Hi guys,

I'm trying to make a low temperature voltage source with around 15V output (I can probably make do with between 12-20V). Basically I'm trying to power a bunch of LEDs, but I want it to be outside. I'll be using a transformer to convert 120V AC to ~20V DC, and I want the output to be stable 15V. The problem however is that I need this to work outside, so it has to operate between -50 to +50 degrees Celsius. LM78x voltage regulators don't seem to have that kind of operating temperature range. Can anyone recommend a part? Also, I can make do with 50 mA but I would prefer at least 100 mA output.

Thanks.

 

Hello,

There are L78XX stabalizers for -55 - 150 degrees.
They are called mil. spec. types.
Here is a datasheet from ST (look at maximum ratings).

Greetings,
Bertus

 

Also, look at the LM317 adjustable regulator which can operate from -40-125 degrees celcious. If you look at the datasheet there are some types which can work from -55-125.

http://www.national.com/ds/LM/LM117.pdf

 

Hey guys, thanks for the tips. I found a LM317 on ebay with the specs I need, but for now I'm using the 7815s for testing purposes.

 

Don't forget that some power will be dissipated by the regulator itself, so in that regards it's self-heating.

It may be sufficient to simply fully enclose it during very cold weather.

Don't forget that your transformer's power source and the transformer itself must be in an enclosure that's rated for outdoor use.

 

Hey,

Thanks for the tip. I just ran a test and had the 15V output drive 12 LEDs. Each LED is arranged in chains of 4 LEDs in series, 3 chains in parallel. I left the LEDs running for about 24h with 12mA going through each chain. These are bright white LEDs with which drop about 3V each. I ran the test inside, with about 25 degrees C room temp and the voltage regulator became very hot, maybe 80-90 degrees C.

Since I live in Canada, the temperature can become about 40C during the summer and -50C during the winter. Should I put a heat sink on the regulator?

Also do you guys have any tips on how to design electronics to for all weather purposes? All I really have is the LEDs, two transistors and a phototransistor. I will try to encase the electronics in a metal case which should hopefully be water tight, but assuming a bit of water gets in, is there a way to design for that?

Thanks.

 

Hello,

It is always recomended to have a heatsink on the regulator IC.
The temperature it dissipates is added to the surrounding temperature.
There are so called IP65 housings, that are dust and water tight.

Greetings,
Bertus