High-Side Switching vs. Low-Side Switching

Nov 2022 R. Kwas  (Comments Added)

High-Side Switching
Troubleshooting a High-Side Switched Circuit
Low-Side Switching
Troubleshooting a Low-Side Switched Circuit
Real World Circuits

Related Links 
   
"H" Bridge for DC Motor Direction Reversal
    Related Practical Application
    Circuit Examples
   
Reference Information
    Chassis to Engine Connection
   
Courtesy Lights are Low-Side Switched
   
Courtesy Light Fader Circuit

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Some questions came up recently which called for the explanation of the difference between High-Side and Low-Side Switching, so here is a quick look at the different manners power can be applied to a load, and controlled. 

Recalling that to energize a load, there must be a complete circuit between it and the power source (on both power [high] side, as well as negative [low] side, such that all the voltage of the power source will be "dropped across" the Load, thereby allowing the power source to drive current through the Load...Voltage is the potential to drive Current, but Current actually flowing is what actually does the work.  This is the explanation is at its simplest...naturally it gets more complicated once we implement the theory practically and on an actual vehicle...because manufacturers/we can play tricks when we take into account the relative location of Power Source, Switch, and Load, and manufacturers/we can also play cute tricks to eliminate a wire, and make it less expensive/simpler to produce if we are the manufacturer, or implement if we are an owner simply wiring one electrical circuit. 

In a car, with its metal body/chassis for instance, it was realized early on, that the conductive body could be used to "save a wire", so we often will find the chassis involved in the circuit...and in Real World Circuits in a car we may also find other connections, and multiple switches...  All of these factors above and beyond a simple circuit can make it tricky when troubleshooting a non-(or worse - whacky!) functioning circuit ...and the more connections we put in a current path, the more opportunities for Voltage Drops somewhere other than the load where ideally it is the only place we would like the source voltage dropped...  See also: Voltage Drops Cause Inadvertent Circuit Interactions! 

 

High-Side Switching:  

This is the simplest way power can be switched...and the way it is often implemented...the Switch has power applied, and based on if the switch is "Closed" or "Open", sends that power along to the Load.  As noted before, we must also have a connection to the negative Source side back to the Battery...this can also be referred to as the "Return Current Path", and note that it can be, and/or include any conductive material, from a copper wire, to the metal parts of the vehicle...including the screws of Fuel Tank or the Taillight Assembly !

An example of High-Side Switching are the Lights (Headlights and also Brakelights).  The associated Switch is supplied by power, and when it is closed, it enables the current path to the Lights, which have their low side tied to chassis for the Return Current Path.

 


High Side Switching.  Switch is in series with power, and thus the Load Current path.

 

Troubleshooting a High-Side Switched Circuit: 

The Voltages we might measure are shown:


Using a Voltmeter to measure circuit Voltages to troubleshoot a non-working Load.

First test we might make is at Battery to see what the Voltage is.  Next we can measure V at fuse input and output to see if we get power at the load when Sw is closed...and so on, all the way to the Load.  Meter Neg can be connected to any handy Batt Neg point including chassis near where other tests are being performed...

 

Low-Side Switching:  

If controlling the Current flowing in the Load is the way we can control the Load, it stands to reason that we can also locate the Switch on the "Low-Side" of the Load, and simply allow or disallow the chassis "Return Current Path".  By doing this, we can control the Load just as well as we could with the Switch in the High-Side, and by again using the Chassis as the conductor of the Return Current, have again saved a wire! 

A simple example of a Low-Side Switch implementation, is the Oil Pressure Indicator Light.  Indicator itself is supplied with (Ign) power at the Dashboard, and the single wire which runs off to the Pressure Switch at engine, can thus control the Indicator by simply opening or closing a contact as a function of engine oil pressure. 

Another example of Low-Side Switching are the Courtesy Lights of many cars, including the 122 and 1800 models.  The switches mounted in the frame of the door simply complete the current path to chassis.

 


Low-Side Switching. 
Switch is also in series with the Load Current, but in this case, the Return Path...we could really have it electrically in any number of places in this path! 

 

Troubleshooting a Low-Side Switched Circuit: 

With a Low-Side switched circuit, when the switch is open, we will measure V even at the low side of the Load.  Measuring V alone is not a sign that all is well! 

Not maybe having the wiring diagram at hand, we might not even be aware that a circuit is Low-Side Switched, but the first hint that a circuit was Low-Side Switched would be that power is measured at both Load terminals, but the Load is still not functioning as expected...this is not a fault condition!  The fact that power is on both sides of the Load should not be overlooked, but remember that power must be applied across the Load for it to function.  Power on both sides of for instance a lamp, means that it will NOT light (this might/should sound familiar to some SW-EM site readers:  Reference:  http://www.sw-em.com/AMP_Indicator_ON.htm  ).  Measuring this condition is also an immediate hint that the switch (or somewhere else in the low-side return path) is open! 


