If you need to seamlessly switch between two DC power sources—like a 5V USB wall adapter and a 5V battery backup—without relying on a microcontroller or mechanical relays, a passive diode OR-ing topology is the most robust solution. When drafting a circuit diagram with diode OR-ing, the default component pick for loads under 2A is the SS34 Schottky diode. It provides a low forward voltage drop ($V_F \approx 0.55V$ at 1A) and prevents reverse current from back-feeding into your inactive power supply.
This guide walks through the exact topology, component selection, failure modes, and breadboard verification for a dual-source 5V OR-ing circuit. We will terminate with a concrete decision matrix so you know exactly which part to drop into your BOM.
Topology and Node Definitions
The OR-ing topology relies on the unidirectional current flow of diodes to automatically select the power source with the highest voltage. Here is the physical layout and node mapping for a dual-source system:
- Node A (Source 1 Anode): Connected to the positive terminal of your primary supply (e.g., 5V USB).
- Node B (Source 2 Anode): Connected to the positive terminal of your backup supply (e.g., 5V boosted Li-ion battery).
- Node C (Common Cathode / Load): The cathodes of both diodes are tied together here. This node feeds the positive input of your load.
- Common Ground (GND): The negative terminals of Source 1, Source 2, and the Load must all be tied to a single common ground bus before the diodes.
How it works: If Source A is at 5.0V and Source B is at 4.2V, Diode A becomes forward-biased, pulling Node C up to roughly 4.45V (5.0V - 0.55V $V_F$). Diode B sees 4.45V on its cathode and 4.2V on its anode. It is reverse-biased by 0.25V and blocks current. If Source A fails and drops to 0V, Diode A becomes reverse-biased. Node C drops until Diode B becomes forward-biased, seamlessly transferring the load to Source B.
Component Selection Decision Path
Not all diodes are created equal for power multiplexing. Standard silicon rectifiers (like the 1N4007) have a $V_F$ of 0.8V to 1.1V, which wastes power as heat and drops your load voltage below acceptable thresholds. Use this decision tree to select the right part number based on your continuous current draw and thermal constraints.
| Load Current | Diode Technology | Concrete Part Pick | Why This Part? |
|---|---|---|---|
| < 1.0A | Schottky (Through-hole) | 1N5819 | 40V PIV, 1A max. $V_F$ is ~0.6V at 1A. Cheap, ubiquitous in DIP kits. |
| 1.0A - 3.0A | Schottky (SMD / Axial) | SS34 (SMA/DO-214AC) | 40V PIV, 3A max. $V_F$ is ~0.55V at 3A. Excellent thermal mass for the footprint. |
| 3.0A - 5.0A | Ultra-Low $V_F$ Schottky | SS56 | 60V PIV, 5A max. Lower thermal runaway risk at higher ambient temps. |
| > 5.0A or High Efficiency | Ideal Diode Controller IC | LTC4359 + N-FET | Replaces the diode drop with a MOSFET $R_{DS(on)}$ drop (~20mV). Eliminates heat. |
The Default Pick: For standard hobbyist and light-industrial 5V/12V applications drawing under 2A, buy the SS34. It offers the best balance of low forward voltage, surge current handling (up to 80A non-repetitive), and physical size. If your load exceeds 3A or you cannot tolerate a 0.55V drop, abandon passive diodes and switch to an Analog Devices Ideal Diode Controller driving an external N-channel MOSFET.
Dynamic Behavior and Extreme Failure Modes
To design a reliable circuit, you must understand what happens when components degrade or fail. The table below maps the behavior of Node C and the current flow under normal and extreme fault conditions, assuming a 5V Source A, 5V Source B, and an SS34 diode pair.
