Re: Looking for replacement for a 7505

Windell Oskay
Yes, that looks like the likely explanation; the standard Arduino ethernet shield can draw 150 mA on its own, so you probably need around 200 mA total capacity.

I don’t know of any device that can burn that much power and can fit in the TO-92 location provided.  If you pull a full 500 mA, then you would be asking the regulator would be to “burn up” (12 V – 5 V) * 500 mA = 3.5 W of power as heat– more than actually goes to power the device, and much more than a TO-92 package can normally handle.    In situations like this, one normally wants to use a switching regulator that will convert the input power, rather than just burning up the majority of it.

As far as similarly-sized linear regulators go, the L4931CZ50-AP will fit in the location provided, and handle 250 mA, but will be thermally limited and likely will not work.  At 200 mA, you’d be dissipating 1.4 W as heat.  The thermal resistance of that device is 200 degrees C/W (the *worst* I’ve ever seen in a TO-92!), so at 1.4 degrees, it would operate internally at 280 degrees C above ambient– well above its rating of 125 C.
If you wanted to use the L4931CZ50-AP at 200 mA, one solution would be to provide it with a lower input voltage, allowing it to dissipate much less heat.    Assuming 25 C ambient, you’d need to keep the thermal dissipation to 0.5 W, for a 100 degree rise.  0.5 W = 0.2 A *  2.5 V, so you’d want no more than 7.5 V input.  To get there, you could drop from 12V to 7.5 V– a drop of at least 4.5 V –with a zener diode. For example, the 1N5340BG (a 6 V zener) might be a good choice.   
(This is very similar to the last example in our article about zener diodes, here:  )
So, there’s no easy, simple solution.  It might be worth mounting an off-board 7805 (possibly with a heat sink), mounting a 7805 with extension leads, or using the zener trick, but none of these is particularly appealing.  You could also make things a lot easier by reducing the input voltage, for example by using an external regulator to step the voltage down.