# Connecting two power supplies and preventing reverse feeding either [duplicate]

Is there any way to prevent current from passing through a certain wire without actually cutting off the power from that line.

To be clear, I'm using 2 power source and because of "even with 0.0001v difference, they are going to fry each other when you connected them in parallel" rule, I used Schottky diode but the voltage drop is too much. My power source is 3.3v but with the voltage drop it went to 2.9v. Is there any work around.

Also, will this be a good way for my application https://www.instructables.com/id/Reverse-polarity-protection-for-your-circuit-with/

Based from the circuit alone, I can say that it will cut the power from the source as soon as it sense a reverse current.

I don't wanna protect my device, I just want to combine the 2 power source without frying them. (Wait that's protection) So that's it.

• Better duplicate of OR-ing power supplies (diode or mosfet), based on the OP’s real question. – DoxyLover Jun 29 at 5:24
• Is what you actually want to do: Automatically switch input between two different supplies? – hekete Jun 29 at 5:35

I'm using two power source and because of "even with 0.0001 V difference, they are going to fry each other when you connected them in parallel" rule.

This is not a rule. There are many cases when power supplies can be paralleled. These include battery, mains DC PSUs and AC transformers.

Many power supplies including batteries, simple mains rectifiers and transformers have internal resistance or, for AC, impedance. The effect of this is that the voltage will droop to some extent with increasing current draw. This in turn means that if one supply is taking the bulk of the load that it's output voltage will drop and the supplies will tend towards balance.

Figure 1. When sharing a load an exact match of power sources is not required. Image source: Loddon Valley Lions Club.

simulate this circuit – Schematic created using CircuitLab

Figure 2. Equivalent circuit of Figure 1. Each horse's internal resistance tends to balance the load sharing.

If your power supplies are tightly regulated but to different output voltages then you may have problems as the one with the higher output voltage setpoint will try to power the load and may backfeed into the second supply. Unfortunately there isn't enough information in your question to address this.

• Your horse analogy falls apart if you disconnect the plow. To make the analogy work, one horse would need to push the other horse backward. This is the dangerous condition when putting power supplies in parallel...that current from one source will attempt to flow into the other source. This can definitely be dangerous if one is a fully charged 10000mAh LiPo and the other is a 1000mAh discharged LiPo. – Elliot Alderson Jun 29 at 14:43
• Agreed. I think I covered that in the last paragraph but your comment makes it clear. – Transistor Jun 29 at 15:04
• What if I used a diode? – Khyles Gibrian Ramos Jun 29 at 16:34
• You answered that in your own question. The voltage drop is too high. – Transistor Jun 29 at 16:55