I am trying to setup a schottky diode oring circuit to switch between my battery and a dc power supply.

I am dealing with some pretty high amperages and a liPo battery, so I need to make sure I get this right or else somthing could go boom. The circuit I am trying to create will be pulling about 20A max from my 11.1v nominal voltage battery and obviously around 10A from my 24v wall supply (if necessary I also have a 12v wall supply that I could use). The LiPo battery charger will hopefully be included in the circuit along with an extra diode to charge the battery.

Iwill attach an image of what I have drawn up, but please note I have absolutely no electrical background so my drawings might be completely off (just note that the triangle things are diodes)

enter image description here


simulate this circuit – Schematic created using CircuitLab

Charger schematic redrawn by kindly neighbour. Note drawing convention of higher voltages at top with general flow from left to right.

  • \$\begingroup\$ I should have said this originally, but I have already bought some schottky diodes that I was planning on implementing: amazon.com/MBR30100CT-Terminals-Schottky-Barrier-Diode/dp/… \$\endgroup\$ – Laessen Apr 20 '16 at 19:13
  • \$\begingroup\$ I also just realized that in this drawing, the voltage drop from the diode after the charger may affect the charging capability, so it may be impossible to use the charger in this circuit. \$\endgroup\$ – Laessen Apr 20 '16 at 19:19
  • 1
    \$\begingroup\$ I've taken the liberty of clarifying your question with the addition of a schematic using the built-in schematic editor. D2 may be redundant. It prevents current feeding back into the charger when it's switched off but it should have that protection internally. \$\endgroup\$ – Transistor Apr 20 '16 at 23:17


simulate this circuit – Schematic created using CircuitLab

Figure 1. Relay voltage source selector.

You might find this circuit less scary than MOSFET switches.

  • A relay with a 24 V coil monitors the 24 V supply.
  • When 24 V supply is present the relay is energised and 24 V is fed to the output.
  • When the 24 V supply fails the relay drops out D1 powers the load from the battery for a couple of hundred milliseconds while the relay switches over and connects the battery directly to the output. D1 needs to be able to handle the maximum current for this short duration.
  • There is no power loss across the contacts. There is some power consumed by the relay coil.

Select a relay with contacts rated for your maximum current and with a DC rating. (AC is easier to switch because the current falls to zero at each zero-cross whereas with DC it is usually continuous.) You have plenty of choice for 24 V relays as they are very widely used in industrial systems but also used in trucks and buses with 24 V electrical systems.

  • \$\begingroup\$ How about adding a Schottky diode in parallel with the relay NO contact and Armature? This keeps the circuit powered while the relay is switching. (cathode to armature). \$\endgroup\$ – Dwayne Reid Apr 21 '16 at 1:44
  • \$\begingroup\$ I actually did build a similiar circuit using a relay such as that, but the switching delay was far too long for my application. \$\endgroup\$ – Laessen Apr 21 '16 at 4:50
  • \$\begingroup\$ Ah, but you didn't tell us your application, did you? ;^) I've added D1 as per comment of @DwayneReid. \$\endgroup\$ – Transistor Apr 21 '16 at 6:16
  • \$\begingroup\$ @transistor Thanks, this solved my problem. I might not even have to buy a new relay! This also solves the problem of voltage drop across diodes, which is very useful to me. I can also use the 12v powersupply I already have rather than buy one with higher voltage to make the diodes work. Thanks again! \$\endgroup\$ – Laessen Apr 21 '16 at 17:39

With those currents you probably want to be using MOSFET-based ideal diodes so that something doesn't melt down. Additionally, using the circuit in AN1149 will ensure that you don't end up trying to pull 30A from your charger.

  • \$\begingroup\$ I am a complete novice, could you link me to components that I would need to buy to do this? \$\endgroup\$ – Laessen Apr 20 '16 at 19:05
  • \$\begingroup\$ Have you had a chance to go over the contents of the links? \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 20 '16 at 19:06
  • \$\begingroup\$ Yea, even those seem intimidating to me. Two diodes in a system is about all I can handle, mosfets are a foreign thing to me. Like I said in the OP I have verrrry limited experience in reading electrical schematics. \$\endgroup\$ – Laessen Apr 20 '16 at 19:07
  • \$\begingroup\$ Also, since the voltage coming out of the charger would be a lower voltage than the powersupply, shouldn't it draw from the powersupply? Isn't that how this setup works in the first place? \$\endgroup\$ – Laessen Apr 20 '16 at 19:08

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