current draw on headlight circuit?

My 20amp fuse blew today on the headlight/backlights circuit. I hooked up a meter and it was pulling 25amps when the bike was running. Is this normal? I think I remember the fuses only being 15amps when I replaced them. The only thing I changed on the circuit where the tach/speed gauge back lights and I changed those to LEDS. What do you guys think? The bike doesn't like to start after sitting for a while so I have a feeling there is a short and its killing the battery.

 

Let's assume you have a 60w headlight (stock on the 650 Maxim was 40 low, 50 high). That's 5 amps (5 amps X 12 volts = 60 watts), or less if your voltage is up to snuff.

I don't think the taillight even on the same fuse, but I'm too lazy to pull out the electrical diagram, so let's assume it is. Taillight draws 8.6 watts (less than 1 amp).

Then add in a couple amps for all the other stuff to be safe.

8 amps total.

You are drawing way too much power on the headlamp circuit.

 

This seems to be the simplest circuit on the bike. Weird that its drawing 25 amps. Should I check the wiring to make sure there isn't a short? This is the strangest problem. I attached a copy of the lighthing schematic for my Xj550. The white part is the circuit where the fuse that keeps blowing lies. 25a on this does not seem right. Hope this helps someone and someone can help me. Thanks, nik

 

Check for corrosion on ALL of the contacts. The corrosion on the pins will raise the resistance, requiring additional current draw. Remove as much of the corrosion as possible with a steel toothbrush.

 

Corrosion drawing 180 watts? That is a lot of resistance. How many watts can the battery/alternator put out while the bike is running? It might just be totally shorted out if it is anything near 200 watts.

 

How did you come up with 180 watts? 25 amps at 12 volts is 300 watts.

Your alternator can put out about 19 amps - but the battery will help until it's dead.

Since watts=volts X amps, if your voltage is low at the bulb it will draw more amps to get the power (watts) it wants.

I'm curious, how did you measure 25 amps? Most of us don't have meters that will measure that kind of DC amperage laying around the garage.

 

actually corrosion on the connectors will lower the total current in a circuit

 

Where your load has a fixed resistance that would be true. Light bulbs don't have a fixed resistance - they will suck more and more current until they reach their rated wattage.

You can check this yourself - pull out your multimeter and check the resistance on a light bulb. You'll find it's darn near 0. As current flows through it the filament heats up - the resistance rises - creating more heat - creating more resistance until it reaches its wattage. Of course it all happens very fast.

The extra resistance from a corroded connector creates a voltage drop. So less volts to the bulb, bulb draws more amps to make up for the reduced voltage.

 

I friend of mine had a mean multimeter laying around. 300watts is what it is pulling now, 120watts is what it should be pulling (generous), 300-120= 180 mystery watts. Sounds like a total short to me. Its a mystery that the headlights even turn on. Are the headlight supposed to turn on when you start the bike or only when the engine is running?

 

The headlight comes on when a certain voltage threshold is met. Mine comes on even if the bike doesn't start.