I'm working with two RFID readers in my circuit and an arduino to control both of them. The whole thing is powered by a very cheap wall-wart Power supply at 12V which is rather noisy to begin with. And a 7805 Voltage regulator to drop the voltage to 5V for the arduino.

All components have proper decoupling capacitors (in my opinion): 470uF+100nF for the two RFID readers, in close proximity. 100nF for the Arduino (not counting the decoupling on the arduino itself) 22uF+100nF on the input for the 7805 and 100uF+100nF on the 5V output. There is one additional component which is a MAX232 which is set up according to the datasheet.

Therefore I'd say, the decoupling is done porperly. However sensitivity aka read range is 5mm with the wall wart. Now if I use my bench-top Power supply (Peaktech 6150, not even an expensive one) read range is around 55mm. The same effect can be had when connecting the scope to the circuit and using the wall wart. So I'd conclude it is not a problem with the noisy wall wart but rather a problem with the grounding as the bench top PSU and the Scope are probably connected to earth(?) and thus provide a "harder" ground I guess?



Update: Using a scope to probe the 0V or +12V (and any other point) with referenece to the scopes GND, revealed that there are two major components of common mode noise: 355V Pk-Pk @ 50Hz from Mains 10V Pk-Pk @ 125kHz from the antenna-drivers (same frequency).

Probing the +12V point with reference to the 0V on the PCB shows 140mV Pk-Pk @ 1kHz which I belive to be the switching frequency of the wall wart. Superimposed I can still observe some noise from the antenna driver at the same 125kHz but now with the scopes earth/gnd connected it's around 4mV Pk-Pk. Which is sufficently low to bring the read range up to where it should be.

Solution (sorta) An accepted solution would be the use of a pre-made power supply with DC 0V connected to PE directly or via Capacitor. Or some other means of grounding the 0V line. However as Andy aka points out, it would be better to take a look at the susceptibility and source of the noise as this is just a fix for a non-ideal circuit.

  • \$\begingroup\$ 4905 regulator??? What does "sensitivity is around +-55mm" actually mean ? \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2019 at 9:52
  • \$\begingroup\$ Whoops i meant 7805! Sensitivity means, successfull read range. \$\endgroup\$
    – Horst
    Commented Aug 15, 2019 at 9:55
  • \$\begingroup\$ Why is it + and -? \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2019 at 11:28
  • \$\begingroup\$ To which part of the circuit are you referring? \$\endgroup\$
    – Horst
    Commented Aug 15, 2019 at 11:29
  • \$\begingroup\$ "+-55mm" is the bit I don't understand - is it a range of uncertainty? \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2019 at 11:58

1 Answer 1


Therefore I'd say, the decoupling is done porperly. However sensitivity aka read range is 5mm with the wall wart.

That assumes that the common mode AC voltage on the DC output of the wall wart is very low. It hardly ever is and the AC content is wiggling around at the low to mid hundreds of kHz. That's the switching frequency on the primary inside the wall wart leaking through to the secondary and wobbling both DC outs up and down together. It may be a few tens of milli volts but it can be enough to upset things.

This output common mode noise of the wall wart can be alleviated (and usually is) by having a class Y/X rated capacitor from the DC output to the rectified DC line voltage on the primary. Many suppliers of WWs use this technique but it comes with a problem.

That problem is a significant AC voltage at mains AC frequency on the DC output - it's harmless of course but, if the target load (your device) is not grounded then it's being shaken up and down by several tens (to hundreds) of volts p-p at mains frequency.

This will inject unbalancing currents into your load and potentially upset this or that. Hard to be definitive but it can (and does) happen.

So, my guess is that the decoupling isn't done properly OR your layout is a little flaky and is thus susceptible to unbalancing currents. Hanging a scope earth on the load 0 volts can be usually enough (in many cases) to solve this problem. Using a bench power supply that has probably got decent common mode (to ground) filters on the DC output can be enough too.

Of course, the wall wart might be just generating too much differential AC voltage at its DC output and this would be the place to review first. If it looks OK then use your o-scope and look at the WWs output voltage with respect to earth - each wire at a time and see what you see.

  • \$\begingroup\$ Thank you very much for the detailed explanation! Just to make sure I understand everything correctly: The voltage of my WWs 0V and 12V line go up and down in sync. The first component - in reference to a "true" gnd - is then at 50Hz which is the mains freq. and ~130khz which is the WWs secondary. This is at least what my scope tells my if I probe both outputs from my WW without connecting the scope earth. 338V Pk-Pk @50Hz & ~133kHz @ 10V Pk-Pk. If I probe the WW (under load) with it's own 0V reference the output ripple is around 140mV @ 1kHz, with a sawtooth shape that I'd expect. \$\endgroup\$
    – Horst
    Commented Aug 15, 2019 at 14:05
  • \$\begingroup\$ Yes, in sync. Yes AC mains frequency (sometimes double) and HF switching frequency. And yes, the output ripple seems reasonable but can still be an issue. \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2019 at 15:08
  • \$\begingroup\$ Thanks for confirming. I did some further testing, and removing the antennas eliminates the 130kHz signal as well. Since that is the antenna frequency, it is most likely the problem causing the read range to go down. Is it possible that the antennas can cause the whole system to ring and thus interfere with itself? If I only use one of the RFID readers the problem still persists, so it's not just cross-interference. I'd exclude induction as there is no difference if the antenna is 40cm or 10cm apart from the board. \$\endgroup\$
    – Horst
    Commented Aug 16, 2019 at 9:33
  • \$\begingroup\$ These sorts of detailed questions require answers that are beyond my ability to predict. Reason: I'm here and you're somewhere else and I have no hands on the hardware. \$\endgroup\$
    – Andy aka
    Commented Aug 16, 2019 at 9:59
  • \$\begingroup\$ I think I'll open another question on common mode noise that is more specific. But to conclude this question based on your comments and a bit more research into SMPS and RFID, an accepted solution would be the use of a pre-made power supply with DC 0V connected to PE directly or via Capacitor? Or making that connection with a 3-prong cable and using a transformer that is meant for use on PCBs. Although I have to do some research if this is allowed and what restricitons apply. \$\endgroup\$
    – Horst
    Commented Aug 16, 2019 at 11:33

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