Ohm's Law - Learn it, live it, love it

[JedidiahStolzfus]

Been reading through several threads here about connecting up non-factory headlights and I see the term "wattage" thrown around a lot.  As though you can take something that is of similar wattage and just connect it without knowing the other variables involved in the circuit. 

Wattage is not set value when it comes to electronics as it can change based upon other values involved.  Please review the Ohm's law diagram.
Wattage is generally calcuated based upon knowing the current and the voltage of the circuit.  So let's look at a basic example for those of you that aren't into electronics. 

Your average hair dryer is 1500W, and in the US it would be running off a 120VAC circuit.
1500W / 120VAC = 12.5Amps
But in Europe and other countries, they use 220VAC
1500W / 220VAC = 6.8Amps
Increasing the voltage supplied lower the current drawn.  Running devices in your home on 220VAC is more efficient as it draws less current.  Less current, less heat.
Now in the cases of headlights I'm seeing lots of values being thrown around without much consideration as to how much current is actually going to be drawn in the circuit.  The headlight circuit in the controllers doesn't supply much, and if you connect something in there that's going to draw more than it can supply you'll quickly burn out the voltage regulator for that part of the controller.  Unless you're really handy with a soldering iron and trouble shooting circuits, you'll be buying a new controller.
Let's do some math, cause math is fun.
You buy a headlight that's an 18W, but it needs 12V to function.
18W / 12V = 1.5A

What if the circuit only supplies .5A or 500mA?  You just tripled the amount of current the circuit needs to supply.  It will work, but only for a little bit before the magic blue smoke is released.
Part of the problem in figuring these things out is what voltage and how much current the headlight circuit is designed to supply.  You can measure the voltage with a standard multimeter and that will give you a start, but unless you can get the manufacturer to tell you how much current the circuit was designed to supply you're just kind of guessing.  And when you're guess, it's best to guess low.
Let's got back to our 18W light.
18W / 36V = .5A  which fits into our example from above.18W / 48V = .375 which again would fit.
You need to know the voltage that the light circuit is supplying before you can make any kind of proper guess if it's going to work without burning out the circuit. 

Most everything in electronics can be expressed as a math equation, even horsepower.  1Hp = 745W, so your 1500W hairdryer is a little more than 2Hp.
How many horsepower does your average horse supply?  Between 20 and 25hp.
Please before you start hooking random lights up to your bike's controller, take some time to measure voltage of the circuit and do the math.  You do not want to release the magic blue smoke of your controller.  Once it's released you can't put it back in.

[Muskie1]

 I agree, I did not see a need or I never will cut into factory wiring. My auxiliary lights work awesome, rechargeable.  I get 3 to 4 hours on high beam. I don't do much night riding anymore more mostly strobe for day.
I am by far a electronics engineer  but I still remember what way a diode went in and the resistor codes. BBRYGBVGW.

[JedidiahStolzfus]

I agree, I did not see a need or I never will cut into factory wiring. My auxiliary lights work awesome, rechargeable.  I get 3 to 4 hours on high beam. I don't do much night riding anymore more mostly strobe for day.
I am by far a electronics engineer  but I still remember what way a diode went in and the resistor codes. BBRYGBVGW.

We learned a mnemonic to remember that.  You're missing some.

Bad   0 Black
Boys  1 Brown
Rape  2 Red
Our    3 Orange
Young 4 Yellow
Girls   6 Green
But     7 Blue
Violet  8 Purple
Gives  9 Grey
Willingly 10 White
Get 0% Gold
Some 10% Silver
Now > 10% No tolerance band
You also don't need to cut into the harness.  You can buy those connectors on Amazon.  They're overpriced, but they're available.

[Tree]

I agree, I did not see a need or I never will cut into factory wiring. My auxiliary lights work awesome, rechargeable.  I get 3 to 4 hours on high beam. I don't do much night riding anymore more mostly strobe for day.
I am by far a electronics engineer  but I still remember what way a diode went in and the resistor codes. BBRYGBVGW.

We learned a mnemonic to remember that.  You're missing some.

