Feasibility of Removing High Voltage Portion of Power Supply Circuitry

Discussion in 'Power Electronics' started by gctseng, Dec 14, 2017.

  1. gctseng

    Thread Starter New Member

    Dec 14, 2017
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    Need a sanity/safety check from you guys. I'm just getting into IoT and am looking at purchasing a bunch of Sonoff Basic WiFi Wireless Switches (link here) (ESP8266-based). They are all AC powered (90-250V). For simplicity of developing a common firmware platform (I'm going to be flashing the existing firmware with my own code), I want to be able to use them both where there are AC and in some cases, DC (battery only). My question is, for the latter case, can I simply remove/disconnect the traces for the AC portion of the power supply circuitry?

    From the circuit diagram (link here), it would seem that I can simply disconnect the 5 and 8 contacts of the EE10 transformer (see section C,3 on page 2) and provide my own DC power. I can't tell from the pictures of the circuit board (link here) what the U2 voltage regulator (see section C,1 on page 2), needs, but I'm hoping to be able to provide either a 5V or 12V).

    The controlled AC output will still seem to be available as the traces from the AC input seem to be unaffected by how the rest of the circuit is powered.

    Before I blow myself up, can anyone see anything wrong with my approach?

    (First time posting, so I didn't know whether to just provide the links to the files or upload; I opted for the former, but let me know if I need to upload the schematics pictures instead of linking them.)
     
  2. dendad

    Distinguished Member

    Feb 20, 2016
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    The SonOff can be run on 5V if you need to.
    Across D5, +5V to "VccBar , D5 cathode, and 0V to D5 Anode.
    But if you are running them on 5V, do you want to switch DC in place of the mains too? Why run them on DC if you will be switching mains anyway? Or will you be switching 12V instead of the mains? I'm a bit confused.
    If you want to disable the mains supply, take the bridge rectifier out altogether just to make it safer. It i not a good idea to disconnect the secondary and keep the primary running.
    I've added the circuit too.
     
  3. gctseng

    Thread Starter New Member

    Dec 14, 2017
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    Ahh. Your solution re:providing the 5V power and removing the bridge is totally logical.

    I should have phrased my questions thusly:

    Can I connect 12 volts to a location in the Sonoff circuit to bypass use of the mains power supply circuitry and therefore eliminate the need to run mains power to the Sonoff device?

    I'm guessing I'd have to see what U2 is to figure out if I'm limited to 5V or if I can use 12V directly.

    To clarify my use scenario(s): I want to use the Sonoff in situations where there is a low voltage (12v DC) distributed power network that will power the Sonoff as well as the device (e.g., led strips) that is being switched. Or, power the Sonoff and the device to be switched with a 5v wall wart. A third situation (more likely than the other two, actually) is to not necessarily switch anything but use the Sonoff as a sensor platform.

    Now that you've mentioned it, it wouldnt make much sense to connect mains to the Sonoff but switch it with a low voltage source. I don't know why I had that in mind.

    In case I didn't address why I don't just buy a low voltage alternative, I believe that the cost of the Sonoff device, including casing, etc., is at a price point where it is less expensive to just pay for the "extra" high voltage components even in cases where I'm not going to use them.

    (I'll probably end up with some extra transformers and other components from the high voltage circuitry of these low voltage application Sonoffs.)
     
  4. dendad

    Distinguished Member

    Feb 20, 2016
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    You won't be able to run the SonOff from 12V directly. For one thing, the relay is 5V so a 7805, with bypass caps and a protection diode, running from the 12V supply could be added and connect the 5V out as mentioned above. Just be sure you don't mix up the mains and 12V versions ;)
    If you pull out the transformer, you could grow the 7805 in there. And even use the bridge and filter cap. The 10R in the mains lead may need shorting out.
    Do you need help on connecting the 5V regulator? Here is an idea...
    12V_SonOffMod.jpg
    This makes use of the existing caps.

    Just remember, the relay will not be rated to switch as high DC currents as it is on AC.
     
    Last edited: Dec 15, 2017
    gctseng likes this.
  5. gctseng

    Thread Starter New Member

    Dec 14, 2017
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    Thanks! I woke up this morning thinking about the relay issue, haha.

    The help is much appreciated!
     
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