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I am trying to construct Automatic switching power supply, which switches between 5 V power source (produced from 9 V DC SMPS Power supply) and 5 V Battery Power from the Boost converter (produced from 3.7 V 18650 Li-Ion Battery) and the Relay output is connected to Vin pin of Nodemcu v1.0 ESP-12E with 160 mA current consumption.

5 V Relay is used to switch between two power sources and the output of 5 V Relay is given as input supply to the Vin Pin of Nodemcu v1.0 ESP-12E. Nodemcu v1.0 ESP-12E consumes 160 mA current during operation.

But due to slow relay switching speed, Nodemcu v1.0 ESP-12E is loosing power for 5 seconds and i have used 4700 Micro Farad Capacitor between Vin and Gnd pin of Nodemcu v1.0 ESP-12E. But the problem still exists.

I have Googled for Solutions. But they are not useful for me.

  1. MCP73871 PowerBoost Chip

    MCP73871 PowerBoost Chip costs around 800Rs which increases the cost of the project. Is there any low-cost reliable solution for switching between two power sources without interrupting power to Nodemcu v1.0 ESP-12E ???

  2. LTC4412 Low Loss PowerPath™ Controller is not available in India. So I can't implement the Automatic switching circuit mentioned in this website.

  3. Using two diodes for OR operation copied from this website, looks problematic, if there is high power consumption from Nodemcu v1.0.

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Please suggest me some working solution which can safely provide 5 V, 500 mA current to Nodemcu ESP-12E Vin pin.

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  • \$\begingroup\$ What are the min and max voltages that the Nodemcu ESP-12E Vin pin can take? I could not understand why you think the diode solution is problematic. Could you elaborate? \$\endgroup\$
    – sai
    Commented Sep 26 at 15:20
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    \$\begingroup\$ Your relay has no flyback network, which is potentially an issue but not the main issue here. \$\endgroup\$
    – Hearth
    Commented Sep 26 at 15:30
  • \$\begingroup\$ @Hearth, i will add the flyback diode, but the problem here is the Mechanical Relays slow switching speed \$\endgroup\$
    – arjun kumar
    Commented Sep 26 at 15:56
  • \$\begingroup\$ @arjunkumar, can your boost converter be programmed to provide 6V instead of 5V? \$\endgroup\$
    – sai
    Commented Sep 26 at 15:58
  • \$\begingroup\$ @sai, the min(is 5v) and max(is 5.5v) voltages that the Nodemcu ESP-12E Vin pin can take without overheating AMS1117 (LDO) present on Nodemcu v1.0(overheating reduces its lifespan) and overheating will also spoils the WiFi chip functionality present on Nodemcu v1.0. the Vin- Vout voltage of AMS1117 (with 3.3v regulator output) at 800mA should be 1.3V \$\endgroup\$
    – arjun kumar
    Commented Sep 26 at 16:01

2 Answers 2

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Do use the 2-diode solution because it's an instantaneous switchover. But replace the diodes with MOSFETs for minimal power dissipation and little voltage drop.

For example, this page shows you one solution.

Two-MOSFET ideal rectifier

{Source: electronicsforu}

In practice, at 5 V, you only need Q1 and Q3 with the gates to ground. The rest of the circuit is not needed for 5 V.

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  • \$\begingroup\$ A question. What are the purpose Q2A and Q4B in general for this Circuit? \$\endgroup\$
    – Tyassin
    Commented Sep 27 at 9:23
  • \$\begingroup\$ To operate in a wide range of voltages. OP doesn't have that requirement because it's a 5 V circuit. \$\endgroup\$
    – Davide Andrea
    Commented Sep 27 at 12:50
  • \$\begingroup\$ Ok i see. So basically the two transistor could also be swapped with diode and have the same function? \$\endgroup\$
    – Tyassin
    Commented Sep 27 at 18:35
  • \$\begingroup\$ >the two transistor could also be swapped with diode. No, that's not what I meant. sorry for the confusion. What I meant was " you only need Q1 and Q3 with the gates to ground. The rest of the circuit is not needed for 5 V". I don't know how to say it more clearly than that. \$\endgroup\$
    – Davide Andrea
    Commented Oct 14 at 11:22
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    \$\begingroup\$ I simulated the circuit and can see how the matching of the transistors are important for higher inputs Thx. \$\endgroup\$
    – Tyassin
    Commented Oct 14 at 11:56
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What you require is a power mux. TPS2117 of Texas Instruments is a low cost solution you can use.

You can use this application note for further reading in detail: Basics of Power MUX

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  • \$\begingroup\$ @ Naveen Kurumaddali, thanks for your response , I have checked the price of TPS2117. It is around 1300Rs . mgsuperlabs.co.in/estore/… \$\endgroup\$
    – arjun kumar
    Commented Sep 27 at 10:56

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