On Mon, 4 Jul 2016 20:27:22 -0700, you wrote: >Nonsense. This is how the vast majority of power supplies work. The voltage >ramps up while the current is maintained at it’s maximum current. When the >voltage reaches the regulation voltage, the current is reduced. What the OP >proposed is that 4A was regarded as a short circuit and hence the power supply >shutdown. This is not normal.
Much of this depends on the type of supply. A bulk supply provides unregulated current to the limit of its capacity. Voltage decreases as current is drawn. There are no voltage or current regulating elements. Obviously not used for the BBB. A voltage regulated supply may or may not have a current limit circuit. If not, then the supply current is limited by the resistance of the parts. From zero, the supply will try to charge up whatever capacitance is on the output. Large startup currents can happen, similar to a bulk supply. The voltage regulator does keep the output voltage from rising past a set point, however. A voltage and current regulated supply may operate in two ways depending on the current limiter design. If the supply is only current limited, then the supply is a constant current supply at startup, supplying the maximum current it can (depending on load) until the nominal voltage is reached (assuming we're charging capacitors). Under normal operation, the supply is a voltage regulated supply. Should current demand exceed (or try to) the current limit, the supply becomes a constant current supply. The voltage drops to the point where the rated maximum current flows. For instance, 5 volts 4 amps. Put a 2 ohm load on the supply and you have 5 volts at 2.5 amps load. Put a 1 ohm load on it, and you will have a 4 volt supply at 4 amps. It is possible for a supply to have foldback current limiting. In that case, the maximum supply current changes on maximum draw. In this case, you may have a 200 ma current foldback limit. Try to draw 4 amps and the supply changes its limit to 200 ma and adjusts the output voltage to maintain that lower limit. Depending on the design of the foldback, the load may have to be reduced or disconnected to restore the regulated voltage. Constant voltage with constant current limiting is common on lab supplies, higher quality wall-wart power supplies, and is not common on straight battery supplies, or bulk supplies, and some older (and cheaper) power adaptors. Power limiting can be built in rather easily, though, if you have the right parts. Harvey > >Regards, >John > > > > >> On Jul 4, 2016, at 6:14 PM, 'Morgaine' via BeagleBoard >> <[email protected]> wrote: >> >> John Syne writes: >> > a power supply that is spec’d at 4A should not shutdown when it sees a 4A >> > load, but rather, it should current limit at 4A. If the power supply is >> > spec’d at 4A, then 4A should not be treated as a short circuit. >> >> That's impossible. You can't recommend that fundamental electrical laws be >> overridden. :P >> >> If a PSU current limits at 4A, it can do so only by reducing its output >> voltage. This may then drop below specification for its load and this can >> have very bad consequences such as non-stop rebooting. There is no way for >> the voltage to be maintained above its minimum spec while still providing a >> current limit. >> >> This is the reason why ensuring that startup inrush transients cause no harm >> must always be handled within the design of the load, ie. the BBB in this >> case. The load is a black box as far as the external PSU is concerned, so >> the external PSU has no means to perform this protective function while >> still maintaining regulation. (Blowing a fuse does not maintain regulation, >> but is sometimes the only practical alternative.) >> >> In other words, a load can demand a minimum current capability under a rated >> voltage specification, but it cannot demand a maximum current capability >> unless it can cope with arbitrary drops in supply voltage. Such voltage >> tolerance is generally not available in electronic circuitry today, >> certainly not in BBB. >> >> >> >> -- >> For more options, visit http://beagleboard.org/discuss >> <http://beagleboard.org/discuss> >> --- >> You received this message because you are subscribed to the Google Groups >> "BeagleBoard" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected] >> <mailto:[email protected]>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/beagleboard/CAM0uzStPOH3i%2B0pH2O4Bz3E_kcggLrJu9yZSaOO4HzZ%2BkLQ3mg%40mail.gmail.com >> >> <https://groups.google.com/d/msgid/beagleboard/CAM0uzStPOH3i%2B0pH2O4Bz3E_kcggLrJu9yZSaOO4HzZ%2BkLQ3mg%40mail.gmail.com?utm_medium=email&utm_source=footer>. >> For more options, visit https://groups.google.com/d/optout >> <https://groups.google.com/d/optout>. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/ogemnbl4qd81t8ojl9sfrn0qvtbsu7jrf3%404ax.com. For more options, visit https://groups.google.com/d/optout.
