Problem in Combining Two Voltage Regulators (AMS1117 3.3) (LM7809)

Thread Starter

Anant Singh 1

Joined Aug 4, 2016
5
Hello
I'm making a robot, for that i need various power supplies.
The problem is my battery is of 15 volts and I need a 3.3 power supply for a part (although I need many more levels but, they are working fine except 3.3) , for that i used AMS1117 3.3 voltage regulator combined with LM7809 as AMS1117 can only take upto 12 volts only. But a lot of heat is being produced as soon as I apply load on the regulator. Current from the IC is around 250mA (which is not a sufficient reason for this much heat). I do not know much about the internal circuits of these ICs. Also it gives 3.3 voltage at no load.

Please help, suggest a better circuit or the cause of the heat production from the regulator..
Feel free to ask for any further details..
 

Bordodynov

Joined May 20, 2015
2,660
P=250mA*(16V-3.3V)=3175mW=3.175W
LM7809 --> P1=250mA*(16V-9V)= 1750mW
AMS1117-->P2=250mA*(9V-3.3V)=1425mW
For these two chips are required to remove the heat radiators. Otherwise, they will be warmer than 100 degrees Celsius.
An alternative is to apply the dc/dc converter. This will improve efficiency and reduce current consumption from the battery 3-4 times.
 

dl324

Joined Mar 30, 2015
11,516
Welcome to AAC!
Current from the IC is around 250mA (which is not a sufficient reason for this much heat).
As indicated, the 9V and 3V regulators will be dissipating several watts. You'd be better off using a switching regulator for the 3.3V supply or use a switching preregulator instead of the 7809.
 

AnalogKid

Joined Aug 1, 2013
8,590
ebay has many low cost buck regulators that can take 15 V down to 3.3 V in one stage with 85% efficiency or better. This would reduce the total regulator power dissipation to less than 0.5 W. The switching device will be warm to the touch, but it won't melt your fingerprints.

ak
 

AnalogKid

Joined Aug 1, 2013
8,590
(melt) haha.....
Sounds like a funny exaggeration, I know. But ... it isn't. Ask anyone who has dealt with power electronics or soldering irons for a while, and you'll see this is a real thing. I takes only a few milliseconds for accidental contact with a soldering iron to melt/singe/fry the skin, and an overheated power transistor or resistor can do it almost as fast. By the time you feel it, it's too late.

ak
 
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