LM317 Router Power

[Guyabano]

Hello,

I‘m Stephan, just registered and been looking for a forum to share & advance some projects with other. So m 1st post.

Currently I‘m working on „project garden WiFi“.
I’ve got 12v solar power available, stored in some larger batteries. So wanted to use it.
There‘s an old DIR-615 router which could work as a repeater, runs on 5V, 1.5A (officially).
But I checked it, it only uses 0.6A maximum.

So made a fairly standard, simple LM317 circuit to reduce 12V to 5V. 0.6A should be ok- so I thought.
Turns out, any LM317 gets up to 60° after 5min, even with a cooling plate attached.

The router itself comes on and remains stable (without going into a reboot loop) at 5.5V....but it’s happy with 5V from a USB outlet.
Below 5V it goes into a reboot loop.

i’m looking for some opinions on why the LM317 gets so warm at such a low amperage and why the router shows such different behaviour between the LM317 and a USB supply. I’m a bit clueless....

Stephan

 

[Diver300]

The LM317 will get warm because of the voltage drop across it, and the current through it. The heat generated is those two numbers multiplied together, so in your case (12 - 7) * 0.6 = 3 W.

The current taken by the router may be more than 0.6 A for very short periods of time, so it may be difficult to measure, but long enough to cause the voltage to drop and the router to reboot.

Any regulator will need input and output capacitors. Generally, the larger the better.

I suggest you look at switch-mode regulators like this:-https://uk.farnell.com/murata-power...1-5-w36h-c/dc-dc-converter-5v-1-5a/dp/2812636
which will take much less power from your batteries, won't get hot, and will be rated to the full 1.5 A. It will need larger capacitors than the LM317.

 

[audioguru]

Maybe your 12V is actually 13V and the output of the LM317= 5V then it heats with (13V-5V) x 0.6A= 4.8W. You need a heatsink with fins (not a little plate) and some thermal grease.

 

[Nigel Goodwin]

Or better still, use a switch-mode regulator, you're wasting a LOT of your solar panel power making the LM317 hot.

You can get incredibly cheap switch-mode regulator modules, which will run MUCH cooler and waste very little energy.

 

[KeepItSimpleStupid]

Do you have a charge controller on your solar panel/battery system?

ebay has plenty of switch mode modules.

 

[gophert]

Get a stack of something like this, about $1 each. Handles 2 amps with 3 amp surge.
Only a tiny fraction of the heat made by LM317 because it uses smps instead of simply converting excess power to heat.

just turn the little pot screw to dial in the voltage you need.

5Pcs LM2596 HW-411 DC-DC Adjustable Step Down Buck Converter Power Supply Module | eBay

Module Spec: Non-Isolated Step-Down (BUCK). Module Size: 42mm(Length) 20mm(Width). If item is defective after 3 months, you can still send it back to us. We will send you a new one after receiving the defective item.

www.ebay.com

many other ebay site sell it. Look for model number HW-411 (it's made by some Chinese shop but many ebay stores sell it).