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Suppose I have a circuit I want to power, nominally 6V DC, 500 mA max. The circuit could be modelled as purely resistive (an incandescent bulb, for example).

Let Vx and Vy be two different voltage sources. Each of these sources might oscillate around 6.2V and 0V (just an hypothetical value, meaning "slightly above nominal, but not much").

I would like to drive current only from the highest voltage source at every time, while turning the other off. The switching between them should be transparent to the powered circuit, so as it doesn't get voltage drops.

I have seen similar problems being solved with two diodes, but my proposed circuit could not afford the voltage drop, since most the time the max available voltage would be very near 6V or slightly below it.

I have thought about using an operatioal amplfier, perhaps combined with, say, some Mosfet(s), but I don't know how exactly that would be done, or if this problem has some better solution.

Also I would like a simple solution with "a handful of readily available generic components" instead of some pre-made module.

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    \$\begingroup\$ (1) What is the output impedance of your two power sources. If it is not zero then switching from one supply to the other will cause voltage droop and may cause switching back and forward between the two. (2) What are you really trying to do? \$\endgroup\$ – Transistor Mar 12 '16 at 17:19
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    \$\begingroup\$ It would be good to quantify the allowed voltage drop. Because there are ultra-low forward voltage schottky diodes with only 160mV forward voltage drops (like SPV1512), and ultra-low dropout voltage regulators (e.g. NCP4641, 200mV dropout at 5.5V). With those you might still be able to realize the diode+regulator solution, simpler and cheaper than the alternatives. \$\endgroup\$ – DerManu Mar 12 '16 at 21:10
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Higher voltage power selector.

A relay offers a means of selecting the power source with very low voltage drop across its contacts.

  • The comparitor compares V1 and V2 and will energise the relay if V2 > V1. The relay contacts change over and V2 is passed straight through to the output.
  • R6 provides a little hysteresis to prevent rapid switching on minor changes in voltage. Increase the value of R6 to reduce the hysteresis.
  • You may wish to add 100 uF or so to the output to prevent dips during relay switching.

The LM324 comparitor should do the trick.

enter image description here

Figure 2. Quad relay board.

A single relay version of these 5 V relay boards sold for Arduino, Raspberry Pi, etc., expansion should do the trick. Some have the transistor and base resistor or an opto-isolator on board. Tons available on eBay. Alternatively a 5 V relay will do the trick.

I forgot D3 to protect Q1 from inductive spikes when switching off the relay. Added now.

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  • \$\begingroup\$ Do you think a flyback diode might be advised across the RLY1 input, or am I missing something? \$\endgroup\$ – DerManu Mar 12 '16 at 18:15
  • \$\begingroup\$ Thanks for your great answer! Pardon my ignorance, but I think a relay is something I would like to avoid for what I'm doing. Do you think I could replace the relay by two Mosfets or Transistors, so that level changes at R5 output would "toggle" which Mosfet/Transistor is active? I believe that, for this to happen, Q1 would have to be replaced by logic gates somehow. What do you think? \$\endgroup\$ – heltonbiker Mar 12 '16 at 18:42
  • \$\begingroup\$ Also, if you might suggest some specific component for the Comparator, and some (small) relay, I would be glad, since I have never used any of these components yet. \$\endgroup\$ – heltonbiker Mar 12 '16 at 18:43
  • \$\begingroup\$ You could replace with MOSFETs. You'd need an inverter to make one work opposite to the other. Comparitor part number added and suggestion for relay if you want to follow up on that. \$\endgroup\$ – Transistor Mar 12 '16 at 19:28
  • \$\begingroup\$ @DerManu: Yes, I forgot the flyback diode. Added now. Thanks. \$\endgroup\$ – Transistor Mar 12 '16 at 19:28

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