Linear Voltage Regulator Basics
Linear Voltage Regulator Basics
(OP)
Hi All,
I'm using an LM317 (TO-220 package) to regulate a supply voltage of 7.5VDC from 12VDC. It's the basic LM317 circuit: 0.1uF cap on the input, 1.0cap on the output, forward diode from output to input, and two resistors on the adjust pin so Vout = 1.25*(1+r2/r1) + Iadj*r2. I'm using 100ohm for r1 and 500ohm for r2. The 7.5VDC is used to supply power to an ethernet hub circuit, and the max. current draw is 1A. The 12VDC input is coming from an unregulated power supply.
The problem is that the regulator is dissipating too much heat for my application, even w/ a large TO-220 heat sink.
What's the best way to reduce the heat from the regulator? Is it by reducing the input voltage (because the heat dissipation is in relation to the voltage drop)? I thought about putting a zener in seried with the input, but is this bad circuit design technique?
Thanks in advance!
I'm using an LM317 (TO-220 package) to regulate a supply voltage of 7.5VDC from 12VDC. It's the basic LM317 circuit: 0.1uF cap on the input, 1.0cap on the output, forward diode from output to input, and two resistors on the adjust pin so Vout = 1.25*(1+r2/r1) + Iadj*r2. I'm using 100ohm for r1 and 500ohm for r2. The 7.5VDC is used to supply power to an ethernet hub circuit, and the max. current draw is 1A. The 12VDC input is coming from an unregulated power supply.
The problem is that the regulator is dissipating too much heat for my application, even w/ a large TO-220 heat sink.
What's the best way to reduce the heat from the regulator? Is it by reducing the input voltage (because the heat dissipation is in relation to the voltage drop)? I thought about putting a zener in seried with the input, but is this bad circuit design technique?
Thanks in advance!





RE: Linear Voltage Regulator Basics
To truly reduce your heat output, a switch-mode power supply is the way to go. Heat output is the reason they were originally developed. A good design with adequate filtering can be just as quiet as a noisy linear. Just a thought.
RE: Linear Voltage Regulator Basics
use the national switcher program to design your switcher
have fun
RE: Linear Voltage Regulator Basics
Good luck and keep us posted!
RE: Linear Voltage Regulator Basics
Thanks for the help. I thought about a switcher, but I need to get something completed by the end of this week. I breadboarded the regulator with three 2.5A diodes in series, and this cut the input down around 9.5V. This noticeably took some of the heat from the 317. Also, I put the diodes a little further away from the 317. Previously, my main concern was the amount of heat being generated within such a localized area. Basically, the output voltage looks good, and I'm not melting the surrounding plastic! Thanks again.
RE: Linear Voltage Regulator Basics
Also, I don't necessarily agree with adding front end capacitance. This would depend on the source of the 12VDC. Adding capacitance will take out more low freq ripple, if it is present (depends on source) and won't help at all with your heating problem (don't think poster was implying it would either).
It sounds like the board layout may have contributed to the heating problems as well. Is the regulator laying flat on the board or standing up?