Lighting Mystery

[Timbone]

On my new build, I want a simple lighting system: click a toggle tied to a 12V SLA battery, headlight and taillight illuminate. Easy and simple.

To that end, I played off the $5 HD headlight idea on this site. Came out pretty well. Made a nice bracket and fashioned an on/off switch. Soldier good quality wire and connectors together solid.

Here's where I went wrong: the SLA battery is 12V 3aH. I figure it will produce 36W for an hour. Maybe. No biggie. I can leave room for error and still have good lights.

For the rear light, I bought a red "clearance light". It has a CR193 bulb in it. Googled it and it is said to be 5W, drawing less than 1 amp. SO, let's be bold and soldier in a 20W headlight bulb! Tests before showed an even weak 12V battery would light it. I even tested with one of those 6W lantern batteries - nice bright light!

But the system will not illuminate the 2 bulbs - I removed the 5W tail light, closed the circuit with a small washer and the 20W head lamp glowed bright!

Ok. I'll get a 12V 10w bulb. That's still a lot of light. Same thing, more or less. The battery will not light both lamps. The only difference is that, with the 10W bulb, the filament starts to glow red, but it won't produce light.

Both bulbs are 12V. In both cases, the 5W tail light always lights. I would think this battery (fully charged) should power both for at least 2 hours.

What am I missing?

Tim

 

[Frankenstein]

It's called ohms law, same reason when pairing multiple leds in parallel you need to add resistors so they light up equally, funny thing about electricity is it likes to take the path of least resistance.

I would also recommend finding the led equivalent of your bulbs at an auto shop, everyone and their brother has an led side-by-side that you can buy nowadays.

Basically you'll need a resistor that will give the same ohms as the headlight when combined with the tail.

Odd though since this doesn't normally happen with good strong batteries. Are you by chance running them in series by mistake or did you run both off a long piece of wire with a jumper clicked in? If you run both on their own independent wires that could fix it. I would test it by stringing each onto about 6 inches of wire and then sticking both at the same time to the terminals of the battery to see if it will light both, if it doesn't your battery may be on the shot-side since sla should have enough amperage to give out that the lights shouldn't be starving one another for power.

 

[crassius]

assuming you wired bulbs in parallel (but "closed the circuit with a small washer" makes me suspect you didn't) - I'd set my meter to amps and check draw of each bulb one at a time - maybe one was miss-marked

 

[Timbone]

My $15 multimeter is useless to me. No matter what the setting, when I check for amps it goes off scale.

This is one simple circuit - both lamps are wired in series. When I hit the interrupter switch to close the circuit, only the tail light (5W) fires. Obviously, current flows through the head light but it will not illuminate.

If I remove the tail light bulb and close the circuit with a washer in its place, the 20W headlight bulb will fire.

I'm have no resisters, regulators, jumpers etc. on this circuit. Neg battery to taillight to switch to headlight to Pos battery terminal.

It's like the little tail light bulb is drawing much more than it should.

 

[5-7HEAVEN]

I'll make it simple.
Forgive me if you already know this.

I believe you need to wire your bulbs in parallel.

You know, all red wires to the (+) side of the power source,
all black wires to the (-) side.

Of course, you can wire your battery(-) side to the bike's frame,
and attach all the black wires of your lights, horns, whatever to the bike frame.

To measure amperage at one bulb,
remove one of its wire from the power source.
Clip one meter lead to the power source,
and the other meter lead to the
wire leading to the bulb.

Then turn on the light and read your ammeter.

Instead of taking your readings from the (=) side/red wires, you could also test from
the ground(-) wires and attach the ammeter there.

When you hook up an ammeter,
the current runs directly through one meter lead,
THROUGH the ammeter(amp meter)
and out through the other meter lead,
then back through the wire and power source(or ground source).

When the bulb lights up, you'll see the current draw,
measured in amperes(amps).

Amps are always measured in series.

 

[5-7HEAVEN]

(QUOTE)I'm have no resisters, regulators, jumpers etc. on this circuit. Neg battery to taillight to switch to headlight to Pos battery terminal.

It's like the little tail light bulb is drawing much more than it should.(QUOTE)


When wired in series, the little tail light bulb is essentially wired to become a resistor.

 

[Frankenstein]

You can't do that in series, 2 12v lights in series needs a 24v source, hook them up in parallel and you'll be golden. Mystery solved.

 

[Frankenstein]

I'll make it simple.
Forgive me if you already know this.

I believe you need to wire your bulbs in parallel.

You know, all red wires to the (+) side of the power source,
all black wires to the (-) side.

Of course, you can wire your battery(-) side to the bike's frame,
and attach all the black wires of your lights, horns, whatever to the bike frame.

To measure amperage at one bulb,
remove one of its wire from the power source.
Clip one meter lead to the power source,
and the other meter lead to the
wire leading to the bulb.

Then turn on the light and read your ammeter.

Instead of taking your readings from the (=) side/red wires, you could also test from
the ground(-) wires and attach the ammeter there.

When you hook up an ammeter,
the current runs directly through one meter lead,
THROUGH the ammeter(amp meter)
and out through the other meter lead,
then back through the wire and power source(or ground source).

When the bulb lights up, you'll see the current draw,
measured in amperes(amps).

Amps are always measured in series.

5-7 is right in this regard, to measure amperage you need to make your multimeter part of the circuit, basically pull a connection apart and stick it in the middle, usually doesn't matter when it comes to polarity but some meters require it be hooked up with the ground probe coming off the positive lead of the circuit.

 

[Timbone]

!!!

Thank you guys so much!!!

It's a dreary, drizzly day so I went to the garage where I disconnected all my wiring. Did some googling for simple schematics for two devices attached with a switch and a battery. In two hours, I had a completely new wiring rig! Both lamps wired in parallel and firing nicely!

I really appreciate the great info. I can only hope that I am able to help others a fraction of how much you guys have helped me. I always wanted a rig like this for my bike!
Thank you!

 

[5-7HEAVEN]

!!!

Thank you guys so much!!!

It's a dreary, drizzly day so I went to the garage where I disconnected all my wiring. Did some googling for simple schematics for two devices attached with a switch and a battery. In two hours, I had a completely new wiring rig! Both lamps wired in parallel and firing nicely!

I really appreciate the great info. I can only hope that I am able to help others a fraction of how much you guys have helped me. I always wanted a rig like this for my bike!
Thank you!

You're welcome!

FWIW, I've learned so much from YOU,
and your innovative ways of cutting and
welding and making stuff work!

We all learn from each other.

That saves everyone time, $$, energy and stress to endure.