Efficient AC Adapter Circuit for Battery-Powered Devices

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Discussion Overview

The discussion revolves around designing a circuit that allows a device to switch between an AC adapter and battery power seamlessly. The focus is on finding efficient and standard methods for implementing this power management in battery-powered devices.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks a standard solution for connecting an AC adapter to a circuit that can switch to battery power when the adapter is unplugged.
  • Another participant suggests using a small power socket with an internal switch that disconnects the battery when the AC adapter is plugged in, emphasizing the need for filtered DC from the adapter.
  • A different participant notes that they cannot use the suggested socket option due to the specific requirement of a USB mini plug for the adapter.
  • Another idea proposed involves using diodes in series with each power source, allowing the AC adapter to take precedence over the batteries if its voltage is higher.
  • One participant mentions integrated circuits known as 'supervisors' or 'power muxes' that can manage the switchover between power supplies, highlighting their hysteresis feature to prevent rapid switching.

Areas of Agreement / Disagreement

Participants present multiple competing views and approaches to the problem, with no consensus reached on a single solution. Various methods are proposed, but each has its own limitations or specific requirements.

Contextual Notes

Some suggestions depend on specific types of connectors or components, such as the inability to use a standard power socket due to the USB mini plug requirement. Additionally, the effectiveness of diodes and integrated circuits may vary based on the specific application and design constraints.

Who May Find This Useful

Readers interested in circuit design, power management solutions for battery-powered devices, and those exploring efficient ways to integrate AC and battery power sources may find this discussion relevant.

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I'd like to stick an ac adapter onto a circuit such that when the adapter is plugged in it runs on the ac adapter, but when it isn't plugged in it runs on batteries. Anyone know of a quick elegant solution?

I'm sure these circuits get made all the time, but I can't seem to find any!

I'm sure I could come up with something, but I was wondering if there was a standard way of doing it.
 
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Yes, you can get small power sockets that have a switch in them.

This switch makes contact unless the AC adaptor is plugged in.

So, you have the battery positive connected through this switch and then to the centre input pin of the socket and then to the circuit.

When the AC plug is inserted, the switch opens and the battery is disconnected. Power just comes from the AC adaptor. This has to provide filtered DC, of course.

The negative leads are all joined together.
 
Unfortunately, I can't use that option. The specified adapter uses a USB mini plug.
 
You might be able to use a diode in series with each power source going to whatever the load is. If the voltage from the AC adapter is slightly higher than the batteries the adapter's diode will conduct in "preference" to the batteries'.
 
There are ICs called 'supervisors' (specifically, supervisors with battery switchover) which allow you to switch between two power supplies. They usually have a little hysteresis so they don't switch-over as soon as one voltage dips below the other.

I think they're also called a 'power mux' by some manufacturers, at least according to a TI datasheet I found on a quick Google search:
http://focus.ti.com/docs/prod/folders/print/tps2110.html
 

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