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mosfet battery protection

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nsted

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Hi Everyone,
This is my first post.
I'm trying to make a battery-powered device that plugs into an AC adapter for recharging. When it is plugged in, the circuit should still be powered from the adapter. I want to prevent the adapter from directly connecting to the the battery (limiting the connection through the dedicated recharge IC). In the past I've just used a schottky diode in series with the high side of the battery, but I've decided that even a few hundred millivolts is too much to waste. I've been trying to find another solution but so far haven't found anything that seems identical to my setup. Most sites suggest using a mosfet, but this is offered as a solution to reverse battery protection. Can I use a mosfet in series with a battery as I've drawn in the following schematic to prevent the adapter voltage (marked 12V) from flowing into the battery? Thanks.

**broken link removed**
 
What about a switched socket for the adaptor to plug into?
 
You can use a MOSFET, but you have to control its gate. As Mneary says, the MOSFET conducts in both directions when turned on. The power connections you have on the IRF7404 is correct, but you have to raise the gate voltage, to turn off the MOSFET, when the mains power is available.
 
I assume you're not bothered about the diode losses from 12V, it's the battery that's the problem?

You need a diode in series with the 12V and to connect the gate to 12V so it can turn off the MOSFET when its connected.
 

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Thanks very much. That answers my questions both about how mosfets work, and about what to do.
nick
 
Hi again,
I realized that hero999's useful circuit would not work in my case, as I need the battery power to reach other parts of the circuit not depicted. Basically I have a bunch of battery-powered devices that plug into the same power bus in parallel thus pooling their electricity. If a power supply with a higher voltage is plugged into the bus then all of the batteries recharge while their respective devices continue to operate. So hero's diode would prevent the batteries from putting juice out on the bus because the diode has to be in the circuit for each device.

A schottky is ideal for this situation except for the voltage drop. The mosfet might still work, and I'm thinking now of using a comparator to compare the voltage on the bus to each battery. If the bus is using batteries then the voltages will match and the mosfet will conduct, but if the devices are plugged in then the bus voltage will be higher and the mosfet will be shut off. I've attached a new schematic depicting this.

**broken link removed**

I have two concerns regarding this. Would the power from the supply conduct through the mosfet and reach the battery before the comparator can shut off access, and what might happen if it does? Second, if the power supply is unplugged will the mosfets automatically start to conduct again...won't the comparator inputs still be uneven and therefore stay shut off?

Maybe it's just simpler and safer to go with the schottky because it will clearly disallow the power supply from directly reaching the battery. What do you think?
Thanks again,
nick
 
Hi again,
I realized that hero999's useful circuit would not work in my case, as I need the battery power to reach other parts of the circuit not depicted. Basically I have a bunch of battery-powered devices that plug into the same power bus in parallel thus pooling their electricity. If a power supply with a higher voltage is plugged into the bus then all of the batteries recharge while their respective devices continue to operate.
Do you want to need to recharge the batteries?

You need a real battery charger.

If not use a separate MOSFET for each battery.

If the circuit will work of 7.2V then why are you worried about a maximum of 1V drop from a 12V battery?
 
Sorry for the confusion.

There is a li-poly charging IC in each circuit which works (MCP73863), but I left it out of the schematic to simplify. It kicks in when it sees 12V (actually 10V would do).

I'm not concerned about a 1V drop from the 12V power supply, but the diode would block the battery from flowing out into the bus when the supply is not attached. Hopefully this new schematic clarifies. So I'm wondering if this comparator-controlled mosfet would stop the 12v from hitting the battery or not? I imagine it would be dangerous if the 12V did reach the battery directly.

**broken link removed**

If it is unclear what I'm trying to do, all I want is to emulate the following circuit without encountering the voltage drop of the schottky diode. I read that mosfets were useful alternatives.

**broken link removed**
 
The comparator circuit won't work because the LM392 has an open collector output so would need a pull-up and the inputs don't work up to the supply voltage.

Use a comparator with rail-to-rail inputs and a pull-up resistor and it should work.
 
Hi Everyone,
I've tested the circuit and thought I'd post the results.
The mosfet solution only kind of works, and not as well as the Schottky. Using the comparator to switch the mosfet on/off works fine, but there appears to be a few millivolts leaking through (150mV), though at worst it trickles through the mosfets 100k+ resistance so any effect on the battery would be negligible I think. Worse though, when the battery power is run through the mosfet with Drain attached to ground (this is opposite the normal switching use) then there is a voltage drop of almost a .6V, worse than the Schottky. This was unexpected, and I think is caused by the diode that connects Source to Drain. I thought that once the Mosfet closed the battery would bypass the diode and there would be no voltage drop, but that does not appear to be the case in my tests. At any rate, I've found that in practice the Schottky will work fine for my application, which is good because it's simpler and cheaper. So I won't be looking into this much more, but I wonder if anyone else will. Thanks again for your help, especially hero999.
 
I agree that using a Schottky is the best solution because it's simple, reliable and if you overrate the diode, a very low voltage loss <400mV.

You can buy two Schottky diodes in the same package in a common cathode configuration which will suit your application.
 
Here is a mosfet controller that my help with the problem. I controls mosfets and overcomes the diode drop.
 

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It looks like an interesting IC and is certainly ideal for this application but iit's in s tiny package so will be hard to solder.
 
Battery/Adapter Switching Circuit

I am also interested in a circuit that will switch between a battery and an AC/DC adapter with minimal voltage drop of the battery. I also want the circuit to charge the NiMH battery while the adapter is plugged in (using an LM317 regulator to provide a constant current to the battery). Will the attached schematic work to block the battery when the adapter is plugged in?

-Jeff
 

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