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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

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  • \$\begingroup\$ 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? \$\endgroup\$
    – winny
    Aug 10, 2016 at 9:34
  • \$\begingroup\$ 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 \$\endgroup\$
    – M.Ferru
    Aug 10, 2016 at 9:44
  • \$\begingroup\$ You could do some calculations about voltages etc. to see that they are (electrically speaking) not the same. \$\endgroup\$
    – PlasmaHH
    Aug 10, 2016 at 9:45
  • \$\begingroup\$ By "it works the same", I meant the load will be drive the same way. Is it wrong? \$\endgroup\$
    – M.Ferru
    Aug 10, 2016 at 9:47
  • \$\begingroup\$ @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? \$\endgroup\$
    – winny
    Aug 10, 2016 at 9:54

2 Answers 2

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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.

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  • \$\begingroup\$ Another useful quality of the low side open collector output - they can be wired together to give a logical 'OR' \$\endgroup\$
    – N.G. near
    Aug 10, 2016 at 10:53
  • \$\begingroup\$ If I add another transistor to the high side switch, I make a logical OR as well. Right ? \$\endgroup\$
    – M.Ferru
    Aug 10, 2016 at 11:18
  • \$\begingroup\$ Is making a logical OR part of the question? "add another transistor" means nothing concrete. \$\endgroup\$
    – Andy aka
    Aug 10, 2016 at 11:31
  • \$\begingroup\$ No, it's not part a question. @Andyaka, thanks to your answer :) \$\endgroup\$
    – M.Ferru
    Aug 10, 2016 at 11:34
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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.

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  • \$\begingroup\$ What make the the high side safer? Is it because of eventual short circuit? \$\endgroup\$
    – M.Ferru
    Aug 10, 2016 at 10:02
  • \$\begingroup\$ Yes. Specially because people touch what they should not... and you as designer need to consider misuse from final user. \$\endgroup\$
    – matzeri
    Aug 10, 2016 at 10:11
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    \$\begingroup\$ @N.G.near OMG ! he is talking about grounding ( circuit ground ) or battery negative terminal which is connected to the chassis . NOT EARTHING \$\endgroup\$
    – ElectronS
    Aug 10, 2016 at 11:04
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    \$\begingroup\$ 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. \$\endgroup\$
    – N.G. near
    Aug 10, 2016 at 11:33
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    \$\begingroup\$ Electrical Ground does NOT mean connected to earth. It can be but it is not a must. \$\endgroup\$
    – matzeri
    Aug 10, 2016 at 12:11

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