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I wonder what is the difference between high side switch and low side switch. Basically it work the same way (electrically speaking) but I don't know if one is better than the other. Is there any safety purpose? What is the best solution if there is big capacitor in the circuit? Capacitor C1 will discharge through the load in the low side switch mode whereas C2 discharge through the ground.

schematic

simulate this circuit – Schematic created using CircuitLab

M.Ferru
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  • In general you go for low side whenever you can for economical reasons, but do you have a specific case? Can you draw a schematic? – winny Aug 10 '16 at 09:34
  • I don't have any specific case in mind, I just ask this question for "knowledge" purpose. I just add a schematic but it do not include any capacitor – M.Ferru Aug 10 '16 at 09:44
  • You could do some calculations about voltages etc. to see that they are (electrically speaking) not the same. – PlasmaHH Aug 10 '16 at 09:45
  • By "it works the same", I meant the load will be drive the same way. Is it wrong? – M.Ferru Aug 10 '16 at 09:47
  • @M.Ferru Excellent schematic. Electrically, your high side example is more expensive due to PNP instead of NPN and an extra transistor. Can you add the capacitor you mention? – winny Aug 10 '16 at 09:54
  • I just edit the schematic. Please mind it's just an example – M.Ferru Aug 10 '16 at 10:00

2 Answers2

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I wonder what is the difference between high side switch and low side switch. Basically it work the same way (electrically speaking) but I don't know if one is better than the other.

It depends on the circuit you are switching. Ignoring safety in vehicle reasons, you might choose to switch on the high side of a signal amplifier because the low side might be the signal 0 V reference (as well as power 0 V) and, routing both signal and power grounds through a common point i.e. the low side switching MOSFET can lead to serious noise and distortion effects on the final signal. If the MOSFET had true micro-ohm (or less) on-resistance then it's less of a problem but you won't find one this good for pennies or pounds.

So, if you can switch the low-side it's generally easier because a micro/controller will normally be logic level referenced to 0 V and it can easily drive an N channel MOSFET (or NPN) but beware of signal lines sharing this common switching device as outlined above.

What is the best solution if there is big capacitor in the circuit?

If you are referring to a big capacitor across the target load's power pins then there are no real implications other than to ensure your switching mechanism can handle the in-rush current.

Andy aka
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The high side approach is used for safety, It is typical of vehicles. Putting the mosfet/switch as high side allows to have the load as low side so connected to the ground that is also the chassis.

matzeri
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  • What make the the high side safer? Is it because of eventual short circuit? – M.Ferru Aug 10 '16 at 10:02
  • Yes. Specially because people touch what they should not... and you as designer need to consider misuse from final user. – matzeri Aug 10 '16 at 10:11
  • @ matzeri How is a vehicle mounted on rubber tyres grounded in any way shape or form? – N.G. near Aug 10 '16 at 10:55
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    @N.G.near OMG ! he is talking about grounding ( circuit ground ) or battery negative terminal which is connected to the chassis . NOT EARTHING – ElectronS Aug 10 '16 at 11:04
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    Precisely! Grounded means connected to earth - hence the name. Ground and chassis are NOT interchangeable terms. It is important to make the distinction, n'est-ce pas? There is no requirement for chassis to be at ground potential, take an aircraft for example. – N.G. near Aug 10 '16 at 11:33
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    Electrical Ground does NOT mean connected to earth. It can be but it is not a must. – matzeri Aug 10 '16 at 12:11
  • We that's not what I was taught. A bit pedantic, perhaps - but chassis, common - call it what you will - is not ground, although the terms seem to be used interchangeably. If you are a designer, be sure to stipulate that your 'ground' is not at earth potential won't you? – N.G. near Aug 11 '16 at 20:35
  • You have never looked on vehicle electric circuits. Do you see the GND labels on figure 1 and 2 of http://www.aecouncil.com/Documents/AEC_Q100-012.pdf . – matzeri Aug 11 '16 at 20:49
  • @matzeri, the new convention is to use grounding to refer *only* to that which is directly connected to earth(so grounding rod/plate/conductor and the conductor attached to it up to the first place it is tapped off), and to use "bonding" to refer to the "ground" in all other parts of the system, with "earth" and "ground" being treated the same. In theoretical and engineering related to systems where an earth bond is not practical, this convention is not always observed, as the oldest definition was much more broad and in these cases, context reveals the correct interpretation. – K H Mar 28 '19 at 22:22
  • So for automotive purposes, the car is only loosely "grounded" by its tires, and all of the interconnects marked with a ground symbol would be bond, which you could refer to as common, but that gets you into a similar bit of non-specificity, because whereas all bonds are common conductors, not all common conductors are bonds. Similarly, "chassis" refers only to a specific section of the bonding system. Because of the admittedly poor connection of the chassis to ground through tires, air and parasitics, the chassis also stays more or less at ground V unless a significant source is applied. – K H Mar 28 '19 at 22:31