Tag: series

We do this exercise when I teach electrical basics where we sit down and connect a 10 watt bulb to a power supply and through a switch. A SUPER SIMPLE circuit, the kind you might have learned about in high school science class.

But then I grab another 10 watt bulb and tell them to connect it in line with the other 10 Watt bulb (series circuit) and BEFORE they can turn the switch on I ask them a series of questions.

  • Will the two lights be twice as bright as the one? the same? or half as bright?
  • Will the circuit draw twice the amps as before? The Same? or half the amps?

Before we move on, I want you to make your choice.

So everyone makes their choice.. we turn on the switch…


The bulbs combined are half as bright, using half the amps and thus half the watts. On my quizzes this is an area where experienced techs and electricians will even get frustrated “If you have X2 10 Watt bulbs that is 20 watts” they will say.

The science is actually really simple. In a light bulb, they may be stamped with a rating wattage but that wattage is just a rated wattage when the full rated voltage is applied. The constant in a light bulb is the resistance in Ohms, not the wattage. When you double the resistance of a circuit by adding in another 10 watt bulb in series you are cutting the amperage in half and therefore also cutting the wattage of the circuit in half.

an electrical circuit is a path between two points that have a difference in electrical potential (Voltage) the amperage (and by extension the wattage) is a function of the total resistance of that circuit between those points. If the resistance goes up, the amperage goes down and vice versa. It doesn’t matter if that resistance is added by a bulb, resistor, thermistor, pitted contactor points, motors etc…

Now when we mix in inductive reactance in motors and other inductive loads that resistance is bit less cut and dry to understand… but we will save that for another tip.

— Bryan

Knowing how to properly combine capacitors in series and parallel is a great, practical field skill to employ when you need to get a customer up and running and you don’t have the exact size.

Increasing in size is easy. Just connect in parallel and add the two sizes together. For example, if you needed a 70MFD capacitor you could easily connect a 50 and 20 in parallel will add up to 70MFD. Connecting in parallel is as easy as making two jumper wires with connectors and jumping one side of each capacitor to the other and then connecting one side like usual.

Series is a little more tricky, it goes like this

Total Capacitance is 1 ÷ (1÷C + 1÷C) = Total MFD When Wired in Series

The result is that the total capacitance will always be less than the smallest capacitor. Let’s imagine a real-world scenario where you need a 3MFD capacitor and all you have is 5 & 7.5 MFD on your van.

The math would be

1 ÷ (1÷5 + 1÷7.5) = Total MFD


1 ÷ (0.2 +.13) = Total MFD


1 ÷ (0.33) = Total MFD


3.03 = Total MFD

Definitely not something you will run into every day but a nice knowledge tool to have in the noggin toolbox

— Bryan


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