Electronic – Jumper out an LM7805

lm78xxvoltage-regulator

I have a 5V hobby circuit that will be externally powered from a wall wart, with way less than 1 amp. For full flexibility, I'm thinking of using a simple LM7805 regulator – but that has a 7V minimum. Since 5V supplies are becoming ubiquitous, I'd like to allow that too – but then I'd need to bypass the regulator.

What would be the impact / effect / damage if I simply provided a jumper that shorted the regulator's input to the output for semi-permanent use with a 5V supply? Use a 9-12V supply: remove the jumper. Use a 5V supply: add the jumper. I'd leave the input and output capacitors to do their thing: it won't harm the 5V case.

Would the regulator be at minimum consumption because the output was already at 5V? Or would it struggle at full load with the less-than 7V input – or worse, short output to ground?

Best Answer

Well, I decided to simply breadboard it. See below for the Summary.

I fully recognise @glen_geek's comment regarding overlooking the jumper - but I figure that's my lookout!
Yes, I realise that this is a single experiment and not to trust this empirical result. However...

Experiment

For those who are thinking of doing the same, I implemented the following schematic: LM7805 schematic

  • TP1: Current measurement (mA)
  • TP2: Voltage measurement (V)
  • LM7805: Regulator temperature (Tr)
  • 20R 5W: Dissipation temperature (Td)
  • JP1: When Batt was 9V, the jumper was OFF.
    When Batt was 4.5V, the jumper was ON.

1) 9V with JP1 OFF:
mA = 250 mA
V = 4.99V
Td = 36C
Tr = 39C

2) 4.5V with JP1 ON:
mA = 200 mA, descending to 180mA over 1 hour
V = 4.08V, descending to 3.51V over 1 hour
Td = 36C
Tr = 22C (ambient)

3) Back to 9V with JP1 OFF:
mA = 250 mA
V = 4.95V
Td = 36C
Tr = 39C

4) 4.5V with LM7805 removed! (JP1 ON):
mA = 180 mA
V = 3.52V (same old batteries as before)
Td = 36C
Tr = N/A

Summary

In short, with a 200-250mA load:

  • The regulator got hot when I expected it to;
  • The regulator was cold when it was jumpered over;
  • The source current didn't change regardless of voltage supply;
  • The regulator did its job again after being abused for an hour;
  • Removing the regulator did nothing for the low-voltage scenario.

I'm going to risk it!

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