Thanks. That helped. Yes, after using ac coupling and a horizontal scale of 10nS there was a ripple that was visible. The peak to peak voltage was about 0.3V.

PIC-User Awesome! So now maybe we're getting somewhere with mystery!

So here's more measurements that could reveal pattern:
◊What's ripple frequency w/100μf Cap?
◊What's ripple amplitude and frequency w/10μF Cap?

 

Can you take a stab at estimating the frequency of the ripple? To me ripple is an artifact of converting AC to DC and should show up as a periodic waveform at twice the power line frequency. Ripple is what you see from an unregulated DC power supply. Once you run the DC with ripple through a voltage regulator, you don't see ripple any more; what you see are other artifacts and their frequency can be a clue to the source and the solution. A scope picture would help immensely.

Papabravo Sry for the crosspost

 

Hi, the frequency of the 5V ripple is the same with the 10uF and the 100uF. The frequency is around 50MHz. The frequency used by the RFID module is 13.56MHz, so maybe the noise is something like 4xF.

I have done some reading about the oscillation relaxation issue mentioned earlier. I tried to fix it with a 10 Ohms resistor in series with the 100uF cap, but that didn't work. According to what I read, this oscillation relaxation has something to do with the cap not getting discharged properly. So, I put a 10k resistor in parallel with the 100uF cap and the circuit seems to be working very well. The module hasn't jammed yet. I know that this represents a leak of 0.5mA, but I can live with that if the circuit works.

 

So, I put a 10k resistor in parallel with the 100uF cap and the circuit seems to be working very well. The module hasn't jammed yet. I know that this represents a leak of 0.5mA, but I can live with that if the circuit works.

PIC-User I totally agree! Cuz almost all SMPSUs need a minimum external load 4 proper regulation and stability! So I say adding capacitor wasn't the real problem cuz it just revealed real problem by like tipping under-loaded psu off _knife's edge_! So even without 100μF cap, psu circuit prolly wasn't 100% reliable loaded just by module. So even if u decide not to use big cap u should still use resistor 2 establish minimum load. So happy it's working 4u now!