Some thoughts about selecting a voltage regulator for the Transistor Tester:
- It should handle 100mA (MCU) plus whatever is needed for display and hardware options.
The MCU does need much less in average but a few checks use high currents very briefly.
- It should be able to handle rapid load changes (sleep mode, normal operation, high current checks, hardware options).
- It should have a good regulation to keep the voltage stable under changing loads.
- It doesn't need to be one with a low output voltage tolerance but such a regulator would improve the tester's accuracy.
We recommend the MCP1702 (250mA, 0.4%, about EUR 0.60). Affordable and much better than a LM78L05 plus TL431.
This brings us to the external voltage reference. The purpose of the reference is to determine Vcc and therefore it should have a tolerance at least one order of magnitude better than the voltage regulator's tolerance. A TL431 is ok for a 78L05, a LM4040 would be better. If you use a MCP1702 you don't need a reference at all unless you're a voltage nut
.
I hope this helps with the selection of the voltage regulator and external voltage reference.