Adding LEDs or other accessories changing amperage

[jwallis]

Edit - subject changed from "doubling amperage" to "changing amperage" in order to make it sound less scary and avoid drawing unnecessary attention.

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Ooooookay, if you're like me and you want to add a light somewhere, you find a wire that's giving you 12V, you slap a light on it and screw the other end to the nearest frame. After watching some electrical videos, I'm pretty concerned that I've been creating parallel circuits, thus doubling the amperage in some pretty small wires, adding the hazards of blown fuses or overheated wires that could melt and short to the frame, etc.

Here's the specific part of a really nice video "Ohm's law for service techs" I'm talking about. He's teaching circuits by having people tape copper ribbon to cardboard and soldering on components. In this video, he obviously cut the copper ribbon between the + and - leads of the switch and lightbulb, otherwise it wouldn't work of course, but he did it earlier in the video and it's hard to tell by looking. You only need to watch about 90s of the video to understand the demonstration.
https://youtu.be/c588objVBs0?t=6m11s

So I'm curious if this doubling of amperage surprises and concerns anyone else...

The more I learn about circuits the less it makes sense sometimes : (
I'm a software engineer and it's surprisingly challenging for me.
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[Wildthings]

If you added an LED to another circuit that only had a similar LED light on it you would double the current flow, but it would still be very small. If you added an LED to the heater circuit it would barely add any load at all percentage wise.

What you really want to do is find an uncritical circuit if you are going to tap into an existing circuit and be very neat with your wiring. The load from a single LED is pretty much going to be trivial no matter the circuit, but I still wouldn't want it added into an unfused circuit like #30 main supply circuit or the #15 ignition circuit. Hooking it into something with similar usage like the dome lights would be appropriate.

[jwallis]

[tim_ha]

The two equations that are of concern here are:

1. V = I*R (V=voltage, I=current, R=resistance)
rearrange to get:
I = V / R
or:
R = V / I

2. Parallel Resistance: R_total = (R_1 * R_2) / (R_1 + R_2)


So... adding LEDs to a circuit that is currently providing 1A.

- Using equation 1. we can solve for the circuits equivalent resistance:
R = V / I becomes R = 12v / 1A and we see that R = 12 ohms

- What is the resistance of the LED?
This one http://www.van-cafe.com/home/van/page_1244_138/led-high-power-interior-light-bulb.html
claims 165mA. So, R = 12v / 0.165A => R = 72 ohms (approximate)

- Now lets find the circuits new equivalent resistance using equation 2.
R_new = (12 * 72) / (12 + 72) = 10.28 ohms

- And finally, the circuits new current
I = 12v / 10.28 ohm = 1.167 A

Interesting, the currents add! Note, there is a rounding error and the result should be 1.165 A.

It can be shown and is indeed a fundamental law of circuits (Kirchhoff's current law) that currents in different loops of the circuit add.
I_new = I_original + I_added (for loads added in parallel)

So, add those lights (in parallel) to your hearts content. You are only increasing the total current on the circuit by whatever your light is rated to draw.

I think your confusion may be coming from the fact that if R_original and R_added are equal, you double the current of the circuit.

I find the analogy of circuits to water systems quite useful...
Voltage is like water pressure
Current is like water flow
Resistance is like a restriction in the pipe
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[chris_d]

More practically.. lookup the current draw of whatever device you're adding. All the wires in the path from battery to this device then to ground will be carrying this current.

Figure out the gauge of those wires, lookup a awg/amp/distance wire chart and see if that gauge of wire can support the added amperage (may have to look at what else is on that path).

And make sure the path you're connecting on is fused. Fuses are both meant to protect the wire and the device.

This is why I don't like the idea of just picking a red wire and splicing when you didn't design the system: difficult to find out the rest of the path, then find all devices on that path, then determine if fuses are in the right place (i.e. when gauges change size on a path!)..
But LEDs are super low draw so it's probably the least risky thing you could do

[shadetreetim]

I would not randomly tap into a circuit to add an LED even though the LED would not add much of a load. I have had an LED go bad and blow the fuse. If I had tapped into a critical circuit, that bad LED could have left me stranded somewhere. On the side of the road is a bad place to start tracing down a short.

I have added LED's to the stock interior light circuit. and have added LED's to a dedicated fuse block just for auxiliary devices. I have a solar panel with aux batteries though.
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[nemobuscaptain]

[jwallis]