Jim Thompson says... > Oooops! I misunderstood your original post. I only > tried reverse voltage protection, not turn-off via the > transistor. > Do you really need to turn off via the transistor rather > than just a switch at the battery? Yes. The whole point is not having to use a manual switch. Besides, it appears reverse polarity protection doesn't work if the power supply is more than 8V, which would include a 9V battery. Well, I'll keep working on this, but for now it looks like two P-channel mosfets will be needed, as others have suggested.
PNP for soft switch and reverse battery protection
Started by ●February 9, 2017
Reply by ●February 11, 20172017-02-11
Reply by ●February 11, 20172017-02-11
On 2/11/2017 7:31 PM, Peabody wrote:> Jim Thompson says... > > > Oooops! I misunderstood your original post. I only > > tried reverse voltage protection, not turn-off via the > > transistor. > > > Do you really need to turn off via the transistor rather > > than just a switch at the battery? > > Yes. The whole point is not having to use a manual switch. > > Besides, it appears reverse polarity protection doesn't work > if the power supply is more than 8V, which would include a > 9V battery. > > Well, I'll keep working on this, but for now it looks like > two P-channel mosfets will be needed, as others have > suggested.It's a long thread, maybe this was stated. Is there a reason why a Schottky diode can't be used for reverse polarity protection? That seems simpler than adding two transistors. Maybe not. Two of the same line item can be easier in fabrication than two line items. -- Rick C
Reply by ●February 11, 20172017-02-11
On Sat, 11 Feb 2017 18:31:18 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>Jim Thompson says... > > > Oooops! I misunderstood your original post. I only > > tried reverse voltage protection, not turn-off via the > > transistor. > > > Do you really need to turn off via the transistor rather > > than just a switch at the battery? > >Yes. The whole point is not having to use a manual switch. > >Besides, it appears reverse polarity protection doesn't work >if the power supply is more than 8V, which would include a >9V battery. > >Well, I'll keep working on this, but for now it looks like >two P-channel mosfets will be needed, as others have >suggested.Did you miss... Message-ID: <6equ9cdnov5br6kbmnd78i70o40ce75jma@4ax.com> ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 11, 20172017-02-11
On 12/02/17 11:50, rickman wrote:> On 2/11/2017 7:31 PM, Peabody wrote: >> Jim Thompson says... >> >> > Oooops! I misunderstood your original post. I only >> > tried reverse voltage protection, not turn-off via the >> > transistor. >> >> > Do you really need to turn off via the transistor rather >> > than just a switch at the battery? >> >> Yes. The whole point is not having to use a manual switch. >> >> Besides, it appears reverse polarity protection doesn't work >> if the power supply is more than 8V, which would include a >> 9V battery. >> >> Well, I'll keep working on this, but for now it looks like >> two P-channel mosfets will be needed, as others have >> suggested. > > It's a long thread, maybe this was stated. Is there a reason why a > Schottky diode can't be used for reverse polarity protection?Sometimes you simply can't spare 400mV, such as in single-cell applications. A MOSFET that will pass (say) 10amps is a lot smaller and cheaper than a Schottky that will. Clifford Heath.
Reply by ●February 12, 20172017-02-12
Jim Thompson says... > Did you miss... > Message-ID: <6equ9cdnov5br6kbmnd78i70o40ce75jma@4ax.com> Yes I did. Thanks for asking. And for the circuit suggestion. But I think if I have to use two devices, then I would rather just use a P-channel mosfet for the polarity protection. It wouldn't need any additional parts since its gate would be tied directly to ground. Then the switch transistor could either be another mosfet, or just a simple PNP switch. With either choice, I wouldn't need the diodes or the extra resistor.
