deacreasing voltage to keep regulator cold

We have:
DDl driver (based on LM317)
Uinput=8.4V
red LD on output
Icircuit=420mA

I know that with these values, LM317 become very very hot (i am talking about SMD dpack version) in few moments. This is normal, because i have minimum 1.2-1.4V extra voltage which LM must convert in heat. I would like to divide voltage between LM and some diodes, in order to keep regulator as cold as possible. If I'll connect 2 x 1N400X diodes in series with input ddl driver, I will obtain a Uinput on driver = 7.0-7.2V! Did anybody tried this solution? Any cons or danger?

regards,
alx

You can use diodes to drop the voltage, in order to reduce heating of the LM317, which could be dissipating a good 2+ watts with your setup. If you have a test load available, use it instead of the LD. Add the diodes one at a time, checking that current to the load is unaffected, and checking for reduced heating of the LM317. The diodes will dissipate some heat themselves as they drop the voltage. If the diodes get warm, switch to 3a diodes. When satisfied, remove the test load and reinstall the LD.

YES -- each silicon diode drops 0.7 volts.  The 1N40xx series work well.

Mike

Are you guys sure? As far as I know, an LM317 has a dropout voltage of around 3V. If the supply voltage is not at least 3V higher than what you need for the load, the output may be unstable.

I would not recommend dropping the voltage down.

691175002 said:

Are you guys sure? As far as I know, an LM317 has a dropout voltage of around 3V. If the supply voltage is not at least 3V higher than what you need for the load, the output may be unstable.

I would not recommend dropping the voltage down.

Click to expand...

dear 691175002 user: did you read the hypothesis (at the beginning of this thread)? Uinput=8.4V! higher then 6V ;-) And yes, my intention is to decrease a little beat voltage on regulator, in order to dissipate less heat and spread the rest to silicon diodes...

and for billg519: yes, i am using a test driver. i'm not fool to play games with my LD. My question, regard especially any problem or some concers if i'll use it, not how to use it ;-)

regards,
alx

You would need to do some experimentation with your Driver setup and diodes..
and you could let us know of your findings... That would be helpful to other
members with the same problem...

Jerry