Light therapy device

[(*steve*)]

You mean that I'll need two heat sinks total, one each for one of the transistors in the two CCDs? I'm not sure which one will heat up most and require the sink.

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.

 

[mattfara]

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.

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 Ohm@1/2 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

 

[(*steve*)]

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.

 

[(*steve*)]

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.

 

[mattfara]

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.

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

 

[(*steve*)]

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.

 

[mattfara]

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?

 

[(*steve*)]

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.

 

[mattfara]

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.

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.

 

[mattfara]

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.

He says the glass plate should be placed directly against the skin, using spot mode.

 

[(*steve*)]

Is this person qualified? Or just some random person on YouTube?

 

[mattfara]

Is this person qualified? Or just some random person on YouTube?

Haha he specializes in this. At Harvard. Michael Hamblin

 

[(*steve*)]

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

 

[mattfara]

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

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

 

[mattfara]

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.

 

[(*steve*)]

Sorry about the delay.

Here is a BD679 https://www.digikey.com/product-detail/en/stmicroelectronics/BD679/497-5776-ND/1300084

It's in a plastic package so you can screw it through the board and onto the heatsink without worrying about shorting stuff out.

 

[mattfara]

Sorry about the delay.

Here is a BD679 https://www.digikey.com/product-detail/en/stmicroelectronics/BD679/497-5776-ND/1300084

It's in a plastic package so you can screw it through the board and onto the heatsink without worrying about shorting stuff out.

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

 

[(*steve*)]

What would you recommend for the circuit board?

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.

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 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.

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?

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)

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?

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.

Lastly, how about THIS as a plugpack?

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?

 

[mattfara]

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

You are right

 

[(*steve*)]

Then we have an interesting task.

Have you ever done surface mount soldering?