Auto disconnect load when charging - need advice

Thread Starter

Cadmium685

Joined Feb 23, 2018
2
Hi all,

New to the forum and new to designing circuits and PCBs. I'm working on a project and I can't seem to figure out how exactly how to do what I'm trying to do. I'm hoping one of you can give me a hand understanding the concept behind a few things I just can't seem to wrap my head around. (P.S. Sorry if this isn't the right place to post this).

Basically, I'm designing a PCB for something that is battery powered but it's in a really tight space so it can't be removed for charging, it has to be charged in its spot. I have some 3.7v 18650 batteries that I am charging off of a TP4056. The batteries are constantly connected to their load and so I need to make a circuit that disconnects their load when the 5v is applied for charging. I'm told a MOSFET will do what I'm thinking but I can't seem to figure out exactly how.

The main thing that is throwing me off about this is how and where the v- comes into play. I've really never dove into this world but now that I have to here I am asking for assistance.

Ultimately this is a much larger design and this circuit has to be repeated 4 more times. (I need to fit this design in a pipe that is only 1.25" in diameter so it's about 7.5" total length after connecting (5) 18" PCBs.

Any advice is appreciated!
 

wayneh

Joined Sep 9, 2010
17,175
The batteries are constantly connected to their load and so I need to make a circuit that disconnects their load when the 5v is applied for charging.
Silly question, but why do you need to disconnect the load? Think about your car. You don’t have to disconnect the headlights while the battery charges.

Another solution is to use a battery charger jack with an integral switch in it to disconnect the load when you plug in the jack. That’s less common these days.
 

AnalogKid

Joined Aug 1, 2013
10,098
The jack switch is by far the most simple and low cost approach. However, this sounds like a rugged industrial application, and most DC power jacks are very low cost parts aimed at high volume/low cost devices in benign environments. In other words, the contact plating is not very good.

Using a MOSFET switch is one way to go, but we need more information about the system. For example, the transistor can go in series with either the + or - battery terminal and the load circuit. Do you care which one? Can you post a partial schematic showing the power section of the design?

ak
 

Dodgydave

Joined Jun 22, 2012
10,359
You could use a 5V relay with a NC set of contacts, so when the pcb is powered up to charge, the relay disconnects the load.
 
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