3
\$\begingroup\$

I am using a linear regulator to step down the voltage for my microcontroller circuit, however, I noticed that when I power my microcontroller circuit from my laptop (at 5 volts) it pulls around 76 milliamps, but when I try to power the circuit with a 9v battery (or my 12v car battery) and the voltage regulator (L7805CV), then the current being drawn is only around 20 milliamps (instead of the 76mA that I get from my computer when powering without the regulator).

The voltage regulator output is 5volts as I would expect, but the current is being limited or something when the max should be 1.5Amps. I thought that maybe I burned out the regulator accidentally or something, so I swapped it for another one but am getting the same results. 

(TOP VIEW)
--------|====== Output (5v)
|       |====== Ground 
--------|====== Input (9v)

When I'm powering from the regulator I am getting 5 volts, but the circuit is only able to draw .20mA and the circuit is not working properly. When I am powering from the USB power supply, I am also getting 5volts but at 76mA, and the circuit works as designed.

EDIT I am not using the .33 uf or .1uf capacitors. When I get a min, I will try adding those, but I have a hard time figuring out how that would limit my current to .02amps when the max should be 1.5 amps.

\$\endgroup\$
14
  • \$\begingroup\$ Is the voltage from your laptop 5 V? Where are you measuring the current? The dropout for that part is 2 V, so if you are not inputting at least 7 V I expect undefined behavior. \$\endgroup\$
    – ricehornet
    Nov 2, 2018 at 2:25
  • 2
    \$\begingroup\$ If the voltage regulator is putting out 5V, then regardless of the current being pulled, it's doing its job. The amount of current consumed at a given voltage is a characteristic of the load, not the source. \$\endgroup\$
    – TimWescott
    Nov 2, 2018 at 2:34
  • 1
    \$\begingroup\$ The real question is, is the voltage still 5V on the output when it's plugged into your microcontroller circuit? And is your microcontroller circuit still operating as expected? \$\endgroup\$
    – DSWG
    Nov 2, 2018 at 5:41
  • 1
    \$\begingroup\$ @ChaseRoberts Ah HA! Now we're getting somewhere. You may want to edit your question (and the title, if Stackexchange will let you). It is most likely that the startup behavior of the power line is different in each case. Second to that, it is most likely that the 7805 is ramping up too slowly for the microprocessor, and possibly overshooting. Typically when this is a problem the micro (or some other complex chip) comes out of reset when the voltage is too low for it to start up properly, and it comes up wedged. If you can manually reset, try doing so and see if it comes up. \$\endgroup\$
    – TimWescott
    Nov 2, 2018 at 15:10
  • 2
    \$\begingroup\$ Sounds like your regulator output might be oscillating, at least to some extent. Have you tried putting in those capacitors yet? \$\endgroup\$
    – Hearth
    Nov 2, 2018 at 17:23

2 Answers 2

Reset to default
1
\$\begingroup\$

For powering from 5V:

The dropout for this regulator is 2V so that means you need at least a 7V input to run the regulator. Since USB is only 5V the regulator doesn't have enough voltage to turn on the transistors inside of it to supply current.

You can buy regulators with lower dropout (like 50mV, I think they go as low as 20mV), however that would mean you'd need at least 5.05V. Another caveat is the usb cables usually have a drop from resistance so if your sourcing 100mA's then you'll get 4.9V or 4.8V at the end of the cable.

If you really need 5V regulation, you'll need a DC DC converter.

enter image description here Source L7805CV datasheet

For powering from 12V:

The circuit should regulate to 5V, you have enough dropout. You may want to check the input to make sure you have at least 7V, if the battery has too much internal resistance (like on a 9V alkaline battery) then you won't be able to pull much current from it.

A car battery should be able to source enough current, but check the output to ground anyway.

You may have the regulator wired wrong it shouldn't power anything from 5V so double check the pins with the datasheet.

\$\endgroup\$
1
  • \$\begingroup\$ I am bypassing the regulator when powering from the USB, so that's not the problem. Although that could be the problem with the 9v battery. \$\endgroup\$
    – Chase Roberts
    Nov 2, 2018 at 17:39
0
\$\begingroup\$

Check the output voltage as well. If you can, do it with an oscilloscope, to check whether there is any ringing.

Also, add a cap to the output. It is a must. A regulator without an output cap is not a real regulator since the cap does high frequency filtering/smoothing. Otherwise, you might get power supply rejection issues. Check the datasheet whether you need a cap for stability. The dominant pole of the feedback loop might be set by the output capacitor in linear regulators (output compensation).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.