By Rob Petkau (Madcow)
“Which resistor do I need?” If you’re new and you’re building your first lightsaber, this question is going to come up if you’re using a resistor with your LEDs. I get asked this question all the time. In this article I’m going to show you how to find your resistor and it’s going to be a lot simpler than you think.
How to Find the Resistor for Your Lightsaber
Choosing the correct resistor helps ensure your lightsaber’s LED receives the proper voltage and current. To find your resistor, you’re going to need three pieces of information:
1) Your Battery Voltage
If you’re using a lithium ion battery like most of us do for sabers nowadays, you’re probably working with a 3.7V battery or a 7.4V battery pack. Those are two of the most common. Now for the purposes of today’s tutorial, we’re going to round the voltage up. The reason we’re rounding up is a 3.7V lithium ion battery fully charged is going to be about 4.1V.
You’re going to see that calculating a resistor, while simple, actually isn’t an exact science. Calculating a resistor is more like horseshoes and hand-grenades than sniper rifles. So what we’re going to be is we’re going to round up a 3.7V battery to 4V and we’re going to round up a 7.4V battery pack to 8V to make it really simple.
2. The Maximum Current (in amps) Your LED Can Handle
Finding the maximum current in amps that your LED can handle is actually really easy. If you’ve purchased your LED from The Custom Saber Shop, just go to the product page where you ordered your LED and right at the bottom there are a couple of key pieces of information. One of them is the maximum current in amps.
You may see the maximum current listed in milliamps (mA). We’re looking for your maximum current in amps. Many Cree LEDs use a maximum 1,000mA, which is equal to 1 amp. Since 1,000mA equal 1 amp, 700mA would equal 0.7amps. You get the idea. Find the maximum current in amps.
3. The Forward Voltage (Vf) of Your LED at the Maximum amps
The third piece of information that you’ll need is the forward voltage of your LED at the maximum amps because the forward voltage (Vf) isn’t a set figure for your LED. The Vf varies depending on the current–the amps. So at your maximum amps, find the forward voltage of your LED. The information will be listed on the product page, if you’ve purchased your LED from The Custom Saber Shop. If not, you may need to look up your LED and find the datasheet.
Applying Ohm’s law
So we’ve got the battery voltage rounded up, we’ve got the maximum current in amps, and we’ve got the forward voltage (Vf) of our LED at that maximum current. Congratulations, that’s the hard part! The rest of this is super easy. The rest relies on an equation called Ohm’s law.
Ohm’s law equation
Resistance = Voltage/Current
R = V/I
So in our application, simple resistance equals your battery voltage minus your LED’s forward voltage at maximum amps divided by your maximum amps.
R = (Battery Voltage – LED’s forward voltage at maximum amps) / maximum amps
In our example scenario:
R = (4V – 3.5V) / 1 amp
R = 0.5V / 1 amp
R = 0.5 ohm
In the above example, a 0.5 ohm resistor is the measurement of resistance that we’re using so we need a 0.5 ohm resistor in this application. Congratulations, you’ve used Ohm’s law to calculate your resistor.
Choosing a resistor for a high power lightsaber LED
When I use a resistor for a high power LED I’m going to use a 3W or 5W resistor. You’ll notice that watts (W) is another thing you’ll have to select when choosing your resistor. So we need a 0.5 ohm resistor. I’m going to go with a 3W resistor.
Now there is a separate calculation to calculate watts. The reason I use 3W or 5W is because watts is a measurement of heat and resistors, among other things, dissipate heat so I might err on the side of a beefier resistor for high power applications like this. If you get a 3W resistor or a 5W resistor it’s not going to change the value very much of the important stuff, it’s just going to manage your heat. So for high power LEDs, I recommend a 3W or a 5W resistor.
Choosing a resistor for an accent LED
If you’re using Ohm’s law to choose the right resistor for an accent LED, like the LED inside your anti-vandal switch (AV switch) for example, then you’re not going to need a 3W resistor for that because an accent LED doesn’t use that many milliamps. It’s probably only a 20mA LED inside the switch so you can use a much smaller wattage resistor, maybe a 0.25W resistor, but you can figure that out now.
Tutorial – FJK’s “Down and Dirty” guide to Ohm’s Law
Ohm’s Law and Resistor Band Chart (with calculator)
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Rob Petkau (Madcow) is a sabersmith and the founder Genesis Custom Sabers, a custom saber based in Canada.
The Custom Saber Shop is a United States based DIY custom saber parts and accessories store.
Note from SaberSourcing: This tutorial is adapted into article form, with permission, from the The Custom Saber Shop video Finding My Resistor.