Resistance Calculation for Series Circuits

The very heart of the matter. Other websites may offer you calculators to work all this out for you, but we reckon its always better to know how to do things yourself so when the machines stop working you can carry on....

You can skip straight to working out Resistance Calculations for Parallel Circuits but if you are new to all this we strongly advise you get your head around this page first...

You can read about Current, Power, Voltage & Resistance here.

To work out the resistor needed for your series circuit;

What is the voltage of your power supply? Call this Vsupply

What is the combined voltage of your LEDs? This will be V1+V2+V3... Call this VLED

What current are your LEDs rated at? (this needs to be the same value for each LED you intend to use). Call this i

You are now ready to calculate the required resistance:

Resistance (in ohms) =  

( Vsupply - VLED )

If Vsupply is 12volts and V1 is 3.4v, V2 is 3.2v, V3 is 2.2v and V4 is 2.8v and i is 0.025 amps then

Resistance needed would be  

( 12 - 11.6 )

  = 16 ohms (Ω)

Look at the resistances available (you can't get them in every value) and pick the exact value if it's available or the nearest higher value.

Okay so far?

Now we also need to work out how much power the resistor is being asked to soak up, so we can use the correct type of resistor:

Power (in watts) is calculated by multiplying current (i) by voltage (v). Simple.

The power going into our circuit is Vsupply

And our circuit is using 0.025 amps because that's what the example LEDs are wanting.

We calculate that the power used will be 0.025 x 12 = 0.3 watts.

Resistors are available in several wattages - we supplied 1/4 watt, 1/2 watt, 1 watt, 2 watt and 6 watt types.

As best practice you would be looking for a power rating of at least 10% more than the value calculated - 0.33 watts, then you'd get the next higher available type, ie 1/2 watt.

For 12v circuits we have found that 1/4 watt resistors have sufficient tolerance to be used for standard 5mm LED circuits of the above wattage, but this is not strictly 'best practice'.

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