Bad   0 Black
Boys  1 Brown
Rape  2 Red
Our    3 Orange
Young 4 Yellow
Girls   6 Green
But     7 Blue
Violet  8 Purple
Gives  9 Grey
Willingly 10 White
Get 0% Gold
Some 10% Silver
Now > 10% No tolerance band
You also don't need to cut into the harness.  You can buy those connectors on Amazon.  They're overpriced, but they're available.

jesus. i hope they've come up with a better mnemonic since then. that's horrid.

[JedidiahStolzfus]

I agree, I did not see a need or I never will cut into factory wiring. My auxiliary lights work awesome, rechargeable.  I get 3 to 4 hours on high beam. I don't do much night riding anymore more mostly strobe for day.
I am by far a electronics engineer  but I still remember what way a diode went in and the resistor codes. BBRYGBVGW.

We learned a mnemonic to remember that.  You're missing some.

Bad   0 Black
Boys  1 Brown
Rape  2 Red
Our    3 Orange
Young 4 Yellow
Girls   6 Green
But     7 Blue
Violet  8 Purple
Gives  9 Grey
Willingly 10 White
Get 0% Gold
Some 10% Silver
Now > 10% No tolerance band
You also don't need to cut into the harness.  You can buy those connectors on Amazon.  They're overpriced, but they're available.

jesus. i hope they've come up with a better mnemonic since then. that's horrid.

Apparently there's a bunch of them: https://en.wikipedia.org/wiki/List_of_electronic_color_code_mnemonics

[Roverdrive]

Bad Boys is the way I learned in college, but that was 1973

[handlebar]

Increasing the voltage supplied lower the current drawn.  Running devices in your home on 220VAC is more efficient as it draws less current.  Less current, less heat.

...


You buy a headlight that's an 18W, but it needs 12V to function.
18W / 12V = 1.5A

Power comes down my street at 9,000 volts. That's for efficiency. Transformers send it to homes as two 120 volt lines, 180 degrees out of phase. That's for safety; the lower the voltage, the less likely it is to kill a grounded person.

My range is on a pair of 40 amp breakers connected to the two out-of-phase supply lines for a difference of 240 volts. Household wiring has to be big enough to handle breaker current with minimal power loss, so it's not for efficiency so much as wiring convenience and fire safety. Eight-gauge copper is adequate for 40 amps. If the range used one side of the transformer line, 120 volts, it would need 80 amps. That would require a 4-gauge circuit. That has three times more copper than 8-gauge, so it's expensive, and it's harder to install. It would be controlled by a single 80-amp breaker. If all that current were to weld the contacts, the house could burn down. The 240-volt circuit has two breakers, and either one will shut off the circuit.

An LED uses about 3 volts. A light advertised for a higher voltage probably contains a buck converter to step down the voltage efficiently. An 18 watt light would probably use approximately 375 milliamps from a 48-volt system.

I wonder if cable extenders for headlight circuits are available. One could separate the conductors in an extender in order to measure the current with a clamp-on meter, then remove the extender to return to the original wiring.

The OEM headlight on my Radrunner would stay on about six seconds after I switched the battery off. Current draw be very low. My Radmission has a much more powerful light. When I switch off the battery, it goes out in less than a second.

[Muskie1]

Bad Boys is the way I learned in college, but that was 1973

The same way I learned it in 1975 , still wish I had met Violet.  I think it is socially unacceptable now. Funny I can remember that in a second.  Dividing fractions no.

[handlebar]

Bad Boys is the way I learned in college, but that was 1973

I heard it in the early 60s, but it's clumsy. By 1973 I was using images.
1 brown shoe
2 red Indians
3 oranges
yellow is 4 cowards
I remember green is 5 but don't remember the associated image.
6 the deep blue sea.
7, 7, purple heaven
8 greyhounds
9 White Sox
0 black as midnight, zero hour.

[Altema]

It's simple math, but yes, the average person will be clueless.
I'd prefer not to cut into factory wiring, but I had no choice with this bike because no one else had done these mods before. I literally had to map out the schematics and make an entire wiring harness from scratch. Hardest circuit I think was having the lights switched on the positive side, but the brake light switched on the negative side, and they used the same feed.