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Light therapy device

Discussion in 'LEDs and Optoelectronics' started by mattfara, Mar 30, 2017.

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In image 3.1 in that guide, the transistor marked as BD679 is the one requiring a small heatsink. If this transistor is in a TO-220 package then the smallest heatsink you can find will probably be more than sufficient if the voltage overhead on not too high. The transistors used are not critical.
     
  2. mattfara

    mattfara

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    Mar 16, 2017
    Sorry. The arrival of spring is distracting me. Gardening.
    Materials:
    ------------------
    **LEDs
    20 TSHG5510-ND
    **surface
    aluminum core printed circuit board
    **linear current drivers
    two heat sinks
    four transistors
    four resistors
    **power
    plugpack rated for 24V at 500mA

    Is this everything I need? If not, please let me know what other materials I might need.

    As for the transistors, I have a few lying around I got from a friend. Please let me know which might be good for this project, and if none, which ones would be right:

    2N3904 (5 pieces)
    2N2222 (2 pieces)
    2N4401 (5 pieces)

    As for resistors, I have three 100kOhm @ 1/2 watt, four 330 Ohm @ 1/2 watt, and five 270 [email protected]/2 watt. If these will not suffice, please let me know what I should get.

    Lastly, if you can make a recommendation for the PCB and plugpack, from digikey (where the LEDs are cheapest), please do.

    Again, thanks very much to you both. I've learned a lot and I plan on learning more.

    Matt
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Any of those three transistors could be used in place of the BC547.

    R2 is calculated as 0.65/I, so for 100mA, the closest available resistor is likely to be 6.8Ω

    Give me a link to the on line catalog of where you purchase your parts and I'll pick a transistor for you suitable as the pass element.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Aluminium core board may be hard to get. You may have to find a company that can produce the board for you. This board will probably also need to be mounted on a heatsink.

    It may also be possible to mount the transistors needing heatsinking to the board and rely on the same heatsink to cool the LEDs and these transistors. To do this you will probably need to use surface mount devices or to surface mount conventional leaded components.
     
  5. mattfara

    mattfara

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    Mar 16, 2017
    I'm wondering about the skin acting as a heat sink -- the device will be placed against the skin during use, and use will be less than 20 minutes at a stretch, I think. Would that obviate a special board or changing the LEDs?

    I like to shop on digikey
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The front of the LEDs will not transmit the heat generated on the site effectively, if it did, the risk of burns would be real.

    I wouldn't place this in contact with your skin. Leave a space of about a half inch to allow the beams from all of the LEDs to spread out.
     
  7. mattfara

    mattfara

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    Mar 16, 2017
    I'm hoping for a little clarification. I've been in touch with a researcher in the LLLT field. He states that THIS device puts out 20mW/cm^2, with each LED putting out 1W. Based on the datasheet, how is this determined?

    Are the LEDs in this commercial device similar to the ones we've been considering?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    At what distance?

    Over what area?

    Is it the flood or spot mode?

    It claims to be a 72W device, but if those are 1W LEDs, there's only 24 of them.

    There is insufficient information to confirm the claims or to know how it was used.
     
  9. mattfara

    mattfara

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    Mar 16, 2017
    I asked him these questions. From a previous email, he said that the electrical power of the device uses 3W per LED (24*3=72), but produces 1W of light power per LED.
     
  10. mattfara

    mattfara

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    Mar 16, 2017
    He says the glass plate should be placed directly against the skin, using spot mode.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Is this person qualified? Or just some random person on YouTube?
     
  12. mattfara

    mattfara

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    Mar 16, 2017
    Haha he specializes in this. At Harvard. Michael Hamblin
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Cool. Presumably he has good reasons backing up what he says.
     
  14. mattfara

    mattfara

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    Mar 16, 2017
    Unfortunately, the device he suggested costs over $1000. Hence, I'm here to learn and find guidance.
     
  15. mattfara

    mattfara

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    Mar 16, 2017
    Anyway, I think DigiKey is the way to go for the LEDs at least, so buying the transistors from there would make sense for shipping.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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  17. mattfara

    mattfara

    62
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    Mar 16, 2017
    Great.

    Materials:
    ------------------
    **LEDs
    twenty TSHG5510-ND
    **surface
    printed circuit board (?)
    **linear current drivers
    two heat sinks(?)
    four transistors (two BD679, two 2N3904)
    four 6.8 Ohm resistors, 1/2 Watt (?)
    **power
    plugpack rated for 24V at 500mA

    What would you recommend for the circuit board? You mentioned that an aluminum board might need to be custom made, but that it would be good for the thermal properties of the device. Do you think a trade off could be made for a pre-made board?

    I assume that using a pre-made board would necessitate using two heat sinks. Would a fan-powered one be necessary for could the type with fins be used?

    I assume that the Wattage rating on resistors tells you the maximum power that can move through a resistor safely, so I'd assume that the power running through these, based on your calculation above, would be 0.65V*.1A = 0.065 W. So the smallest rating should do?

    Lastly, how about THIS as a plugpack? It rates at 24V, 12W. I assume that I can calculate the current it can handle from power = volts*current, so current = 12/24 = .5 = 500mA
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I would recommend you design a circuit board that is single sided and mounts directly to a heatsink about the same size as the board. You'll need several holes in the board to attach it to the heatsink, probably 4 in the corners and a few along the centreline. (oh, and additionally those for the transistors to bolt to the heatsink).

    Ideally you would drill then tap the holes to fit a suitable sized bolt.

    Any through-hole components (such as the leads for the transistors and terminals to attach power) will need to be in a part of the board that is "off" the heatsink so as not to short out the connections.

    I think that's a carryover from when I thought the components would be dissipating 70W of heat. You're down to under 12W now, so aluminium core board won't be needed.

    No, a fan won't be required. you might even be able to get away with a thick sheet of aluminium with a few bent up fins around the edges. If the heatsinked aea of the board is 2 inches by 4 inches, you might be able to use something like this. But that is pretty expensive, and you may be able to raid some old PCs for heatsink(s) with a large flat area. Another alternative is to look for some extruded aluminium that is the correct width , has a reasonably thick (and flat) base and has extrusions that look like fins. Here is an example of the type (that I couldn't find on digikey)

    Yeah, you are best using surface mount resistors, so choose 1206 size resistors (because they're big and easy to handle) 0.1W or higher should be fine.

    That looks fine. if you can find a PCB mount socket for the connector, even better. Here is a surface mount connector. Here is a through-hole version.

    The next thing will be designing the board. I assume you've never done that before?
     
  19. mattfara

    mattfara

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    Mar 16, 2017
    You are right
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Then we have an interesting task.

    Have you ever done surface mount soldering?
     
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