4 ms·

Reverse polarity protection is often not done with diodes for the reasons explained. It is more common to use a p-channel MOSFET (or ideal diode controllers whe

by guan 11y ago

Reverse polarity protection is often not done with diodes for the reasons explained. It is more common to use a p-channel MOSFET (or ideal diode controllers when there are multiple power sources) when more than a trivial amount of power is used by the design. They are not cheap when properly sized, but are not ruinously expensive either and are not too large for most designs.

deleted 11y ago
[deleted]
mindslight 11y ago
There should be a crowbar diode between the rails. If the design is too cheap for an input fuse, the source will limit/disable the short current.
- klodolph 11y ago
  Crowbar isn't a diode, and it doesn't protect against reversed polarity, and it's a bad idea on the input because you're relying on the other device to limit current properly, when it's the other device which got the voltage wrong in the first place.
  - mindslight 11y ago
    It acts like a literal crowbar, so why not call it a crowbar? There are likely already plenty of diodes across the rails (the body diodes of the chips), so without a fuse the power is already limited by just the source. IMHO you might as well try and protect the circuit from foreseeable power-limited situations like frayed cables and the OP.
    - klodolph 11y ago
      I'm saying a crowbar isn't a diode, and you're saying that you should call a crowbar a crowbar? Color me confused. Usually a crowbar is a circuit with a triac or SCR in it designed to protect against overvoltage. This isn't about frayed cables. Frayed cables don't do this. This is about a cable which is wired the opposite of the way it should be. A minimal protection circuit is probably going to cost around $0.20 in quantity (Si2323 on Mouser). For a $500 Chrome Pixel C, the protection circuit is worth the effort if more than 1 in 2,500 ($500 / $0.20) laptops would fail due to charger cables with reversed polarity. If you think that fewer than 1 in 2,500 laptops will fail this way, then don't bother.
      - mindslight 11y ago
        Crowbar is a general concept - a short across a supply. A diode across the rails acts as such, so I referred to it as a crowbar. I'm familiar with SCR-based active crowbar circuits - what we're disagreeing about is narrow-prescriptive versus wide-descriptive usage of a term. Most frayed cables are going to simply short or open, but the right combination of such would cross wires. But sure, they're going to be rare. On a $500 device I would indeed expect a modicum of robustness against negligently designed cables and other weirdness. I mean, I'm not the only one that doesn't have a cable tester and buys cables without a connected-equipment warranty, right? Historically, electrical connections have been designed to deal with weird shit. Do you think you could kill an RS-232 port with any permutation of connections? Of course the cost metric explains why devices are no longer robust. In fact the manufacturer only has to worry about in-warranty failures, and furthermore only the ones they can't blow off as "user error". Just because devices are designed for the 4 month (or whatever it's down to) upgrade cycle, doesn't mean that informed consumers shouldn't call out designers of such equipment as the cheapskates they are!
        krapht 11y ago
        Ugh. Am I the only one who appreciates cheap things for what they are? Over-engineering something is a bad thing, just as under-engineering it is too. It makes things more expensive than they have to be. There's an art to designing something to last for no longer than it has to, and to use not a single part that isn't necessary. How much worse would our lives be if we had to pay industrial/commercial prices for all of our goods?
        simoncion 11y ago
        > It makes things more expensive than they have to be. Consumer electronics are -generally- ridiculously cheap these days. I welcome more "overengineering", even if it drives up the cost somewhat. :)
        XorNot 11y ago
        The problem is cost != quality in any reliable way any more.
  - nathancahill 11y ago
    You guys might want to check in with EEQ. Lots of diode knowledge over there.
klodolph 11y ago
Doing the math, a reasonable PMOS might be the Si2323, although it can only handle ~5A. Costs $0.20 in quantity. Therefore, on a $500 laptop, don't bother if it doesn't save you more than 1 in 2,500 laptops. That is, assuming that you are only trying to save the company money, not make a reliable product. PR can always blame the faulty cable / charger.

by guan 11y ago

deleted 11y ago

[deleted]

mindslight 11y ago

There should be a crowbar diode between the rails. If the design is too cheap for an input fuse, the source will limit/disable the short current.

klodolph 11y ago
Crowbar isn't a diode, and it doesn't protect against reversed polarity, and it's a bad idea on the input because you're relying on the other device to limit current properly, when it's the other device which got the voltage wrong in the first place.
- mindslight 11y ago
  It acts like a literal crowbar, so why not call it a crowbar? There are likely already plenty of diodes across the rails (the body diodes of the chips), so without a fuse the power is already limited by just the source. IMHO you might as well try and protect the circuit from foreseeable power-limited situations like frayed cables and the OP.
  - klodolph 11y ago
    I'm saying a crowbar isn't a diode, and you're saying that you should call a crowbar a crowbar? Color me confused. Usually a crowbar is a circuit with a triac or SCR in it designed to protect against overvoltage. This isn't about frayed cables. Frayed cables don't do this. This is about a cable which is wired the opposite of the way it should be. A minimal protection circuit is probably going to cost around $0.20 in quantity (Si2323 on Mouser). For a $500 Chrome Pixel C, the protection circuit is worth the effort if more than 1 in 2,500 ($500 / $0.20) laptops would fail due to charger cables with reversed polarity. If you think that fewer than 1 in 2,500 laptops will fail this way, then don't bother.
    - mindslight 11y ago
      Crowbar is a general concept - a short across a supply. A diode across the rails acts as such, so I referred to it as a crowbar. I'm familiar with SCR-based active crowbar circuits - what we're disagreeing about is narrow-prescriptive versus wide-descriptive usage of a term. Most frayed cables are going to simply short or open, but the right combination of such would cross wires. But sure, they're going to be rare. On a $500 device I would indeed expect a modicum of robustness against negligently designed cables and other weirdness. I mean, I'm not the only one that doesn't have a cable tester and buys cables without a connected-equipment warranty, right? Historically, electrical connections have been designed to deal with weird shit. Do you think you could kill an RS-232 port with any permutation of connections? Of course the cost metric explains why devices are no longer robust. In fact the manufacturer only has to worry about in-warranty failures, and furthermore only the ones they can't blow off as "user error". Just because devices are designed for the 4 month (or whatever it's down to) upgrade cycle, doesn't mean that informed consumers shouldn't call out designers of such equipment as the cheapskates they are!
      - krapht 11y ago
        Ugh. Am I the only one who appreciates cheap things for what they are? Over-engineering something is a bad thing, just as under-engineering it is too. It makes things more expensive than they have to be. There's an art to designing something to last for no longer than it has to, and to use not a single part that isn't necessary. How much worse would our lives be if we had to pay industrial/commercial prices for all of our goods?
        simoncion 11y ago
        > It makes things more expensive than they have to be. Consumer electronics are -generally- ridiculously cheap these days. I welcome more "overengineering", even if it drives up the cost somewhat. :)
        XorNot 11y ago
        The problem is cost != quality in any reliable way any more.
- nathancahill 11y ago
  You guys might want to check in with EEQ. Lots of diode knowledge over there.

klodolph 11y ago

Doing the math, a reasonable PMOS might be the Si2323, although it can only handle ~5A. Costs $0.20 in quantity. Therefore, on a $500 laptop, don't bother if it doesn't save you more than 1 in 2,500 laptops. That is, assuming that you are only trying to save the company money, not make a reliable product. PR can always blame the faulty cable / charger.