How does a wall adapter supply more current than its rated for.

Thread Starter

Lorvuk

Joined Jan 21, 2021
5
For this question I will use a tutorial linked below that uses 6 solenoids connected to a raspberry pi model 3B powered by a wall adapter as an example of how something is powered(and works, I dont know how). From my understanding, the wall adapter used to power the raspberry pi model 3B outputs 5 volts at 2.5A, the typical bare board current consumption for a pi 3b is about 600mA so that's fine, but how does it provide enough current for six 5V solenoids that each draw 1.1A of current which is 6.6A total(if they were all on at the same time). Doesn't that exceed how much current the charger can provide? (Sorry if this is a stupid question)

https://www.hackster.io/hitherejoe/braillebox-braille-news-reader-e86060
 

wayneh

Joined Sep 9, 2010
17,152
For this question I will use a tutorial linked below that uses 6 solenoids connected to a raspberry pi model 3B powered by a wall adapter as an example of how something is powered(and works, I dont know how). From my understanding, the wall adapter used to power the raspberry pi model 3B outputs 5 volts at 2.5A, the typical bare board current consumption for a pi 3b is about 600mA so that's fine, but how does it provide enough current for six 5V solenoids that each draw 1.1A of current which is 6.6A total(if they were all on at the same time). Doesn't that exceed how much current the charger can provide? (Sorry if this is a stupid question)

https://www.hackster.io/hitherejoe/braillebox-braille-news-reader-e86060
I didn’t see any mention of the power supply, so it’s hard to be sure. The link to the solenoids also is broken, so I can’t confirm the current draw.

That said, the design might “get away” with it by relying on the fact that the time-averaged current will be below the rating of the power supply. The power supply can survive temporary overloads without overheating.

If that’s the case, I’d say it’s a poor design. Murphy’s Law teaches us that the overload situation will happen eventually. A good design anticipates the worst-case scenario.
 

MrChips

Joined Oct 2, 2009
23,524
There are many reasons why this doesn't add up and still works.

1) If the manufacturer is conservative in rating the device (AC adapter in this case) then there could be some headroom. For example, the adapter could be quite capable of delivering 5V @ 3A instead of the rated 2.5A.

2) Usually it not advisable to operate power devices at their limits. This could likely result in higher than designed for temperatures and shorter lifespan. When a power supply is pushed beyond its rated power limit (5V x 2.5A = 12.5W) the output voltage may fall. Hence the voltage could be falling to say 4.5V @ 3A = 13.5W, and still operate the load properly.

3) Assuming that all six solenoids are energized, based on the information provided,
(6 x 1.1A) + 0.6A = 7.2A

You may have noticed that on average, only three solenoids are energized.
(3 x 1.1A) + 0.6A = 3.9A

Now, we are making the assumption that the solenoids actually draw 1.1A. That appears to be quite high for a simple tactile device.

4) The solenoids are only required to be on for a short length of time for a braille output device. One can imagine that a 20% duty cycle may be very adequate.

5) Solenoids require a higher current for initial energizing and the current can be reduced to about 50% after the initial turn period. The designer could have chosen to modulate the power in order to reduce power demand and hence internal temperature.

When all of the above is taken into consideration it is highly possible that the average current draw could be 20% of the max, i.e. 1/5 of 7A = 1.4A which would be within the specs of the 2.5A adapter.
 

Thread Starter

Lorvuk

Joined Jan 21, 2021
5
I didn’t see any mention of the power supply, so it’s hard to be sure. The link to the solenoids also is broken, so I can’t confirm the current draw.
Here's another question then hypothetically if you had a total current draw of 4.6A on your raspberry pi powering various things and you hook up a 5000ma/h power bank via usb-micro port to the pi would that exceed some type of max current that the micro usb is rated for. Because I always see ratings for chargers like 5v/2A or 9V/3A for certain usb cords. In other words would the coord stop the battery from supplying enough current?
 
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MrChips

Joined Oct 2, 2009
23,524
A USB power charger has its own bag of tricks. The original USB was not designed to charge your smart phone etc. It was capable of provide just sufficient power in order to operate a simple electronic circuit that needed no more than 5V @ 100mA. That was 0.5W for USB Version 1.0.

As the demand for more power came along, USB 2.0 was made capable of supplying 5V @ 0.5A = 2.5W. But you don't get it without asking. The device plugged into a USB 2.0 port had to ask for the higher current draw otherwise only 100mA was available.

Now a USB charger is a different beast. The amount of current available is entirely up to the designer of the charger, so long as the connector and USB cable can handle the higher current without burning up.

You can read more about USB charging here.
 
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