# Resistors before every led?

I'm working on lighting up an entire LEGO city from my nephew. But the problem is I need a lot of leds with different amperes and voltages ( I'm gonna put them in parallel). So my 2 questions are :

1. Do I need to put a resistor before every led? or can I just put a resistor in front of a bunch of leds ( I have around 15-25 leds per resistor)?

2. I need for most of my leds about the same value of the resistor (around 150 Ohms). Can I put the resistor of exact 150 Ohms (1% tolerance) in front of the leds? or do I need to set the value a little bit higher with an extra resistor?

EDIT: also in the comments: 41 yellow and red leds have 2 volts and 20mA About 30 white leds have 3.5 volts and 20mA and about 10-20 leds have 2.6 volts and 70mA

• what colours and how many in series. – Sunnyskyguy EE75 Sep 16 '17 at 19:27
• It would help if you drew a diagram and supplied some more details on the LEDs. – David Sep 16 '17 at 19:27
• I'm sorry for that I didn't give any information about the leds. So here is the information: most of the leds ( 41 to be precise) are leds with 2 volt and 20mA. There are also about 30 leds with 3.5 volts and also 20mA. And and about 10 leds with 2.6 volts and 70 mA. Hope that is enough. If not just ask what you want to know. – Eli Van der Burght Sep 16 '17 at 19:32
• laymen explanation: a resistor-per-led allows precise brightness matching. LEDs with identical ratings can share a common resistor, as long as the total dissipated watts are held in-check and you don't mind subtle brightness differences from manufacturing defects. – dandavis Sep 16 '17 at 20:39

You should use a resitor for every series string of LEDs. Put as many LEDs in series as can be driven by the available supply voltage, minus a little. Then size the resistor so that you get the right current thru the string. Only string together LEDs that you want to run at the same current.

For example, let's say you have a 12 V power supply. You have a bunch of LEDs that you want to run at 20 mA, and that drop about 2 V in the process. 6 in series would come out to 12 V, leaving nothing for the resistor. Therefore, you'd put 5 in series. Those should drop 10 V. The resistor drops the remaining 2 V. You find the resistor value by Ohms law. (2 V)/(20 mA) = 100 Ω. Since 20 mA is the maximum spec, and you therefore don't want to exceed it, the next larger common size of of 110 Ω would be appropriate.

LEDs voltage at rated current has a rather large dispersion, so if you wire them in parallel, some will be a lot brighter than others. It is better to start with a high enough yet save voltage (like 12-24V) and wire the LEDs in series with one resistor per group.

• Thanks for the info. I think I have a spare car battery somewhere in my workshop. One last question: I want to connect the leds from the different buildings ( around 12 buildings) with a screw terminal, is that a problem? – Eli Van der Burght Sep 16 '17 at 20:38
• @EliVanderBurght Beware that a "spare car battery" laying around somewhere is likely no good. Lead acid batteries will destroy themselves if left alone too long. (The charge slowly leaks away and in the discharged state they slowly convert to a form that will not take a charge.) – Loren Pechtel Sep 18 '17 at 4:41

You can put leds with the same current requirement in series and use a single resistor or current regulator to drop the last bit of voltage.

Though the total voltage drop over the leds may then become too large to be safe. (keep it under 30V).

You can also put sets of leds with the same forward total voltage drop in parallel and sum up the current of each set and size the resistor to that.