HF49FD/005 relay control

Relay HF49FD/005 closes and opens until 240 V AC goes through, then it does not open, staying closed. 3V3 goes from logic unit WeMos D1 Mini. I cannot switch the relay off. It seems like 5V continues to go through the 2N3904 transistor to the ground.

This same diagram works with Songle SRD-05VDC relay, but not with this relay. The load is 240VAC 0.15-2.2vA 3D printer.

  1. What is wrong that I cannot switch off under AC current?
  2. Is the 1N4007 diode alright or I should use a 1N4001?
  • 2
    \$\begingroup\$ We don't know what is wrong with that info. Most likely the reason is not visible in your schematics, as according to the schematics the 3.3V will always be there and the relay will always be on. From where does the 3.3V come from, how do you control it? \$\endgroup\$
    – Justme
    May 19 at 8:46
  • \$\begingroup\$ 3V3 goes from logic unit WeMos D1 Mini and If you use the same schematic for relay Songle SRD-05VDC-SL-C, it works. \$\endgroup\$
    – pyramidak
    May 19 at 9:40
  • 1
    \$\begingroup\$ What load are you switching? 240VAC, how many amps? Is the load inductive, capacitive or resistive? \$\endgroup\$
    – Justme
    May 19 at 9:59
  • \$\begingroup\$ Load is 3D printer stand by 0,15A warm up 2,2A. \$\endgroup\$
    – pyramidak
    May 19 at 10:10
  • \$\begingroup\$ The printer is not the load. The load is the power supply of the printer. It likely has tens of amps of input inrush current. \$\endgroup\$
    – Justme
    May 19 at 11:45

1 Answer 1


It appears that the 3D printer is drawing high inrush current that causes contact welding in Hongfa relay HF49FD/005.

The contact of Songle relay SRD-05VDC does not weld, most probably because it's suitable for 'high rushing current use' application.

enter image description here

Generally, for such an application as yours, one would need to select a relay from 'high-inrush current' models, considering the TV rating mentioned in its data sheet.

The TV rating, as per UL and CSA standards, is an assessment of the inrush current withstanding capability of relay contacts as follows:

TV3 - 51A, TV4 - 65A, TV5 - 78A and TV8 - 117A, TV10 - 141A, TV15 - 191A.

  • \$\begingroup\$ Thank you, I think this explaining the strange behavour of relay. This is reason the songle is so favourite, but I need smaller relay only with two contacts, not three. I go to search. \$\endgroup\$
    – pyramidak
    May 19 at 11:23
  • \$\begingroup\$ My pleasure, pyramidak! \$\endgroup\$
    – vu2nan
    May 19 at 11:49
  • \$\begingroup\$ I will use this relay HF46F-G-5-HS1T (TV-3). Please do you know how choose the correct diod? Thank you. \$\endgroup\$
    – pyramidak
    May 19 at 16:33
  • \$\begingroup\$ A 1N4001 diode should be good enough as a freewheeling diode for a 5 V DC relay. PIV of the freewheeling diode should be at least equal to the coil voltage i.e. 5V (PIV of 1N4001 is 50 V). Average rectified output current of the diode should be at least equal to relay coil current i.e. 40 mA (average rectified output current of 1N4001 is 1A). \$\endgroup\$
    – vu2nan
    May 19 at 17:12
  • \$\begingroup\$ Thank you, now I´m sure about the logic. \$\endgroup\$
    – pyramidak
    May 20 at 4:45

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