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I'm trying to control the speed of a 12V(0.14A max current) fan using three relays. It should have four states which are turned off, low speed, medium speed and high speed. What's the correct wiring for the relays and the fan? Here is my try:

schematic

simulate this circuit – Schematic created using CircuitLab

I use ULN2003 for driving the relays which is controlled by esp8266.

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  • \$\begingroup\$ What problems did you observe with your first try? Are your relays really rated for a 3.3 V coil voltage and capable of being safely driven from whatever is driving them? Is there a rationale for why PWM (pulse width modulation) can't be used instead? \$\endgroup\$
    – nanofarad
    Sep 1 at 21:38
  • \$\begingroup\$ How are A,B and C controlled? as nanofarad also mentioned, have you thought of more dynamic FSC? or is it strictly 4 mods? you can use a positional switch for that \$\endgroup\$
    – NeuroEng
    Sep 1 at 21:40
  • \$\begingroup\$ What is driving 3.3V/0V to the relay coils A, B and C? \$\endgroup\$
    – Justme
    Sep 1 at 21:41
  • \$\begingroup\$ @nanofarad I haven't tried that yet because I'm not sure it works. Also I'm told to do the speed control using three relays. So PWM can't be done. \$\endgroup\$
    – S.H.W
    Sep 1 at 21:42
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    \$\begingroup\$ @S.H.W ULN2003 is not a high side driver, it can't drive 3.3V to the relays. However the ULN2003 is perfectly capable of driving the low side of the fan directly so you can just remove the relays. Although if you used a single transistor with PWM you would have say 256 speeds with one IO pin, instead of wasting 3 IO pins for 4 speeds. \$\endgroup\$
    – Justme
    Sep 1 at 21:54
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Modified circuit.

If your fan can be controlled that way then you only need two resistors.

  • RLY3 slow.
  • RLY2 medium.
  • RLY1 fast.

schematic

simulate this circuit

Figure 2. Low-side switching of relays. The diodes aren't needed as there are diodes in the ULN2003.

schematic

simulate this circuit

Figure 3. Direct drive by the ULN2003.

Observe the maximum current rating of the ULN2003 and pay attention to the overall current rating if switching multiple loads simultaneously.

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  • \$\begingroup\$ Thanks for the answer. As Justme said, ULN2003 is capable of driving the low side. So how the circuit should be changed? \$\endgroup\$
    – S.H.W
    Sep 1 at 22:02
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    \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Sep 1 at 22:26
  • \$\begingroup\$ Thanks for the great answer. Can we use relays with coil ratings other than 3.3V? I think we can and 3.3V voltage source should be replaced by the desired coil rating for example 5V or 12V and the logic which controls the ULN2003 can be 3.3V or 5V. \$\endgroup\$
    – S.H.W
    Sep 2 at 0:00
  • \$\begingroup\$ you could use RLY1 to short circuit R4 ... same with RLY2 and R3 ... that way you gain one more speed \$\endgroup\$
    – jsotola
    Sep 2 at 2:27
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    \$\begingroup\$ @S.H.W, yes you could use higher voltage relays. The ULN2003 COM, pin 9, for the diodes would then be connected to the +5 or +12 V supply. \$\endgroup\$
    – Transistor
    Sep 2 at 6:13

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