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I'm thinking what would be the better option to get fixed 5v output from car when car is running. I want to have fixed 5v for my Atmega328p and some other parts of the circuit, and the max current drawing will be around 150-200mA.

I saw on some forums that LM7805 dissipate more heat than LM2937, but I didn't have a chance yet to try it.

I know that SMPS solution would be the best option, but for linear solutions, where I can easily just use any of these two since even the pinout is the same, which one is better \ safer to use?

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  • \$\begingroup\$ At 150-200mA there will be no heat. Linear is the right part for the job. Either regulator will work for you neither is better than the other in your case. \$\endgroup\$ – Misunderstood Mar 19 '17 at 3:20
  • \$\begingroup\$ @Misunderstood A running car can be close to 15V, that's 10V dropped in the regulator. At 200mA, that's 2W of heat dissipation, enough to get most TO-220 parts to overheat free-standing in 25°C air, nevermind in the potentially cramped and hot environment that is a car... \$\endgroup\$ – marcelm Aug 6 '19 at 17:49
  • \$\begingroup\$ @marcelm That's why you add a power resistor between the +12 supply and input to the 7805 to dissipate the excess heat. \$\endgroup\$ – Misunderstood Aug 12 '19 at 16:28
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I saw on some forums that LM7805 dissipate more heat than LM2937, but I didn't have a chance yet to try it.

No, this is nonsense. The LM7805 and LM2937 are both linear regulators; they will dissipate however much heat it takes to drop their input to 5V. Neither part has a significant quiescent current.

The primary advantage of the LM2937 in automotive applications is its hardiness. It can withstand input voltage transients of up to 60V, whereas anything above 35V is likely to destroy a LM7805. These sorts of transients are frequent in an automotive environment, especially when cranking the engine.

Note, however, that the LM2937 has a significantly lower current output (500 mA, vs 1.5 A for the LM7805). This appears to be within your design requirements, but it's worth keeping in mind regardless.

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Assuming you are running from a 12 V battery voltage, the efficiency difference is negligible. The LM2937 is a low-dropout regulator, which means it can be operated with a lower supply voltage for more efficiency, but when run from 12 V will be basically the same. There is also probably a difference in the current needed to operate the regulator, but that will be << 150 mA, so won't have much impact on the overall figure.

When selecting for automotive applications, there are other factors to consider. Temperature extremes are common, especially extreme heat. Vibration is another killer, and the electrical environment in a car can be pretty ugly in terms of spikes, surges, dropouts, and RF interference. Anything plugged into the car needs to be able to handle that.

Finally, remember current limiting -- normal bench supplies and wall warts are usually current limited to a couple amps. A car battery can supply 100s of amps, although you will presumably be plugging into an existing line with a fuse, that could still easily be a 20-30 amp fuse. If you use thin gauge wires, make sure it is protected by an appropriate fuse. What you don't want is a surge from the starter motor killing your regulator in a short circuit fault, followed by a fire in the wiring harness.

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  • \$\begingroup\$ So, the LM2937 would be "safer" to use, but not more heat efficient...and how about SMPS solution, like LM2595 for example? \$\endgroup\$ – ShP Mar 19 '17 at 3:30
  • \$\begingroup\$ Basically. Of course ruggedness includes many more factors than regulator IC -- such as capacitors, transient suppression, etc. I wouldn't use a switching regulator in this application -- car alternators produce plenty of juice, and you aren't using much power. \$\endgroup\$ – Evan Mar 19 '17 at 3:45
  • \$\begingroup\$ Thanks for your help. One more off-topic question how about R-785.0-0.5 will this one be tolerant enough to be used in automotive applications? \$\endgroup\$ – ShP Mar 19 '17 at 13:29

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