In a Low-Side Switched Circuit, Voltage measured on both sides of the Load, when Switch is Open and Load is un-powered, will be 12V!  This may be confusing to a troubleshooter inexperienced with such a circuit.  Explanation:  It is Voltage across the Load which causes Current to flow...with Switch open, Voltage may be on both sides of the Load, but no Current is flowing, so the Load will be unpowered and inactive!

 


Low-Side Switching, with a low-side Fault, and resulting Voltage measurements, added.  When the switch is closed, we still measure 12V on both sides, and the Load is non-functional!  If we know this to be a Low-Side Switched circuit, we can immediately suspect an open connection to chassis!  

 

Another example of a Low-Side Switched circuit is the Ignition!  Here the switch is the "Points" and the Load is the Ign Coil Primary.   When using a test light between chassis and the Points node, the light will be OFF when Points are closed (NO Voltage at node!), and ON when they open (Voltage at node!)  See also:  Link to Ignition Tech Article

 

Real World Circuits: 

In the Real World, circuits can have multiple switches and connections points, all which can be a potential location for problems in the current path (such as opens, or poor connections)...for instance, a damaged wire, hardware or terminal looseness, corrosion caused Resistance, all which we must watch for and take into account when troubleshooting!

 


The Flex-connections referenced in the graphic are Chassis to Engine and insulated-mounted Wiper Motor examples(shown below!). 

 

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Related Links: 

"H" Bridge for DC Motor Direction Reversal:

By combining two High-Side and two Low-Side Switches, an "H" Bridge circuit can be made.  This circuit is particularly useful with a DC motor in that is can be used to effectively change the polarity in which power is applied to the motor, thereby changing its direction of rotation (notice the motor is not connected to ground in order to allow this circuit to do its magic!).  This circuit might be employed for instance, for an electric window lift. 


"H" Bridge circuit gets its name from the way wiring is often drawn.  Graphic source:  https://www.engineersgarage.com/dc-motor-control-using-h-bridge/  Marked up!

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Related Practical Application:  

Do the Voltage measurements here look familiar to the reader? ...they should!  Dropping Resistors have been added into the current path of High-Side Switched 6V Loads for the purpose of dropping half the applied Voltage, and thus allowing the 6V rated Loads of a PV444 to be powered by 12V!  Electrical Kung-Fu at its finest!

Link to:  Dropping Resistors in the Tech Article:  Vintage Volvo 6V to 12V Conversion

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Circuit Examples: 

High-Side Switching:

Windshield Wipers: 


122 Wiring Diagram Excerpt showing it is a High-Side Switched circuit.  I've added the Flex-connections in the negative Current Return Path, which accommodates the soft-mounted Windshield Motor.  Without this link, there will be no current in the Wiper Motor!  In this situation, the Flex Connection serves to hop current across the non-conductive soft-mounts!
 

I recall troubleshooting a Wiper function failure after repairs to address "Wipers Parking on the Wrong Side"... [Wipers parking on wrong side-email exchange] and an intermediate finding was measuring 12V on all the Motor wires, (including including the Motor Housing, which should have been connected to chassis!) just as shown above inTroubleshooting a Low-Side Switched Circuit  ...indeed, closer inspection revealed the cause to be the flex connection to the Wiper Motor was loose and electrically open...just as if we had made the circuit a Low-Side Switched circuit!  


Flexible conductive link at Wiper Motor Assembly hops across non-conductive rubber motor-mount to complete low-side current path to chassis.
See Link for further details.

Low-Side Switching: 

Excerpt from an e-mail response to a question about DoorJamb Switches:

"Door contact are mounted in chassis sheetmetal in doorjamb, so they make a contact to chassis, so the Courtesy Light gets power on one side, and the and other side runs off to the Doorjamb switches (and also any other additional switches which can supply chassis and thereby energize the lamp).  On 1800s this is uniquely done with a "local" contact at the fixture...by lifting the Lens, the contact to chassis is made and Courtesy Light lights!  ...and there are variations!  See also: http://www.sw-em.com/Courtesy_Light_1800_Notes.htm

The 122 is much simpler of course...Fuse 4 supplies the fixture lamp, and the switch at the fixture either provides chassis connection OR lets the Doorjamb switches do this.  See: http://www.sw-em.com/Wiring_Diagrams_and_Related.htm#122_Wiring_Diagram_Excerpts

