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Hello EE Stack Exchange!

I am a mechanical engineering student that needs some help with a circuit. I am have an extremely limited background in circuits so I have to apologize in advance for this question. This is the circuit I was able to get together to turn a light on and off using a transistor and relay: Old Circuit

This is great and I think it would have worked but the requirements changed before I could test this. I now need 2 LEDs in parallel. This isn't ideal since I only have access to a power supply of 6v for two 3.3V LEDs in parallel. This is on a device that has a 6v power supply and a 3.3v arduino. Two LEDs in parallel can consume 1.4A to 2A at this voltage. What is the best option here for getting parts to support a circuit like this? Do I get a transistor like this? Do I need to change the circuit further?

Any help is appreciated.

Specific parts used in previous version are:

KEMET EC2-4.5NU Relay

LD1117V33 Linear voltage regulator

CREE XML2-W318 LEDs

2N3904 Transistor

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  • \$\begingroup\$ What makes you think 6V can't drive two LEDs in parallel? How are you controlling the LED current in the existing circuit? \$\endgroup\$
    – Finbarr
    Apr 30 '17 at 21:29
  • \$\begingroup\$ @Finbarr 6V can drive two LEDs in parallel. I simply made note of it since I was unsure if it would or not. I tested the LEDs in parallel using a power supply so I could get an accurate assessment for the current draw. Previously there was a 4.7 Ohm, 3W resistor limiting the LED to 700mA. Currently there is no resistor in the circuit due to the lumen rating that we get when we tested it under these conditions. \$\endgroup\$
    – onyex
    Apr 30 '17 at 21:37
  • \$\begingroup\$ What did you use to make the diagram? \$\endgroup\$
    – user2497
    Apr 30 '17 at 21:46
  • \$\begingroup\$ You need to decide how much current you want going through your LEDs and design your circuit to control it. Other than that I'm not really sure what you're asking. \$\endgroup\$
    – Finbarr
    Apr 30 '17 at 21:47
  • \$\begingroup\$ @user2497 That was drawn in Autodesk AutoCAD. I have tried a variety of circuit drawing software, none of it worked well or did what I needed so I decided to draw it instead. \$\endgroup\$
    – onyex
    Apr 30 '17 at 21:48
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This circuit could do what you're looking for. It scraps the bulky relay and replaces it with a very cheap MOSFET transistor.

The central point is your 6 V driving these high-current LEDs with very short flashes from a battery. You haven't given actual timings for the pulse width, though it's likely to have been 5 ms or longer because of your relay switching time. But you might want it to be a lot shorter in the future one.

A 6 V to 2..3 A precision current-limited DC-DC converter could be used to get your LED's current most efficiently. However, that will draw a small standby current and you mentioned this being battery-powered. For the time being, I've shown 1 ohm series resistors from a 3.3 V nearly-3A starting point derived from the LED datasheet. These could be on all the time so are derated by about a third.

I don't know what your plans are but they sound experimental. You might want to build this and get some better data from trial of you application. You could go to a third generation once you have better timings and make more efficient use of LED power than dumping half of it in resistors like this.

schematic

simulate this circuit – Schematic created using CircuitLab

Q1 is a MOSFET with logic-level drive capability. It can conduct 8.3 A of load when driven with 2.5 V min which suits your Arduino General-Purpose Output (GPO). The 8.3 A is plenty for your load, which I've shown as LED1 and LED2.

R1 limits the GPO current flow when charging and discharging the MOSFET's gate capacitance as GPO switches to high or low. R2 ensures that Q1's gate is pulled to an 'off' state when GPO is high impedance. This will be the case on power-up, until the Arduino is out of reset and software has set the I/O pin mode.

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  • \$\begingroup\$ Ok, so MOSFETs are new to me so I just want to make sure everything is ok here. Minor error of 6V in to LEDs, to MOSFET. MOSFET can take in 12V and 9.3A. Drive voltage required is 2.5V but the 3.3V from the arduino output should be ok, correct? \$\endgroup\$
    – onyex
    Apr 30 '17 at 22:47
  • \$\begingroup\$ @onyex, see above edits. \$\endgroup\$
    – TonyM
    May 1 '17 at 9:59
  • \$\begingroup\$ Indeed the pulse width has not yet been determined. I can say that they would be between approximately 25 ms and 250 ms however this also depends on testing. This is quite a wide range but there is noting I can do until it undergoes testing. This is very much experimental. It is entirely possible that this may be the final solution for the issue since the power consumption is reasonable and there are other aspects of the project that need attention. Thank you. \$\endgroup\$
    – onyex
    May 1 '17 at 14:56
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You have to at least put a current limiting resistor between your Arduino and the base of that transistor, otherwise you're effectively making a short circuit to ground when your Arduino pin goes high, which your digital pin output won't thank you for... maybe 100 would be ok.

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Do I need to change the circuit further?

depends on your application.

if you can be sure of perfectly matched LEDs, precision power supplies, and constant temperature on the LEDs, your original circuit will work and you can simply parallel them.

otherwise, you will need a led driver. for best efficiency, pick a smps-based led driver.

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  • \$\begingroup\$ So if I'm honest, this is more complex than I believe is necessary. It's not that I am not concerned about the fluctuations of current and voltage through the LEDs, however the fluctuations appear to be minor and do not effect the luminous output. I am however woefully ignorant in this entire area. If I am underestimating the impact this may have, let me know. \$\endgroup\$
    – onyex
    May 1 '17 at 14:34

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