Current for LED

Discussion in 'General Electronics Chat' started by beeson76, Oct 30, 2013.

  1. beeson76

    Thread Starter Member

    Apr 19, 2010
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    I need an LED to allow about 90 to 100 mA through it. It is simply an indicator to show that the button is pushed to start a motor. But I need the LED in series with the switch and motor. The LED I am looking at is a SSL-LX3044GC. Its a little 3 mm Green LED made by Lumex. I could put it in a parallel circuit with the switch and motor with a resistor on that leg with the LED but it doesnt light when I push the switch. But when I put the resistor on a the same leg as the switch and motor I get the LED to light but the motor doesnt run.

    So I am thinking about going with a high current LED, but I cant really find any after searching for about an hour on the internet and Catalogs. This is about the only one that fits my needs but the Steady current is rated at 30 mA and peak is 150 mA. Would I have any problems with using this LED. I can possibly get away with a 5 mm LED if need be.

    Any suggestions or help is greatly appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    Assuming you just need an indicator, you don't need 100mA. Even a 5mA LED can be very bright.

    You haven't mentioned the voltages at the switch. Let's say the LED has a voltage drop (in its specifications) of 3V. You need a current limiting resistor to limit the current to, let's say, 10mA for a generic LED. The resistor is calculated using ohm's law: ∆V = IR and so R = ∆V/I = (V-3)/0.01. V is your available voltage at the switch, ∆V is the voltage drop across the resistor.

    So if your available voltage is 12V, you need a (12-3)/0.01=800Ω resistor. You could choose a nearby standard value of 820Ω or go a bit lower to 750Ω.

    Note that the voltage is relative to some other nearby point, usually a ground or chassis. The other end of your LED-and-resistor in series, connects to ground.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Could you show a quick sketch of the configurations you've tried?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    What is the supply voltage that powers your circuit?
     
  5. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
    1
    Here is the schematic for this circuit.

    Here is some requested information I got from the circuit. When checking voltage the circuit provides 37 volts without the switch activated. When activated it drops to .02 volts. This is without the LED in the circuit. The circuit provides about 103 milliamps without the LED in the circuit.

    Circuit #1. This is the only way I could find for the circuit to work with lighting the LED. But at this point The LED is drawing about 40 milliamps with a 400 ohm resistor installed. For this circuit to work I need at least 40 milliamps for the motor to work. Anything less and the motor doesnt work. I chose the 400 ohm resistor because this was the first resistor that allowed the motor to run. Any high ohm resistor wouldnt work. I can go with no resistor but the milliamps through the LED was about 90 milliamps. That was the reason for the initial question on whether there is a high milliamp LED that would allow about 100 milliamps through it.

    Circuit #2. This would be the best way of doing it, but when the switch is pressed the LED doesnt light.

    Any help is greatly appreciated. I really do enjoy this forum a lot:):)
     
    Last edited: Nov 1, 2013
  6. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    You're very close! In #2, take the downstream leg of the LED to the other side of the motor, (which I assume is ground?)

    Be sure the resistor is a large value to protect the LED from excessive current.

    This gives you two separate circuits in parallel - the motor and the LED+resistor - both controlled by the switch. This same configuration will work even if the motor needs 100X more current than the LED does. Trying to run the motor through the LED - the series configuration - rarely works because motors usually need much more current than an LED.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    969
    Use Crkt #2 BUT...
    The cathode side of the LED needs to go to ground (basically the other side of the motor that you did not draw in)

    Then just calculate the resistor based on the input voltage.
    Don't forget to calculate for the wattage rating required by the resistor and then multiple it by 2 (for a safety factor)

    damn..wayne beat me to it...
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    And this time I may have actually got it right. ;)
     
  9. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
    1
    Wow. Thanks guys for the input. I will certainly try it out. Thanks again:):)
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    beeson.. The reason the LED wasn't lighting with #2 is because the voltage on both sides of the LED was equal so the LED was essentially shorted or at the same voltage at both ends and no current could flow.

    By putting it in parallel with the motor the one side is at input voltage (37volts or whatever it might be) and the other side at 0 volts (ground).. Hence the LED then sees a voltage difference from one side to the other thus allowing current to flow through it..
     
  11. BobTPH

    Active Member

    Jun 5, 2013
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    Unless, of course, he wants the LED next to the switch, and ground is not available there, which was the impression I got from what he had tried.

    Bob
     
  12. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
    1
    Yes. BobTPH is correct. I don't really have access to the ground from the Motor. But I do understand the concept of why Circuit 2 wasnt working now:) There wasnt a voltage difference between the 2 legs.
     
  13. BobTPH

    Active Member

    Jun 5, 2013
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    What current and voltage is the motor running at?

    Bob
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Some schematics online for a mains current draw indicator show 3x 1N5401 in series to drop a voltage suitable for driving the Vf of a LED, some show about 10 - 22R in series with the LED as each silicon diode can drop as much as a volt at close to their rated current.

    Use silicon diodes that can carry the load current - so the LED doesn't have to. If its AC; you need an extra diode to clamp the LED reverse voltage.
     
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