Low current 78L05 voltage regulator - Page 1
Forum section: electronics / beginners

author: civrays
author_rank: Newbie
timestamp: December 27, 2020, 09:05:40 pm
content: I'm adding a low current voltage regulator to supply my circuit but I'm unsure of what capacitors to put on the input and output pins on the 78L05. The data sheet specifies a minimum of 330nF on the input and 10nF on the output. So I started with those, although they seem very low values.I put an oscilloscope on the the input and output, but I don't really have much experience. It looks quite noisy but I don't know what is normal. One thing that struck me was the peak to peak voltage variation of between 180mV and 280mV. I was expecting much lower values.I experimented with some electrolytic capacitors on both the input and the output, ranging from a around 3.3uF to 470uF. I even tried two 470uF capacitors, but to be honest there seemed no change what so ever. I even tried removing the 330nF and 10nF capacitors and again no change.So my conclusions was that adding additional capacitors is pointless. My current is very low around 10mA probably less. In real use it could go up to 50mA or more, but the spec on the 78L05 is 100mA absolute max. The circuit is powering an I/O IC and a couple of LEDs and transistors. So it's just a logic switching circuit, not anything delicate like an audio amp.My question is, with such low currents should I worry about the capacitors? My main concern is that if there is a sudden change is current requirement, that the voltage drops and the IC may reset. I think that's unlikely because the currents are so small and the IC can operate at 3.3volts anyway.

author: innkeeper
author_rank: Unknown
timestamp: December 28, 2020, 12:49:45 am
content: so the input capacitiance can be any value, but, the idea here is to reduce any noise going into the device, and therefore they specify that cap should be as close to the regulator as possible.because it is on the input of the regulator, you do not ahve to worry about using higher capacitances. however, you might not get the filtering needed.some people stagger values to get a wider range of noise filtering like using .01 and .33 an 10uf.the same can be done at the output.as for worries about too much load on the device when powering up when there is capacitance at the output, there is current limiting on the regulator, and it can handle higher pulse currents.I've seen commercial designs with 10uf output caps.there are some traps here...theres so much a regulator can do with unclean input power. so make sure that's well filtered before the regulator.there is also a lower limit to the input voltage before the regulator has trouble regulating, this is the dropout voltage. so be sure your input is at least 2v above your regulated output voltage.i am curious what kind of noise your seeing, .. frequency, and amplitude.another gotcha is that regulators if improperly set up, can oscillate.

author: srb1954
author_rank: Super Contributor
timestamp: December 29, 2020, 08:06:52 am
content: Quote from: civrays on December 27, 2020, 09:05:40 pmThe measured voltages of 180mv to 280mV peak to peak are way higher than what you would expect with a 78L05 regulator, even with the minimum recommended capacitor values.What was the frequency and waveform of the measured voltage variation?Did the frequency, amplitude or waveform change in any way as you varied or removed the capacitors?How did you connect the scope probe to the regulator output? Incorrect connection of the scope probe can introduce spurious noise into the measurement.Did you also probe the input voltage to the regulator to ensure that it doesn't drop below the minimum 7V input voltage required by the 78L05.