Two led circuit questions

[cyrusthevirus]

Ok ok i get it now. When determining the W output of the lm317 in your calculation where did you get the number 3.5 from?

 

[Hero999]

3.5 = vin - vout = 56 - 15*3.5

 

[audioguru]

How will you cool the LEDs?

 

[cyrusthevirus]

Why Vin of 56? Is that for the fV of the lm317?

The LEDs will be cooled by an aluminum "C" channel attached with thermal epoxy.

 

[audioguru]

You will not be able to solder the LEDs to aluminum.

 

[Hero999]

I thought your power supply voltage was 56V?

 

[cyrusthevirus]

The PS is 58v and 1A. Does that change anything?

 

[Hero999]

The PS is 58v and 1A. Does that change anything?

What do you think?

 

[cyrusthevirus]

Ohhh good answer. Okay let me see...

Well if I run 16 LEDs @ 3.5V = 56V but the LM317, but the LM317 needs at least 3.25V to work properly so I will need to remove 1 LED.

So, I would need to run 15 LEDs @ 3.5V = 52.5V with 5.5V remaining. This would output 5.5V*0.7A = 3.85W which would require a heatsing which could dissipate (60-20)/3.85 = 10.4 degrees per Watt.

(Max. Heatsink Thermal Resistance = (60 - AmbientTemp) / Power)

The LM317 would still require a 1.8ohms 1W resistor to maintain a current of .7A.

Is this correct?

 

[Mike - K8LH]

You might consider building a "constant current" regulator specifically designed for high current LED lighting applications. Here's an interesting link for a 1.5 Amp regulator;

 

[audioguru]

You are just guessing that your LEDs are 3.5V each. Some might be 3.3V and some might be 3.7V. You won't know unless you measure each one.
15 LEDs at 3.7V each is 55.5V plus 3.25V for the LM317= 58.75V.

 

[Hero999]

The link to the instructable is broken.

It's highly unlikely (read near impossible) that all of the LEDs are going to have the highest possible voltage drop so it's reasonable to assume the typical, plus maybe a bit more, which is what we have.

You should be fine with a 58V power supply for powering 15 LEDs with an LM317 - there should be more than enough headroom.

 

[Mike - K8LH]

It seems there are a number of purposed 700-ma "constant current" SMPS regulators out there specifically for LED lighting.

Here's one I came across from Sure Electronics for $4.99 (free shipping) that might be worth a look; **broken link removed**

Regards, Mike

**broken link removed**

 

[Hero999]

That's not what the original poster needs though.

It's step-down when he needs step-up and it's 700mA but he only needs 20mA.