In both cases, it's called "Low-side switching" in other words, the switch is not supplying power, but it is completing the circuit to chassis, thereby completing the current path [a no less important part of the circuit!].  Switches don't always have to switch power...they can also switch the "low side"!  This is particularly handy, like in this case, when the switch is mounted in the sheetmetal of the car *which we know is also Batt Neg...the switch can simply make the connection to chassis, and done!  ...and with only ONE stinkin' wire!  ...now that is an efficient use of wire too! "

 

Reference Information: 

Chassis to Engine Connection

The flex connection between chassis and engine is important, and must meet several special requirements!  It must be capable of carrying high (starting) and Batt (charging) currents, as well not deteriorating, spending its service-life, hopping between the steady chassis and vibrating engine.  A flat braided cable with many fine conductor strands fulfills these requirements perfectly! 

Picture of absolute first quality restoration work as is typical, by George A. Minassian in Australia, and used with his kind permission.  (View forward along exhaust downpipe, from below an 1800).  The braided cable is clearly evident, tying chassis and Engine/Bell Housing together, ready to pass high current and not mind constant vibration!  A little loop holds it clear of the exhaust. 

 

Courtesy Lights are Low-Side Switched!

Because the Doorjamb Switches are mounted in the body sheetmetal, its easy to see that they are used to complete a circuit to chassis...so it is immediately clear that these will be used in a Low-Side Switched circuit: 


ROOF-LAMP (Courtesy Light) wiring excerpt for the 122.  Wiring maybe by way of a circuitous route, including the Hex-Connector under Dashboard (and which connects to harness which runs up A pillar and toward rear of the vehicle) and this should not confuse a troubleshooter, but it is an example of Low-Side Switching.  A Black wire supplies power.     

 


Early 1800 Wiring Diagram Excerpt showing (dual) Courtesy Lights (TAKLAMPA).  The Violett wire supplies power.  ...as noted in the linked Tech Article, there are variations!  See also: http://www.sw-em.com/Courtesy_Light_1800_Notes.htm

 

Courtesy Light Fader Circuit: 

I ran across this in my travels...its a "soft" Low-Side Switching circuit.  It uses the original Doorjamb Switches to chassis, but the interfacing circuit between Switches and lamps cause it to gently increase and decrease the brightness.  This might be of interest to the vintage Volvo driver who is also and electronic experimenter who might like this feature, and be inclined to construct such a circuit.  From the parts and partscount, it looks like it would easily fit onto a 2" square circuit board, and at a modest cost.  Once wired in, your 122 Courtesy Light would transition slowly: 

Automobile Interior Lights Fader

Copied from:  http://www.bowdenshobbycircuits.info/page5.htm

This circuit is used to slowly brighten and fade interior lights of older cars.

The circuit is based around the LM324 low power opamp which draws around 3mA of current, so it won't bother the battery if left connected for extended periods.

The top two opamps (pins 1,2,3 and 5,6,7) form a triangle wave oscillator running at about 700Hz while the lower opamp (pins 8,9,10) produces a linear, 5 second ramp, that moves up or down depending on the position of the door switch. The two transistors and associated resistors serve to limit the ramp voltage to slightly more and less than the upper and lower limits of the triangle waveform. These two signals (700 hZ. triangle wave and 5 second ramp) are applied to the inputs of the 4th opamp (pins 12,13,14) that serves as a voltage comparator and generates a varying duty cycle square wave that controls the IRFZ44 MOSFET and lamp brightness. The 5 second fade time can be adjusted with the 75K resistor connected to the door switch. A larger value will increase the time and a smaller value will speed it up.

When the door switch is closed (car door open) the voltage on pin 8 slowly rises above the negative peaks of the triangle wave producing a short duty cycle output and a dim light. As the ramp moves farther positive, a greater percentage of the triangle wave will be lower than the ramp voltage producing a wider pulse and brighter light. This process continues until the ramp is 100% above the positive peaks of the triangle wave and the output is maximum. When the door switch is open, the reverse action takes place and the lamps slowly fade out.

The IRFZ44 shouldn't require a heat sink if the total load is 50 watts or less but the temperature of the MOSFET should be monitored to insure it doesn't overheat. The on-state resistance is only 0.028 ohms so that 4 amps of current (48 watts) is only around 100mW. For larger loads, a small heat sink can be added to keep the MOSFET cool.

 

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External sources attributed.  Otherwise this information is Copyright © 2022-23.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here (also replace Lucas automotive electrical component when they fail in the most spectacular ways, some detectable from space by IR sensing sattelites).  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!

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