Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: Southern Sailor on April 12, 2020, 02:58:49 pm

Title: LEDs in parallel
Post by: Southern Sailor on April 12, 2020, 02:58:49 pm
Good day folks.  I'm not an electrical bloke, but thought I would share something I found the other day while getting my lights on Portgarth set up. I connected my nav lights in parallel but the one would not light.  It seems that because red and green operate on different voltages, the lower voltage light limits the voltage to the other as well so the other does not light because of under voltage.  I still have to re-connect these to see if that is the problem.  Any comments? Thanks. Brian
Title: Re: LEDs in parallel
Post by: leachim on April 12, 2020, 04:32:33 pm
I had exactly the same problem with 3 white and 1 yellow LED in parallel on the same circuit. With the yellow connected only the yellow would light up, the whites going out. Remove the yellow one and the whites lit up ok. The only way I found to resolve the problem was to provide the Yellow LED with its own positive feed, all were connected to a common negative.
However I didn't have any problems running red and green on the same circuit.

Title: Re: LEDs in parallel
Post by: LJ Crew on April 12, 2020, 04:40:51 pm
Hello Brian, I assume that you have connected the LEDs in parallel with a series resistor. If you connect the LEDs in series with a suitable resistor they will all light provided the forward voltage drop across the LEDs is less than the supply voltage. Alternatively, feed each colour with a separate resistor in series. I hope this helps. John.
Title: Re: LEDs in parallel
Post by: leachim on April 12, 2020, 04:49:44 pm
It could well be worth checking you have the correct series resistor, it is just possible your resistor isn't allowing sufficient voltage through to light both LEDs.

Title: Re: LEDs in parallel
Post by: malcolmfrary on April 12, 2020, 06:46:02 pm
One of the properties of LEDs is that in one respect they act like Zener diodes.  In an environment with limited current, they don't pass any current until the voltage presented to them goes above their forward voltage.  A Zener just passes current at that point, and regulates the voltage across itself atwhatever voltage it says on its label.  Very handy when making a voltage regulator.  An LED also regulates its voltage, but lights up as well.
Putting two or more in parallel, the one with the lowest forward voltage wins and lights up.  The rest never see their "lighting-up voltage", and don't.
Having one, single, resistor of too high a value will, even if all the LEDs get to their illuminating voltage (assuming they all want the same) they will light up, but not as brightly, because the current through the resistor will be distributed among the LEDs, so each one gets less.
If the LEDs have a wide enough spread, thenthe lowest voltage one will win for a time, then it will die, and the process will repeat until there are no LEDs left in circuit.
If anybody wants to understand basic electronic components, then ( has a free download option.  Although a technical book, it is in real English, rather than college technical.
Title: Re: LEDs in parallel
Post by: Southern Sailor on April 12, 2020, 09:10:12 pm
Wow guys.  As usual a wealth of knowledge and plenty of willing helpers and useful links.  Thank you. It will give me something to think about during the shutdown. Happy sailing. Brian