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I've found this combination in many devices—for example, power supply redundancy modules, Ethernet-powered devices (PD of PoE), and RTC in circuits. In this circuit, the voltage source with a larger voltage only supplies current, but the other does not. Can anyone explain why?

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    \$\begingroup\$ If you add a voltage probe at the common node (left side of R1), you will see that the voltage there is 23.3V. So D2 is in the reverse bias region (cathode more positive than anode). Current will be negligible. If you put in "real" diode models, you may see a small amount of reverse current flow here. Things get more interesting if the difference between the input voltages are closer together: say 24 and 24.1V. With ideal diodes, all the current will flow through one diode. With actual diodes, more current will flow through the one with a higher voltage. \$\endgroup\$
    – Troutdog
    Commented Apr 12, 2023 at 14:24
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    \$\begingroup\$ Not “the diode” (singular) but rather two diodes, here in common cathode configuration or when speaking of multiple voltage sources, OR configuration. \$\endgroup\$
    – winny
    Commented Apr 12, 2023 at 15:36

4 Answers 4

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Let's say we have 2 ideal diodes, to make it easier to understand:

schematic

simulate this circuit – Schematic created using CircuitLab

You know that a diode passes current when its anode is positive relative to its cathode, and it blocks current when its cathode is more positive than its anode.
In this ideal case, if only the 23V source is turned on, we will have +23V where the 2 diodes are tied together (at their cathodes).
But when we turn the 24V source on, the +24V passes through the diode D1 because its anode is at +24V while its cathode is at +23V, and the +24V appears on its other end.
This will make the D2 reverse polarized and it will block and prevent the +24V passing through it towards the +23V side.

In the case of real world diodes, there will be a small voltage drop across them, but the higher potential side will be more positive and thus its diode will conduct.
Below is the same circuit with 2 separate loads (and real world diodes) to make it easier to see what is going on:

schematic

simulate this circuit

As you can see, the 24V side will have a potential higher by 1V relative to the +23V side, which would make the diode D2 reverse polarized and thus blocked if we connect their cathodes together.
In order for the D2 diode to conduct, its cathode would have to be at about 0.7V lower potential than its anode, which is 23-0.7=22.3V, which would happen when the +24V side drops to about +23V, in which case ideally equal currents would flow through both diodes. Any lower than that, and the D1 will keep going into a turn-off state sharply.

This kind of a circuit is useful, for example, in the case of a line adapter supplying a circuit with a backup battery. As long as there is line power coming in, there is no current going from the battery, but as soon as the adapter voltage drops below the battery voltage, the battery will automatically start supplying all the power.
Except for the small voltage drop across the diode(s), this is the simplest and most effective backup power circuit, which also has practically zero delay in switching between the two power sources.

This kind of a circuit is also useful in similar situations where there are two or more redundant or spread out power supplies on the line to both reduce the voltage drop between supplies and loads (if placed on different ends of the line) and to ensure the power remaining on if one of the supplies fails.

P.S.: I've used standard silicon rectifier diodes as a practical example. Different types of diodes will have different forward voltage drops, for example about 0.3-0.4V for a Schottky diode vs. about 0.7V for a standard rectifier diode.

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  • \$\begingroup\$ Nice story about the famous diode configuration... \$\endgroup\$ Commented Apr 12, 2023 at 15:55
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Because the diode 'blocks' it, think about it. On D2 you have 24V on the blocking side and 23V on the other side (or ~23.3V for a non ideal regular diode), so the current wants to go from high to low voltage, but the diode blocks the current.

In reality there is a bit of leakage current, but it is very small and negligible in most applications

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It's called diode OR-ing or auctioneered power. If the voltage at the "central node" between the cathodes of the diodes and the resistor is higher than the voltage at the anode of a diode, it will be in reverse bias and no current will flow.

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I will add a few more ideas to the other answers to expand on this diode configuration.

Basic idea

Connecting diodes in series to voltage sources eliminates conflicts between them. This makes them unidirectional, preventing current from being passed through them in the opposite direction.

Diode as a "comparator switch"

In these applications, diodes work as "comparator switches" that close when the voltage drop across them is positive and open when it is zero or negative. This can be illustrated by the circuit of an "ideal" diode made by an op-amp driving a transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

As you can see, the circuit is very intuitive and gives a functional idea of what the diode does when we apply a positive or negative voltage to it. We can make sure that it is almost perfect from the graphs below of a DC sweep simulation...

