Electrical – Use AC to illuminate DC LED’s

A transistor is basically a switch that can disconnect ground or 5V. There are transistors of all sorts, and some can switch truly huge loads. 50 LEDs is no big deal.

So, the question then is this: Can a single 2N3904 switch 50 LEDs?

To answer that, we'd need to know first what kind of LEDs. But let's assume you are using the usual nothing-special variety. A reasonable estimate of the maximum current of these is \$20mA\$. If you have 50, then the maximum current is \$20mA \cdot 50 = 1000mA \$. Looking at the datasheet I see the maximum collector current for 2N3904 is \$ 200 mA \$. So the answer is no: you can not switch 50 LEDs with one 2N3904.

I suppose you have several options:

• use multiple 2N3904
• use fewer LEDs
• use the same LEDs, but drive them with less current
• use a bigger transistor (TIP121 is very easy to find)
• use some other switching device

Of these, I think reducing the LED current or using a larger transistor is probably the most likely solution. Other switching devices (like a relay) are probably more expensive and slower.

If a module has only two terminals, but has some parallel LEDs internally, you can probably assume there's something inside the module to balance the current between each parallel circuit. This is made somewhat easier when all the LEDs are in one module because they can share a heatsink, and thus the temperature of each is likely close, and temperature variations are a big factor in why parallel LEDs will not share current equally. Each LED was also likely manufactured at the same time, so have very similar characteristics, which you can't guarantee when taking discrete LEDs out of a jar. The module may also include some small resistance in each parallel circuit to help balance. Point being: assume the manufacturer has taken care of this, and all you need to worry about is supplying the correct current to the two terminals provided by the module.

Your particular module is really like three modules, one for each color. It looks like there is one common terminal on the bottom, and three separate terminals on the top, one for each color. I can't find a datasheet that specifies if the cathodes or the anodes are common, so you may have to figure it out for yourself. It looks like maybe you can cut the common terminal apart if you want, but again, I see no datasheet, so you might have to experiment for yourself.

There is no special kind of LED driver that can drive parallel LED circuits. If an LED driver's job is to pump electrons, there's no way for it to tell some electrons to go down one circuit while telling others to go another way. The electrons decide which way to go by going whichever way minimizes their potential.

So, what you want to do with this module is power each of the R, G, and B sub-modules with a suitable driver (or if you can find it, one box that actually has three drivers in it). What you don't want to do is try to put the R, G, and B sub-modules in parallel and drive them all together. Since each color has a different forward voltage, this won't even remotely work: the color with the lowest forward voltage (red) will take very nearly all the current and all the power, and possibly be destroyed. At best you just won't get the other colors to light.