Creating a Simple Circuit for 12V bulb?

Discussion in 'General Electronics Chat' started by raziiq, Nov 12, 2009.

  1. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Hi there. I am very new to electronics. I am trying to light up a Car headlight bulb, i think its 12V through some external supply like a Car battery. Do i need some resistors in between to make this circuit work? Or i can connect them together on a breadboard to make them work?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    What do you want to use as a power source (remember, not all countries use the same voltage and freq, so don't assume).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    An automotive headlamp requires a LOT of power to illuminate it. They are typically rated for around 50 to 70 Watts when operating at a nominal 13.8 Volts. This means they will require in the neighborhood of 5 Amperes of current.

    A breadboard is not designed for that much power; it will burn up. The small-gauge wiring that you must use to connect to a breadboard will burn up very quickly; if you happen to be holding the wires at the time, your fingers will get burned.

    Even brake light bulbs can require around 27 Watts of power; they will get quite warm.

    I suggest that for experiments on breadboards, small LEDs are much better. They don't require very much power to produce a good bit of light.
     
  4. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Thanks for the replies. Infact i have just completed the LED project and was trying to move to bulbs which are much more powerful. Never knew car lights are that powerful.

    Actually i am trying to make a Lamp for my Room, that can light up to a reasonable brightness. I know getting a power of normal 60Watt bulb is not possible from small bulbs but its fine if i can get a bulb which will take 5 - 10 V voltage.

    Anything like this possible?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, an automotive headlamp or fog/driving lamp is right in that range, but they require quite a bit of current.

    Your room light runs from AC @ 120v; 60 Watts at that voltage is 60w/120v 0.5A current.

    A 60W incandescent bulb rated for 12v will require 10x the current; 5A.

    LEDs have become much more efficient and much brighter in recent years. Some of the high-power LEDs by Cree, Phillips and other manufacturers are extremely bright for their power consumption.

    Be careful when working with such high-intensity LEDs; don't look at them directly as your vision can become permanently damaged very quickly.
     
  6. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Actually my room is not very big one, thats why i was thinking that even a low power bulb will be sufficient for it , thats why i was thinking on car light bulb, Furthermore i got this junk car headlight from my Uncle's Junkyard but its working fine, so thought why not to use the bulb inside it to make a lamp for my room.

    So what should i go with? LED's or Bulbs? I am also planning to connect this with my Arduino board laters to control it through my computer, but first step is to totally understand how to light up the bulb as a standalone project.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a typical 60W 120v bulb designed for home electricity has a very wide illumination pattern; it's almost spherical. An automotive headlamp has a relatively flat illumination pattern; perhaps 10° high and 80° wide. It would not be nearly as soft and pleasant as a frosted 120v 60w bulb.
    Makes sense - but trust me, you won't like it for long. It will be very inefficient, too.

    LEDs.

    You'll need a heavy-duty power supply. If you have an old ATX-form-factor computer supply around, you might be able to convert it to a bench power supply.

    Do a Google search on "ATX bench supply" for lots of ideas.
     
  8. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Waoo thanks, that clears a lot of my questions.

    Alright then , i ll be going for a LED lamp now, first gonna build a simple lamp and then will try to connect that to my Arduino. BTW Arduino is a magical device ;)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Any sufficiently advanced technology is indistinguishable from magic.

    Once you thoroughly understand it, it becomes science. ;)
     
  10. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Very well said, i agree :)
     
  11. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    BTW how to know that an electric device like an LED or Bulb requires how much current and voltage to operate? Is it that its hard coded on them?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Incandescent lamps have a filament (usually tungsten) which has a low resistance when the bulb is cold, and increases in resistance when it heats up. There is a large surge of current when power is first applied to an incandescent bulb; as it heats up, the current drops. You might notice other lights flicker when you turn on a light; this start-up surge is why that happens.

    Low-voltage bulbs are usually just marked with a number. You need to look them up on a table to find out what their rated voltage and current is.

    LEDs are not generally marked. However, most small LEDs nowadays are rated to operate with 20mA current. If you don't have a datasheet for them, you can figure out their Vf (forward voltage) by passing 20mA current through them.

    If you don't have a constant current supply, it is easy to make one out of an LM317 regulator and a 62 Ohm fixed resistor.
    1) Connect the 62 Ohm resistor from the OUT terminal to the ADJ terminal.
    2) Connect your load (the LED you are testing) from the ADJ terminal to ground, or the negative terminal of the power supply.
    3) Connect the IN terminal of the LM317 to your power supply.

    For testing most LEDs with this constant-current supply, you will need a voltage supply of at least 7v, up to 12 volts.

    If you don't feel like building a constant current supply, you could use a 510 Ohm resistor in series with the LED, and a 12v power supply. It won't be as accurate as the constant current supply, but it will be reasonably close.

    When the LED is lit, measure the voltage across the LED. It could be anywhere from 1.6v to 4v for an LED that emits visible light.

    After you have determined the Vf @ 20mA, you can figure out what current limiting resistor you will need for operating it on many low-voltage DC sources.
    The general formula for this is:
    Rlimit >= (Voltage_Supply - Vf_LED) / Desired_Current

    Let's say you measured a red LED to have a Vf of 2.2v with 20mA current flowing through it, and you want to operate it from a 5v supply.
    Rlimit >= (5v - 2.2v) / 20mA
    Rlimit >= 2.8V / 0.02A
    Rlimit >= 140 Ohms.
    You will need to select a standard value of resistance that is equal to or greater than 140 Ohms to keep the current at an acceptable level.

    A table of standard resistance values is on this page:
    http://www.logwell.com/tech/components/resistor_values.html
    Bookmark it.
    E12 series resistors (the yellow columns) are commonly available. If you have a large electronics store locally, they may also carry E24 value resistors (the green columns).

    Looking at the table, 150 Ohms is the closest standard value that is >= 140 Ohms.
    Let's see what Iled (the LED current) will be with the 150 Ohm resistor:
    Iled = 2.8v/150 Ohms
    Iled = 18.7mA - that's a bit low, but it is OK. The LED will have a long life.

    Let's see what wattage rating we will need for the resistor.
    Watts = Voltage x Current
    Watts = 2.8 Volts x 0.0187 Amperes
    Watts = 0.0523
    For reliability, we double the result: 0.0523 x 2 = 0.1046 Watts, or 105mW
    You could use a 1/10 Watt resistor, as it is just 5% over the "rule of thumb" for reliability. However, a 1/8 Watt resistor would be very reliable.
     
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