Ideal diode DC sweep

... and time domain simulation.

Ideal diode sin_1

Ideal diode sin_2

We also see that the diode switch cannot be absolutely perfect (VF = 0 V) because it will not work; it needs some, albeit very small, voltage difference across its terminals to be able to switch.

More applications

Below I have added more applications of this famous diode configuration. Since my schematics are conceptual, I have used ideal diodes.

Maximum voltage selector

If we connect more diodes and voltage sources with different voltages, the highest voltage will appear at the output.

schematic

simulate this circuit

Diode OR gate

Here, the input voltages have only two values - 0 V and 5 V. It is enough that only one of the sources has produced a high voltage (5 V or connected to Vs), and it appears at the output.

schematic

simulate this circuit

Diode AND gate

The dual diode gate works as a "minimum voltage selector" - it is enough that only one of the input sources has produced a low voltage (0 V or connected to ground), and it appears at the output. But here there is a peculiarity - because the diodes are reversed but the input voltages are positive, something has to provide the high output voltage. For this purpose, we connect a "pull-up" resistor R to an additional 5 V voltage source.

schematic

simulate this circuit

Diode AND as OR gate

A typical application of the diode AND gate acting as an OR gate with active zero voltage can be seen in the input circuits of car security systems. Diodes isolate the inputs by preventing them from influencing each other.

schematic

simulate this circuit

It is enough to open a door, front engine cover or rear trunk, and a signal with an active zero level is applied to the input of the alarm system (but the other lamps will be not turned on).

Isolating diodes

Diodes play a similar role in the outputs of the alarm systems controlling the turn signals and side lights of the cars. They call them all kinds of names - isolating, blocking, separating, decoupling, etc.

schematic

simulate this circuit

Switching without diodes

Sometimes some of the diodes are not needed.

Diode rectifier:

For example, the rectifier below has diodes (D3 and D4) that block the battery current; therefore a diode is needed only for the backup battery.

schematic

simulate this circuit

Try the simulation for two values of the input AC voltage - 10 V (there is a mains supply) and 0 V (there is no mains supply).

Emitter-coupled pair:

In this configuration, two "voltage sources" - the emitter followers Q1 and Q2, fight to drive the common load R3. But there is no need for isolation diodes because the "pull-up" transistors can only source current.

schematic

simulate this circuit

Try the simulation for two values of the input voltage V1 - 100 mV (Q1 supplies R3) and -100 mV (Q2 supplies R3).

Diode switching

Finally, in addition to switching voltage sources, diodes can switch themselves. The rule is similar:

If we connect two or more diodes with different voltages in parallel, the current will flow through the diode with the lowest voltage; the other will simply be turned off.

schematic

simulate this circuit

This effect can be used to switch LEDs, Zener diodes and others, using simple SPST (ON/OFF) switches instead the more sophisticated SPDT ones.

schematic

simulate this circuit

Generalization

A relationship between the first and last configuration can be seen. In the first, strings of voltage sources with different voltages and diodes with the same VF in series, are connected in parallel; in the latter, only diodes with different VF are connected in parallel. Connecting a voltage source (V) in series with a diode (VF) can be seen as artificially increasing its VF to V + VF (like a Zener diode).

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  • \$\begingroup\$ The circuit with Mains is a nice application, but instead of 1 kHz I would rather think of 50 Hz (or 60 Hz). What is a diode of 1V, or 3V? How to get a reading of 1.000V, that is an unrealistic accuracy. A voltage source of 0 V is just a short circuit, or a straight copper wire, so two of your diagrams are identical. Nice elaboration, but I prefer the short answer of Spike \$\endgroup\$
    – Roland
    Commented Apr 13, 2023 at 11:07
  • \$\begingroup\$ @Roland, I take your comments and explain. These are conceptual diagrams with selected convenient values (1 V, 1 mA, 1к...) without details. About the mains frequency, yes... I can fix it. 1.000 V is what the simulator shows me and I don't know how to rough it; it annoys me too. I am a practical person but for the purpose of an intuitive explanation I have simplified quite a few things; for example the elements are not specifically labeled and their values are not always standard. My goal is to show the general idea, not the specific implementation. \$\endgroup\$ Commented Apr 13, 2023 at 12:11
  • \$\begingroup\$ Also, I agree that a voltage source with zero voltage is just a piece of wire but I suppose that readers will experiment with the simulation by changing the voltage. \$\endgroup\$ Commented Apr 13, 2023 at 12:48

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