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I am soliciting your help regarding a project I am working on. I have done my research but can't decide which solution is optimum for my needs. By optimum, in this case, I mean will consume the less, while allowing easy adjustment of the cut-off voltage (turning a trimmer or reprogramming is fine, swapping resistor values is not).

General description of the object :

The object in question is a homemade portable speaker, with a micro-controller connected to it to allow remote control of tracks, volume, etc.

Information related to the question :

  • Power : via a 20V Black and Decker drill battery, and a 24V power supply. When the power supply gets plugged, the battery gets disconnected. When the power supply gets disconnected, the battery connects.
  • Items that will get powered : An amp + micro-controller + speaker

The question :

What is the best circuitry to use in order to :

  • switch the power from the 24V power supply to 20V battery pack
  • prevent over-discharge of the battery pack, while allowing easy adjustment of the cutoff voltage (threshold would be 15V)

Initial idea :

As a tinkerer (and not a professional) with electronics, I initially got drawn by the idea of using a micro-controller and 2 relays (one latching, one non-latching). The only consumption should be the micro-controller (which can go as low as 4uA with the watch dog timer) and the tension dividers.

Here are two drawings. The first one, "situation actuelle" is how it is working today. No over-discharge protection. The second one shows my two initial ideas using micro-controllers + relays.

After some research, most of the help online redirects people to specific chips like DW01 or others.

Note that I don't have access to each cell individually for the monitoring part. Also, with the 20V battery, some regulators might be needed which might add to the consumption.

This specific project uses B&D battery pack which only provides the positive and negative contacts of the total 20V.

Also, there is no charging considerations in this project. The user removes the battery and charges it on a regular B&D charger, no hacking involved.

I have a feeling the most elegant solution revolves around a Mosfet and a dedicated monitoring chip, but I would really appreciate having your thoughts on this.

Thanks a lot for reading!

All the best,

present situation intitial thoughts

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  • \$\begingroup\$ How about something like this, reworking the numbers for 20 V rather than 12 V. \$\endgroup\$ – Neil_UK Apr 7 '20 at 20:13
  • \$\begingroup\$ Hi Neil, thanks for your suggestion. I guess I need to add a linear regulator to power the TLV431? 12V maybe? \$\endgroup\$ – fab_adtp Apr 7 '20 at 20:45
  • \$\begingroup\$ No, connect it in cascode with a BJT to increase the voltage. \$\endgroup\$ – Neil_UK Apr 7 '20 at 21:07
  • \$\begingroup\$ Thanks Neil. Unfortunately, the BJT in cascade is outside of my knowledge. I looked it up on google but it looks pretty cryptic to me. I might stick with my micro-controller option as this one is inside my abilities. \$\endgroup\$ – fab_adtp Apr 7 '20 at 21:35
  • \$\begingroup\$ Unless you mean to connect the TLV to the emitter pin of a NPN? \$\endgroup\$ – fab_adtp Apr 7 '20 at 21:42

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