OR-ing Circuit Notes

First Published Oct 2017, R. Kwas 


An OR-ing circuit is a circuit where two inputs combine to activate a single function.  Sometimes, this can be as simple as tying two switches to activate a light, but sometimes it gets more complicated as all (normal) operating conditions (as well as possible failure conditions) must be carefully considered and accommodated...it's called engineering! 

Readers of the SW-EM site will have run into several places where this kind of circuit may occur in a vintage Volvo...either from the factory, or in later retrofits or modifications.  In this article, the OR-ing circuit is considered more closely, because sometimes, simply tying the two activating functions together can bring with it some unwanted conditions...that's where OR-ing diodes come in... 


Samples of OR-ing circuits: 

Courtesy Lighting ("Roof - Lamp" on 122 Wiring Diargam):  Opening Either the Right OR Left Door OR switching the manual Switch applies power to the Light.

Excerpt of the 122 Wiring Diagram.  Door switches are connected together at Junction Unit Terminal 1.  Closing either one will complete circuit and allow current to flow through Lamp.  This is the simplest ORing circuit and no blocking diodes are necessary.  


Brake Indicator on late Amazons and other models with dual Brake Systems:  Either a difference in the pressure of the two Brake Systems OR Handbrake away from its rest-position causes the Light to be activated...no blocking diodes required here either! 

Brake Indicator on an Amazon, located 45mm above Wiper Switch. 
Picture by Börnaut of the Volvoniacs Forum used with thanks and his permission.


Electric Cooling Fan control.  Either the Temperature Sensor OR the Manual Dashboard Switch can apply power to operate the Fan, but as a relay is doing the power switching, and it is controlled by chassis switching, no OR-ing diodes are necessary here either!  (The diodes shown in the circuit are strictly Snubbing Diodes).  LINK to:  Cooling System Electric Cooling Fan


So why and when do OR-ing Diodes need to be used?  In the above simple examples, no further components were necessary because the two inputs completed the circuit to negative.  When the two circuits supply power however it is not simply enough to tie the two inputs together, because that would tie the two circuits together by way of the indicator and cause them to interact.  This explanation is somewhat simplified.  A complete theoretical explanation would be more involved, and perhaps not as clear as studying some more practical examples.

In the example below, an indicator is intended to show that either of two functions are powered.  Simply tying both of those functions together at the indicator means that activating one function (Load 1) would inadvertently activate the other function (Load 2) as shown below, center, through a "sneak" current path because of the connection at the indicator.  The way to prevent this is by use of the OR-ing (also Blocking) Diodes.  These allow either function to activate the indicator while still keeping the two functions under separate control and from interacting. 

Requirement:  One Indicator for two Loads.  Simply connecting the two loads to the indicator is not enough!  ORing diodes need to be added to block the sneak current path. 


Another practical Example where ORing Diodes become necessary:  Power to Electronic Ignition Module:  Either Ignition Power terminal (54) of Ign Sw OR Solenoid power (50) must supply power during Starter cranking when term 54 is depowered.  At least one Blocking Diode is required here (the second one is not absolutely necessary).  LINK to:  Ignition Modifications 


LINKS:  An OR-ing Diode and a Snubbing Diode may be the same component, but they are used in very different ways.  See also:  https://en.wikipedia.org/wiki/Flyback_diode


External material sources are attributed.  Otherwise, this article is Copyright © 2017.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the unique metaphor and probably (likely) wise-a** comment. 


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