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I am building a battery charging box for my son's RC batteries. I have a 1000W 24V Meanwell power supply to supply input voltage to a pair of DC chargers. Sometimes we don't have access to AC but do have access to 24V DC. I would like to add a port to his box so that I can take the 24V DC input and use it in place of the power supply. I thought about putting a relay in and triggering it off the AC power supply so that if the AC power supply was on it would switch from the DC input to the power supply. Is there a simpler way? Ideally, I would like the option to use the DC input as a DC output for someone else's DC charger if I was on AC power. But this is not a requirement.

Thanks!

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  • \$\begingroup\$ If your "access to 24V DC" is actually to a battery, note that it may be more like 28V, so ensure everything else can tolerate that. \$\endgroup\$
    – user16324
    Oct 17, 2021 at 19:22
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    \$\begingroup\$ @user_1818839 The chargers will be fine. They are designed to run off said batteries. \$\endgroup\$
    – DKNguyen
    Oct 17, 2021 at 19:30

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If you know that no one will try connecting both power sources at the same time and you know the Meanwell can handle voltage on its output when it has no power input then the simplest way is to just connect the two lines together directly.

If you cannot guarantee this then the "simpler" way is to use a diode. Just connect the Meanwell output to the DC port through a diode. But at 40A, you will be dissipating 28-40W under maximum load and will require something like a stud diode screwed into a heatsink, possibly with a fan.

enter image description here

From Vishay

I would probably go with the relay to be honest. It's a good approach. Then you can skip the heatsink.

Ideally, I would like the option to use the DC input as a DC output for someone else's DC charger if I was on AC power. But this is not a requirement.

Your proposed solution already does this (so long as you use the relay only connect/disconnect the Meanwell output and the direct DC port is always connected to the charger input. The SINGLE diode solution described here does that too, as does the direct connection solution described.

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  • \$\begingroup\$ Can you help me understand how a relay would accomplish the secondary goal of using the internal power supply as an output? Maybe I am thinking about it wrong but if the relay was switched to the power supply as an input wouldn't the DC input connection be disconnected? This is the relay I was considering: citrelay.com/view_relay.php?series=A2H \$\endgroup\$ Oct 17, 2021 at 19:51
  • \$\begingroup\$ Well I suppose it depends on whether you're using the relay to literally swap between Meanwell output and direct DC input (relay with NC/NO terminals), or whether you're just using it to connect/disconnect the Meanwell output and the direct DC port is always connected to the charger inputs (relay with just a NO terminal). The relay in that case is just to stop direct DC input from backflowing through sneak circuits in an unpowered Meanwell. \$\endgroup\$
    – DKNguyen
    Oct 17, 2021 at 20:00
  • \$\begingroup\$ That relay does not have an AC coil. Though I suppose you could get the Meanwell to energize the coil instead instead of having AC energize it directly. Your choice. \$\endgroup\$
    – DKNguyen
    Oct 17, 2021 at 20:06
  • \$\begingroup\$ Thanks @DKNguyen that makes total sense. I didn't think about connecting it that way. I really appreciate your help on this. I felt a relay was the right answer I was just struggling to figure out how to connect the DC input. I really appreciate your help! \$\endgroup\$ Oct 17, 2021 at 20:15

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