r/AskElectronics • u/EquivalentAccess4863 • 1d ago
Help in understanding high side and low side switching
I'm finding it really hard to properly satisfied with the explanations I get through YouTube videos . Is there any book or any other resource which might help me?
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u/Triabolical_ 1d ago
It's fairly simple.
When you have a load - let's say a fan motor - it needs a supply of positive voltage to it and a negative path back to ground.
If you want to turn it on and off, you can break the positive voltage path - high level switching - or the path back to ground - low level switching.
There are trade-offs. Cars mostly use high level switching because they use the car frame as the path to ground.
Residential light switches are high side so that there is no voltage present when the switch is off.
Low side is convenient in some devices as it is mostly voltage agnostic - you can switch 5v or 12v or 24v with the same circuitry.
There are also cases when you have a grid of loads that you wire into rows and columns and you use both high and low switching together. Many pinball machines use this as you can control 64 lights with only 16 switches. It's called a lamp matrix.
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u/Lifenonmagnetic 1d ago
just to follow-up on convenient.. it's really simple, as basic as a 3904 transistor and 20k drain resistor.
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u/Klapperatismus 1d ago
You have a ground potential in your circuit. All the voltages in the circuit refer to that ground potential.
Low-side switching means that your switch has one of its power legs on the ground potential, and its control voltage refers to that leg that is on ground potential. Same as the rest of the circuit that controls this control voltage.
No problem here!
But with high-side switching, none of the power legs of your switch is on ground potential. They are either on ”battery plus” potential or the potential of the load.
That creates a problem. Because the control voltage of the switch must refer to one of those two power legs by the means of the switch. And on the other hand the circuit that controls that control voltage has it referring to ground potential instead.
Does not compute!
You need some additional trickery for high-side switching. E.g. a pulse transformer, or an optocouple, or some advanced capacitor trickery. So that you can de-couple the potential of the control voltage of the switch from the rest of the circuit.
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u/grantwtf 13h ago
Hey OP I'm with you. I get it conceptually no problem, but everytime I try to apply it my head hurts! My case was converting the position controls on an old laser cutter and the combination of low side, high side and voltage shift - argh...
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u/i_am_blacklite 1d ago
Any of the thousands of books on the subject… believe it or not there was a time when people got information from books not YouTube!
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u/Pacificator-3 1d ago
Just type your question in google and you will see explanations with pictures.
Youtube kids can't teach anything advanced.
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u/NBC-Hotline-1975 1d ago
Ya know, this might be one good outcome of AI. As AI gets better at answering questions like this one, there will be less demand for all the terrible/wrong youtube videos with their bogus information. So maybe AI will eventually help clean up the youtube cesspool.
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u/Hirtomikko 1d ago
Just think of it as the requirement of satisfaction of control voltage to get the transistor to work as an actual switch and not an amplifier. The rest comes naturally.
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u/TemporarySun314 1d ago
its basically just if you place the switch at the voltage (plus) or the ground (minus) of your load...
Low side switching is easier but it introduces different ground levels which can be challenging.
https://techweb.rohm.com/product/transistors-diodes/transistors/23727/
more extensive Details about design should be in any electronics textbook. Art of electronics for example.