About Electrical Wiring in Magnetic vs. Permanently Screwed-Down Toplights
We frequently get into discussions about magnetic toplights and the wiring inside of them, especially
if your need is for marker lights for hired indication, or for panic lights. Let's look at the generics of
a light bulb circuit.
In order for a lamp (light bulb) to light up, we need to put power to it. Whether
it has the usual filament in it, or if it is one of the newer LED's (light emitting diodes), it has to have
a power wire, and a "return" circuit, commonly called a "ground" or "earth" for our Brit friends.
|The LED unit at the left is actually 13 led's mounted in a "bayonet base" so it can replace a standard light bulb in an existing socket. On the light bulb diagram, notice there are two wires; Red is the A+ wire which comes in through the tip of the lamp. Gray on the diagram is the outside shell of the socket, and that's connected to chassis ground. Electricity runs in through red, through the filament, and out through the shell on gray to ground.|
SOCKET MOUNTED LED'S
Now let's think about toplights. If you put up a toplight that is basically a small sign, it will normally
have one circuit. The light bulbs in the sign are all connected to one red wire (for example) and one ground
connection in the sockets of the lightbar.
|The mounting screws that hold this lightbar to the roof are also the path for the ground circuit. Metal roof is grounded, screw is metal, lightbar is steel, sockets are connected to lightbar, so ground connections are made. All three light bulbs are connected to the one red wire in the picture. Connect the wire to power, and lamps go on. That lights up the sign.|
Now suppose you want to use a toplight with a panic light on top of it. A panic light is one which the
driver can light with a hidden switch. It can be mounted on the side or back of the light, or right on
top. When the police see that light on, the driver is in trouble, and they stop the cab with guns drawn.
|To make this light work, you of course need a second circuit. It is the wire connected to the panic light socket in the toplight, and that wire is connected to the panic button switch in the car. You simply run an extra wire through the roof when you set it up. No problem. The ground for the bulb is the same as the ground for the sign-light bulbs, connected to the lightbar and the roof through the screws.|
Now, lets notch it up again: Let's use a megnetic toplight - not permanently screwed to the roof, but
held there by strong magnets. We power those up with a portable cable very much like heavy lamp-cord,
and connect them to power by means of a plug with goes into the cigarette lighter socket on the dash.
|This works a little differently since there are no screws holding the toplight on the vehicle, and of course, they are not carrying the return path ground either.We use that two-conductor cable with one wire carrying the A+ lead like the red wires above, and one carrying the ground connection like the gray one above. You put the light on the roof, pull the cable over the rubber of the door, and shut the door. Plug in the connector at the dash and the light comes on. Great! But... what about the panic light? How do you light that up?|
Good question, actually. We furnish lights like that with a third wire taped to the lamp cord. Down at
the cigarette lighter end, there is the cigar-lighter plug and this extra wire, not connected to anything.
|What's a brothah to do? Well, several choices. You can mount a switch down at the plug (Careful, it can be seen there!) and tap into the hot side of the cable for power, with the other side of the switch connected to the 3rd wire (green in our picture).|
You can also pull the green wire out of the last couple feet of the run, and direct it to a hidden switch
mounted to the side of the seat saddle (plastic skirt below driver's seat) or under the dashboard out of
sight. Instead of getting power at the cigarette lighter cable, get A+ power from any good source in
driver's area, such as pin #16 on the OBDII connector, or at the fuse box if appropriately located.
Let it be said, choosing to use a magnetic light is always a poorer choice than a permanently mounted
toplight, because of wind problems (see The Wind Speech" on our websites,) and the obvious lack of
permanent ground in the magnetic version. We always try to talk our clients into using permanent
screw-down toplights because they cannot come off so easily, but also because they give you the full
selection of extra features like VACANT light on top, marker lights, roof-top turn lights, panic lights,
even on-duty off-duty panels. With permanent lights, you just run an extra wire for each extra circuit.
Call us if you have more questions, 760-345-4347, or e-mail Fred@TaxiCabElectronics.com. :-)