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schematic

simulate this circuit – Schematic created using CircuitLab

enter image description hereI have a micro-controller based relay switching circuit, powered by AC/DC SMPS module based on THX203H controller. The module is isolated flyback type. They circuit is powered from 220V/50Hz outlet.

The phenomenon I see it that sometimes the module shuts down momentarily when they relays switch a 220V load, even very light one (such as indication lamps). The phenomenon never happens when no load is connected. After experimenting, I came to conclusion that the switching noise on the lines is somehow able to destabilise the SMPS: 1. If I add a ferrite bead on the A/C phase, the phenomenon diminished significantly. 2. If I power the AC/DC from 220v/120v transformer and the load switching phase directly from 220V outlet, the phenomenon disappears completely. Unfortunately, I have not been able to measure the hot side yet to see what actually happens. Analysing the module compared to the reference design in the datasheet (attached) I have found that X and Y capacitors as well as varistor and input choke are missing. Also the input reservoir is much smaller than recommended (I tried to double it - no effect). The rest seem to follow the recommended design. Did somebody have similar experience before? Any suggestions about what can cause the issue and how to resolve it ? Datasheet of the controller: https://lib.chipdip.ru/030/DOC001030452.pdf

Thanks!

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  • \$\begingroup\$ Caveat emptor \$\endgroup\$
    – Andy aka
    Feb 1, 2018 at 17:13
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    \$\begingroup\$ Impossible to answer without speculating wildly. Please update the question to show at least a wiring diagram, better yet a schematic of the relay switching circuit and how it is connected to the power supply output. Photos would help too. \$\endgroup\$ Feb 1, 2018 at 17:38
  • \$\begingroup\$ Document download failed. \$\endgroup\$ Feb 1, 2018 at 18:23
  • \$\begingroup\$ Added simplified schematics and reference design from the datasheet. \$\endgroup\$ Feb 1, 2018 at 18:52
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    \$\begingroup\$ It SEEMS that the cause is relay load side related BUT relay coil operation is also a possible cause. Place a reverse polarity diode across the relay coil (1N400x or whatever). Ensure adequate filter and bypass cap AT microcontroller supply pin. A say 100 uF or more PLUS a say 0.1 uF ceramic in parallel AT uC Vdd to uC ground should help. It can help to add a small series R in V+ feed to uC with above caps on uC side of it. Dimension R so that V drop is say 0.1V at Imax for uC. \$\endgroup\$
    – Russell McMahon
    Feb 1, 2018 at 20:10

2 Answers 2

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It SEEMS that the cause is relay load side related BUT relay coil operation is also a possible cause.

Place a reverse polarity diode across the relay coil (1N400x or whatever). This helps clamp relay inductive spike on turnoff. While you are apparently seeing turn ON issues, there may be loading related inductance change effects that cause relay coil voltage spikes.

To ensure that the uC (microcontroller) supply is noise free, ensure adequate filter and bypass cap AT the uC supply pins.
Using a say 100 uF or more capacitor PLUS a say 0.1 uF ceramic in parallel AT uC Vdd to uC ground should help.
Ensure that uC V+ supply line and ground run to power supply directly and do not share any wiring run with the relay or load.
It can help to add a small series R in V+ feed to uC with above caps on uC side of it. Dimension R so that V drop is say 0.1V at Imax for uC.

On diagram below ensure that uC ground wiring runs from B directly to A and not via B. Shared wiring A-C allows voltage drop in A-B to appear in uC power feed. uC Vdd supply method is not shown but is unlikely to be 12 VDC. The probably 5 VDC or 3V3 V Vdd is probably provided via a regulator from the 12V supply. Any regulator needs also to be as isolated from coupling to relay supply as reasonably possible. Have adequate caps at regulator input, route 12V wiring (V+ AND ground) direct to regulator with no commonality with relay wiring. Add the small resistor mentioned above to the regulator Vin instead. Having say 1V drop across resistor allows some RC filtering of noise but the regulator input-output noise rejection capability should handle much of this.

enter image description here

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  • \$\begingroup\$ Russel, thank you for the detailed reply. It has been verified and re-verified that the problem is not on the MCU side. There are diodes on the relay coils. The MCU is behind 2 regulators with proper bypassing everywhere. And the main thing - I can see on the scope that sometimes the SMPS shuts down - the output can go down as low as 5V or even less and only then it catches up again. I am also completely positive that the issue does not ever happen if no load connected to the relays. \$\endgroup\$ Feb 1, 2018 at 20:44
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Late to the party, but it SEEMS I run into the same situation (though with slightly different circuits): "very light load" + "relays switch" -> "SMPS shutdown"

Investigation:

  • When NO, NC and COM is unconnected, relay and SMPS operates normally
  • I've connected live and neutral to NO and NC, COM is unconnected: this is zero load, but SMPS shuts down randomly when relay is operated
  • I've connected an external manual switch, switching between live and neutral to the COM (NO and NC unconnected), relay/MCU powered but does nothing, operating the manual switch: SMPS shuts down
  • I've measured 20-30pF between NO/NC/COM and relay coil pins, desoldered the relay, measured the relay and PCB separately, no parasitic capacitance measured, so I suppose some minimal capacitances added together

Conclusion:

  • It SEEMS the voltage rise and drop on the COM/NO/NC pins went through the parasitic capacitances to the optocoupler through the "fast lane" and these spikes caused the SMPS shutdown (I think L2 was also missing in your SMPS as in mine)

Solution:

  • Using a better SMPS (choke, NTC, MOV, X and Y at primary side, L2 and better 431 related circuitry on secondary side) solved the problem, relay is operating normally with load
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