4 replies to this topic

[dielated]

Question: Is it safe to run two 12v LED bulbs on a single NPN booster circuit, and control both lights with a single Pinscape output/ DOF device. Going further, is it safe to operate 4 LED's from the same Pinscape output/DOF signal? Do I need to increase the amount of resistance on the ground wire going to the Pinscape board?

 

Context: I am putting two light fixtures on my back-box, each with two 12v LED bulbs inside. I would like to connect the pairs of bulbs in series and essentially treat them as a single light. Could I wire two, or even 4 LED bulbs in series to save wiring complexity? I know I can 'daisy chain' the 12v power line going to each light, but is it safe to 'daisy chain' the ground and control wires as well?

 

I am using a standalone kl25z board, and using the NPN booster circuit mentionned in the Pinscape guide.

 

Thanks in advance for any guidance!

 

Mitch

 

 

 

[mjr]

Yes, you can do that.  You just have to be sure you have enough voltage for the combination of LEDs, and use the right resistor value.

 

The limiting factor in the booster circuit is generally current.  (There's a voltage limit as well, which depends on the type of transistor you're using for the booster, but for typical NPNs it's high enough that it won't be a factor, usually 40V or higher.)  When you connect LEDs in series, a single current stream flows through all of them, so the total current that the booster has to handle is the same as the current for a single LED.  That means that you can in principle connect infinitely many LEDs in series, and you'll still have the same current as one LED.  However, the trick is that you have to increase the voltage to match.  Each LED has a "forward voltage" (often written as "Vf") value that you can find in its data sheet, and when you connect multiple LEDs in series, you have to add up all of the single LED voltages to get the total overall voltage.  The supply voltage has to be slightly higher than that total.  So, for example, if you're connecting 4 LEDs with Vf of 2.2V, the supply voltage will have to be higher than 8.8V.  That'll work with your 12V supply.  If you have four LEDs with Vf of 3.4V, the total is 13.6V, so 12V isn't enough to power it.

 

You'll also have to choose the appropriate resistor size for the combination of LEDs.  There's a nice online calculator here that will do the math for you:

 

http://led.linear1.org/led.wiz

 

That will also show you the wiring diagram for the serial circuit, to double-check that you have the right wiring in mind.

One additional note: You mentioned daisy-chaining the power and ground connections to the LEDs.  "Daisy-changing" usually means that you're connecting all of the like connections together - connect 12V to all of the LED (+) sides, and connect the booster port to all of the LED (-) sides.  If that's what you had in mind, you're actually talking about a parallel topology for the LEDs, and what I said above doesn't apply to that.  In a parallel connection, the current through the booster will be the sum of all of the single LED currents, so the current limit on the transistor is the limiting factor.  To check if that's safe, you should take the "forward current" values for all of the LEDs that will be connected to the same booster circuit, add them together, then make sure they're less than the maximum collector current (Ic) for the NPN transistor you're using.  The common NPNs like 2N4401 can handle about 400 mA, which is enough for many small (20mA) LEDs, but only enough for one of the large "star" LEDs (those run at about 350mA per channel).

[dielated]

Thank you - I was indeed thinking of a 'Parallel' circuit while asking about 'series'.

The lights I'm using don't have a spec sheet to go from. (Cheap 'T10' automotive LEDs from amazon), they only give me these specs:

(Luminous Flux: 80Lm/bulb) - (Voltage: 12V-DC)- (Output：60MA) - (Wattage: 0.72W/Bulb)

 

Just checking my understanding:

-For Parallel wiring - The output of 60mA/ bulb is well below the .400mA ballpark current limit, meaning I should be fine connecting 4 bulbs in parallel to one booster circuit?

-For series wiring - If I proceeded to wire in series and it 'didn't work', the lights simply would not light up? And without proper specs I'm not really able to guess if my 12v power supply would actually light all the lights - so I could try just wiring up 4 bulbs in series and seeing if my power supply made them light?
-Re Resistors - my T10 automotive lights likely already have some kind of resistor built into them (I have these wired up for flipper button lights and did not insert a resistor into the circuit), - would this mean I likely do not need additional resistors whether going series or parallel?

 

[mjr]

> The output of 60mA/ bulb is well below the .400mA ballpark current limit,

> meaning I should be fine connecting 4 bulbs in parallel to one booster circuit?

 

That should work - 4 x 60ma = 240mA, so you should be safe going with the parallel setup.

 

And yes, you will definitely want to use parallel wiring for these.  Any LED like that that's sold as a replacement for incandescent bulbs has to be plug-and-play in a regular circuit, so it has the necessary resistors built in.  That means you can't connect a bunch of them in series, because for a series setup you always need to choose a different resistor based on how many LEDs are in the series, and you can't change the resistors when they're built into the bulbs.   Which also means you're correct that you don't need additional resistors, no matter how you're wiring them - single or multiple in parallel.

Edited by mjr, 06 November 2020 - 10:27 PM.

[dielated]

You're the man MJR, thanks again.