8 Volt and Led's

[wi0v]

I want to add one of those stop light bars that you put in the back window. I am running an 8 volt system in the car. Should I use the 12 Volt or 6 Volt light bar. Maybe neither one work with 8 Volts? Led's are a little too new for me to understand.

[ken ct]

I would get rid of the 8 v system first of all,nothing but a bandaid and nothing but troubles in the long run. omo ken ct.

[jrelliott]

LEDS only need a few volts to operate. There is a resistor in series with the LEDS inside the bar to limit the current through the LEDS. Expect that a 12V LED bar will work on 6V system. You will probably not detect any difference in brightness with the lower voltage.

[mccsix]

I tested some 12v leds on 6v and they work just fine, can't tell the diff in output.

[MikeK]

Originally Posted by wi0v

I want to add one of those stop light bars that you put in the back window. I am running an 8 volt system in the car. Should I use the 12 Volt or 6 Volt light bar. Maybe neither one work with 8 Volts? Led's are a little too new for me to understand.

This is a crap shoot. When multi element LED lights are made, it is NOT so simple as adding a resistor, which limits current (not volts as most think).

A key semiconductor property called forward voltage drop is taken into account to eliminate or reduce the amount of energy wasted as heat through resistors. Typical forward voltage drop is 1.6 to 3.3 volts per LED. That number is primarily determined by the color of the LED, which dictates the semiconductor material and doping.

Red LED's are generally 1.7V forward voltage, so a 12V light may contain one or more series strings of 6 or 7 plus a single current limit resistor. A properly engineered 12V device would never light at all, not even verry dim, on less than 10.2V. You could use a 6V LED lamp assembly on 8V with a resistor, but you would need to very carefully calculate the required additional current limiting resistance needed to prevent overdriving (Poof!) the device, and it would be very sensitive to voltage rises, like regulator voltage spikes.

If it were me, I'd add three or four 3A barrel diodes in series with the 6V lamp. Each silicon rectifier barrel diode will add a true 0.6V forward voltage drop (not a current limit!). Your 8V battery is actually 8.4V. Minus the three added barrel diodes your 6V LED's will 'see' 6.6V, or 6.0V with four diodes added. Safest, surest, brightest way to go if you have 8V. You might be able to use 1N400x diodes, but you may be pushing their wattage (heat) dissipation limit if the LED lamp assembly draws more than 0.8 amps.

[columbiA]

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I have 12v neg grd 3rd brake lites on my 6v pos "A",s & they work just fine.If your car is pos grd,you will need to use the grd wire as the hot wire for them to work.

[wi0v]

Lots of great information, thanks to everyone that answered my question.

[Cool Hand Lurker]

I had a handful of red 10mm LEDs and made a 12V light bar for the rear of my car trailer. I hooked seven of them in series for tail lights and six of them in series for brake lights. The six were brighter than seven. They work well, I just don't know for how long they will last without a current limiting resistor.
I have also used a single one plus a resistor for converting a 6V Model A tail light bulb into an LED bulb. I filled the bulb base with epoxy right around the wiring and simply put it into the same socket it came out of in the Model A tail light. Don't know how long that one will last either.

[JoeWay]

If you know the specifications of your LEDs, you can use one of many on-line calculators to help design a circuit. Here's one.

Joe

[B_man_Al]

For a series limiting resistor with the LEDS you can do the following. You will need a DC voltmeter to make the measurements.

1-Determine the battery voltage when running. For an 8 V battery, this is around 8.8v ( @ 2.2v/cell)

2- Determine how much current you want through the LED array .... As an earlier post indicated, say .8 A

3- Estimate the LED series voltage ..... say 3 LEDS/string at 1.7v each = 5.1v

4- Calculate the required resistor as follows :
R series = (Vbat-VLED array )/ I desired = (8.8-5.1)/.8 = 4.6 ohms

5- Calculate the required power rating as follows
P resistor = (Vbat-VLED array)*I desired = (8.8-5.1)*.8 = 3W

6- Hook it up and confirm the battery and LED array voltages with a voltmeter. Then recalculate the actual current . Lets say the Vbat actual= 8.4v & the VLED array=5.5V. Then the actual LED current will be (8.4-5.5)/5 ohms = .6A

7- If you want them dimmer, decrease the desired current, and increase the resistor, using the same process to do the calculation.