| Condition / Fault | Node C Voltage | Current Flow Path | Physical Result & Hazard |
|---|---|---|---|
| Normal (Both Sources Active) | ~4.45V | Source A supplies 100% of load. Source B is blocked. | Diode A dissipates $P = 0.55V \times I_{load}$. Needs airflow if $I > 1.5A$. |
| Source A Drops to 0V | ~4.45V | Source B takes over instantly. Diode A is reverse-biased by 5V. | Seamless switchover. Load sees a momentary microsecond droop. |
| Diode A Shorts (Catastrophic) | 5.0V | Source A and Source B are now hard-tied together. | HAZARD: The higher voltage source will back-feed the lower one. If Source B is a battery, it may overcharge and vent. |
| Diode A Opens | 4.45V (from B) | Source B supplies the load permanently. | Loss of redundancy. System continues to run, but primary source is ignored. |
| Source B Reversed (Wired backward) | ~4.45V (from A) | Diode B is heavily reverse-biased by Source A + Source B voltage. | If $V_A + V_B$ exceeds the 40V PIV rating of the SS34, Diode B avalanches and shorts. |
Step-by-Step Breadboard Verification Protocol
Do not solder this circuit until you have validated the switchover behavior and voltage drops on a breadboard. You will need two adjustable bench power supplies, a digital multimeter (DMM), an SS34 diode, and a dummy load (like a 10Ω 5W power resistor).
- Verify Diode Orientation: Set your DMM to diode test mode. Place the red probe on the anode and black on the cathode of the SS34. You should read ~0.300V to 0.400V. Reverse the probes; it should read 'OL' (Open Loop). Wire the diode into the breadboard with the silver band (cathode) pointing toward the load rail.
- Test Single-Source Baseline: Connect Power Supply 1 to Node A. Set it to 5.00V. Leave Node B unconnected. Connect the 10Ω dummy load to Node C and GND. Measure Node C with your DMM. It should read approximately 4.45V. Calculate the load current: $I = 4.45V / 10\Omega = 445mA$.
- Introduce the Backup Source: Connect Power Supply 2 to Node B. Set it to 4.80V (simulating a slightly depleted battery). Measure Node C. It should remain at ~4.45V. Measure the voltage across Diode B (Anode to Cathode). Your DMM should read -0.35V (4.45V cathode minus 4.80V anode), confirming Diode B is safely reverse-biased.
- Simulate Primary Failure: While monitoring Node C on the DMM, turn off Power Supply 1. Node C should instantly drop to ~4.25V (4.80V from Source B minus the 0.55V drop of Diode B). If you have an oscilloscope, probe Node C during this transition; you should see a voltage droop lasting less than 5 microseconds before the backup source clamps it.
- Thermal Audit: Increase Power Supply 1 to 5.0V and drop the load resistance to 2.5Ω to pull 2A. Let it run for 3 minutes. Carefully touch the SS34 plastic body. If it is too hot to hold (exceeding 60°C), you must add a heatsink, use a larger package (like a D2PAK), or switch to an active MOSFET OR-ing controller.
Why Passive OR-ing Wins (and When to Upgrade)
When evaluating power multiplexing for a PCB or enclosure, you generally have three choices. Here is how the passive Schottky circuit diagram with diode components compares to the alternatives.
| Criteria | Passive Schottky OR-ing (SS34) | Mechanical Relay Switchover | Active Ideal Diode IC (LTC4359) |
|---|---|---|---|
| Switchover Speed | Instantaneous (< 1µs) | Slow (5ms - 15ms bounce) | Fast (~10µs to 50µs) |
| Voltage Drop | High (0.4V - 0.6V) | Negligible (Contact resistance only) | Ultra-Low (~20mV) |
| Quiescent Current | Zero (excluding reverse leakage) | High (Coil draw: 30mA - 100mA) | Low (IC quiescent: ~1mA) |
| BOM Cost (1k qty) | $0.15 (Two diodes) | $1.50 - $3.00 (Relay + driver BJT) | $3.50 - $5.00 (IC + 2x FETs) |
| Best Use Case | Wide-input loads (buck converters, 12V systems) | High-current AC mains or 12V/24V auto | Strict 5V/3.3V logic, high-current battery backups |
The Verdict: Choose the passive Schottky topology when your load can tolerate a 0.5V drop and your current is under 3A. It requires no control logic, has no moving parts to wear out, and costs pennies. However, if you are OR-ing 3.3V rails where a 0.5V drop would cause a microcontroller brownout, or if you are pushing 10A through a server backup node, the $I^2R$ heat and voltage loss of a Schottky diode become unacceptable. In those extremes, upgrade to an active Texas Instruments Power Multiplexer or an Ideal Diode Controller.
By selecting the correct Schottky part number, bonding your grounds properly, and verifying the reverse-bias leakage on the bench, your diode OR-ing circuit will provide years of silent, fail-safe power redundancy.
