Watts Law

What is Watt’s Law? _ “PIE ! That’s what comes to mind. Ok, that’s a joke. Actually, this is how I remember. This electrical law”

P = I x E => (PIE)

It defines the relationship between Power, Intensity of flow, and Electromotive Force.  Now we’ve seen before, that Intensity was measured in Amps and E in Volts.  Ultimately, the purpose of electrical flow is to do work.  In our uses, we need to provide the incandescence we see in a light bulb (by the way, they put glass around it so it doesn’t burn anything); a magnetic field that will rotate motor….  These are the most common applications on a vehicle.  Other less common application to create heat for various systems.

Here is how it works: The more Pressure there is, without too much opposition to Flow, the more Power we get.  In electrical terms: more Volts or more Amps => more Power.  One thing we should think is that in order to get more Flow or Amps, we need less Resistance.  As you can see, Ohm’s Law completely defined.

Automotive light bulbs are rated in Watts. One of the most common bulb was the dual filament Brake/Tail light bulb. This bulb is rated (on motorcycles) 12V 21/5W. Meaning, one filament is for 21W and the other for 5W.

Let see if you understood:

What filament has more flow? => the 21W or Brake light.

What filament has the most resistance? => the 5W of Tail light.

Needless to say, that if you manage to install that bulb 180º reversed, you’ve got a few problems.  One, it will blow your tail light fuse.  Two, the brake light will be too dim.  One of the most common mishandling of such is that it gets installed not quite 180º, but somehow across the two socket prongs in a way that it creates a short.  And then starts the head scratching about a fuse that shouldn’t blow?

Let’s get back to our application. Just a little Math.

How many Amps will flow in the tail light circuit?

P = I x E  =>  P / E = (I x E) / E  or  P / E = I

The result will be: 5W / 12V = 0.41A

So, more than likely the tail light fuse will be a 1Amp or more depending on what other circuits share that fuse.  Typically there will be a few more circuits sharing it, and what you’ll have is a 5A.  This is the type of application that concern us technicians.  Let say you want to add a new circuit with a 5W bulb, then you would know what fuse you would install in that circuit to protect it (installed in series of course).  One of the most common installations done on a bike are additional head lights.

Warning: Head lights will require the use of a relay to by-pass an original harness.  Any attempt to run 1 or 2 head lights through the OEM harness could result in melt down of the bike’s harness and switches.  We will address the use and application of relays at a later time.

And in the same way, we can calculate that the Brake of Stop lamp circuit would flow 1.75 Amps.  That is if we only have one lamp.  On many high end motorcycles, they install 2 Tail/Stop lamps, to get a safety extra bulb, just in case one burns.  Now, those bulbs are connected in Parallel.  Ok, we did not discuss the difference between a connection in Series, and one in Parallel.  Well, that will be the next topic.

On the troubleshooting side, one test can be performed to verify if a dual filament bulb is defective.  Using an Ohmmeter, you can check the resistance of the bulb.  It will require to test both filaments.  Notice how the two filaments have a common ground or negative side.  This ground is soldered to the metal part of the bulb’s body.  This body in turn is installed by a push twist action into the socket of the light’s housing.  To test each filament, you must probe from the body of ground tho one of the contacts.  Then from the same ground to the other contact.  You will get two different resistances. For the example of a 12V 21/5W or an 1157 (type of bulb), we could calculate once and for all for all 1157’s, what it should be.

For the Stop: R = E / I  or  12V / 1.75A = 6.8 Ω.  That’s in theory.  In reality it will be more like 1 Ω or less.

For the Tail: 12V / 0.41A = 29.2 Ω => in reality about 2-3Ω

One thing for sure, the tail will have more resistance than the stop.  And for a resistance test to give you an Open…  It all depends on how your meter indicates infinity. That’s for you to read your owners manual once and for all.