## What voltage for LEDs is not the question to ask...

phyrephish
King Tiger Commander
Posts: 135
Joined: Sun Mar 26, 2017 10:24 am

### What voltage for LEDs is not the question to ask...

I have been asked so many times" What voltage for LEDs?" that I thought it would be best just write one response and put it on the forum.

The first thing to realise is that is not the question that needs to be asked. LEDs are NOT voltage rated they are CURRENT DRIVEN. LEDs will work with any voltage greater than their Voltage drop rating which can be as low as 1.5 V or as high as 3.6V. Brightness is instead more proportional to current. As a general rule of thumb most common LEDs used with R/C vehicles require about 20 milliamps to run and have a voltage drop of 1.5 to 2.0 volts. A LED running off 5 volts with 20 milliamps of current will be brighter than a LED running off 12V and only 5 milliamps of current. It is when you supply TOO much current that the LEDs will "blow". So it is CRITICAL to LIMIT the CURRENT going to the LED. Yes, putting too much voltage through a LED will also destroy it but not the voltages 3v - 24v that we typically come across in this hobby.

So the more correct question to ask is How do I get enough current to my LEDs with voltage X volts? That is simple - use OHM's Law V=IR. So if you have 1 LED needing 20 milliamps, a voltage drop of 2.0 volts and a 5 Volt supply OHM's Law will tell you

5.0 volts - 2.0 volts = 0.02amps x R ohms

R ohms = 3.0/0.02

R ohms = 150

So if you have a single normal LED and you want to plug it in to a 5 volt power supply then you need to add into the circuit a resistor of about 150 ohm to LIMIT the current running through the LED.

If you want to power the SAME LED with 220 volts your resistor would need to be R= 218.0/0.02 = 10.9 KOhms. I would not do it as your usual hobby sized resistor would glow hotter than the sun.

Since this is a common question in general you can find many LED calculators online
http://www.ohmslawcalculator.com/led-re ... calculator

NOW FOR THE HARD BIT

LEDs Wired in Series

The above calculations are fine for a SINGLE LED, like say wiring up a single headlight/notek/searchlight, machine gun LED or IR LED. HOWEVER, more than likely you will need to wire up multiple LEDs as the case for dual head or rear lights and things are a little more complicated here. Heng Long et al., will typically wire up a pair of headlights in SERIES and hence the negative (cathode) of one LED will directly connect to the positive (anode) of the other LED.

A good calculator for LEDs in series can be found here:
https://www.hobby-hour.com/electronics/ledcalc.php

If you have a play with the calculator you will see that you can ONLY have 2 LEDs wired in series and a 5 volt supply. That is because of the LEDs voltage drop. Once the "voltage" goes through one LED it is reduced. So after the first LED there is only 5v-2v=3volts left for the second LED. Since the 2nd LED has a voltage drop of 2 volts that is fine, we have 3 volts. However, once going through the second LED the voltage drops again 3v-2v=1volt. So there is only now 1 volt left meaning we can not power a third LED.

This is a trap for many a newcomer to the ways of wiring up LEDs, everything is going good add in another LED and nothing. To wire up multiple LEDs the they are best wired up in PARALLEL.

LEDs Wired in Parallel

So now we get the the BOMB of LED calculators, this baby will even show you the circuit you need to build.
https://www.amplifiedparts.com/tech-art ... calculator

I really don't need to add to much more, the calculator above will explain and show it all

Happy LEDing
P-fish