them to work - or at least not for long.

If I had £1 for every time I heard someone say that I'd be one rich man indeed!

When above their forward threshold voltage an LED has little or no resistance. The current that is allowed to flow through it is near infinite. With no resistor in the circuit too much current will flow and your LEDs will explode. Literally. This is a picture of an LED that I removed from a circuit that someone had designed badly by not having any resistors:

So correct your thinking on that front.

Now, assuming you create your LED circuit properly, there is little or no risk to the rest of your setup, as long as the power supply can provide enough current to drive both circuits.

An LED circuit is very passive and doesn't induce any noise or spikes or anything like that back into the power supply circuit.

Answer 3

LEDs aren't 'rated for' a voltage. They drop a voltage that is relatively constant. If the supply voltage is lower then the LED will draw almost no current, and if it's even a little higher the current will increase dramatically. You need a resistor to limit current. You can calculate the value required using Ohm's Law (V=I*R). For example, to run three 3V LEDs in series from 12V the excess voltage is 12-(3x3) = 3V. If you want the LEDs to draw 20mA then the resistance required is 3V / 0.02A = 150 Ohms.

You may be confused because LED strips are often rated at 12V and don't need resistors, but that's because they already have resistors built in. Individual LEDs always need a current limiter.

Splitting the supply off as you describe should have no effect on the printrbot circuit. LEDs draw pure DC current so there won't be any spikes. The worst that could happen is if you short out the power supply. So include those resistors - and make sure your wiring is neat and properly insulated!