Reply by ●February 12, 20172017-02-12
Clifford Heath says... >> It's a long thread, maybe this was stated. Is there a >> reason why a Schottky diode can't be used for reverse >> polarity protection? > Sometimes you simply can't spare 400mV, such as in > single-cell applications. > A MOSFET that will pass (say) 10amps is a lot smaller > and cheaper than a Schottky that will. I need very little current, so a Schottky would be cheaper. But it's mainly the voltage drop. Anything that produces a voltage drop just shortens the life of the battery - assuming the circuit will only operate properly down to a certain supply voltage. Also, Schottkys aren't known for having all that great reverse leakage values, which might make them problematical for protecting against reverse polarity. From everything I've read, the P-channel mosfet, installed backwards, is the school-approved solution for reverse polarity protection. They are more expensive, but for my needs, the TP2104 should work fine, and it's 60 cents at Digikey, versus maybe 40 cents for a Schottky.
Reply by ●February 12, 20172017-02-12
On 12/02/17 16:26, Peabody wrote:> Clifford Heath says... > >> It's a long thread, maybe this was stated. Is there a > >> reason why a Schottky diode can't be used for reverse > >> polarity protection? > > > Sometimes you simply can't spare 400mV, such as in > > single-cell applications. > > > A MOSFET that will pass (say) 10amps is a lot smaller > > and cheaper than a Schottky that will. > > I need very little current, so a Schottky would be cheaper. > But it's mainly the voltage drop. Anything that produces a > voltage drop just shortens the life of the battery - > assuming the circuit will only operate properly down to a > certain supply voltage. > > Also, Schottkys aren't known for having all that great > reverse leakage values, which might make them problematical > for protecting against reverse polarity. > > From everything I've read, the P-channel mosfet, installed > backwards, is the school-approved solution for reverse > polarity protection. They are more expensive, but for my > needs, the TP2104 should work fine, and it's 60 cents at > Digikey, versus maybe 40 cents for a Schottky.I have some IRLML6302 that I bought for soft power-switches and reverse protection in single-LiPo applications. $0.37 in singles from Digikey.
Reply by ●February 12, 20172017-02-12
On Sat, 11 Feb 2017 23:14:17 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>Jim Thompson says... > > > Did you miss... > > > Message-ID: <6equ9cdnov5br6kbmnd78i70o40ce75jma@4ax.com> > >Yes I did. Thanks for asking. And for the circuit >suggestion. > >But I think if I have to use two devices, then I would >rather just use a P-channel mosfet for the polarity >protection. It wouldn't need any additional parts since its >gate would be tied directly to ground. > >Then the switch transistor could either be another mosfet, >or just a simple PNP switch. With either choice, I wouldn't >need the diodes or the extra resistor. >Yep, like this... <http://www.analog-innovations.com/SED/Peabody_Switch_PMOSx2.png> I used this scheme about 20 years ago for a LiIon battery charge chip design I did for California Micro Devices. I don't know if I was the first to do so or not. I actually used only one FET and switched the body connection... the chip was not only a charger, but also a discharge controller. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 12, 20172017-02-12
Den søndag den 12. februar 2017 kl. 06.26.22 UTC+1 skrev Peabody:> Clifford Heath says... > > >> It's a long thread, maybe this was stated. Is there a > >> reason why a Schottky diode can't be used for reverse > >> polarity protection? > > > Sometimes you simply can't spare 400mV, such as in > > single-cell applications. > > > A MOSFET that will pass (say) 10amps is a lot smaller > > and cheaper than a Schottky that will. > > I need very little current, so a Schottky would be cheaper. > But it's mainly the voltage drop. Anything that produces a > voltage drop just shortens the life of the battery - > assuming the circuit will only operate properly down to a > certain supply voltage. > > Also, Schottkys aren't known for having all that great > reverse leakage values, which might make them problematical > for protecting against reverse polarity. > > From everything I've read, the P-channel mosfet, installed > backwards, is the school-approved solution for reverse > polarity protection. They are more expensive, but for my > needs, the TP2104 should work fine, and it's 60 cents at > Digikey, versus maybe 40 cents for a Schottky.if you don't need the gnd and battery negative to be the same you can use nfets on the low side instead
Reply by ●February 12, 20172017-02-12
On Sat, 11 Feb 2017 23:14:17 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>Jim Thompson says... > > > Did you miss... > > > Message-ID: <6equ9cdnov5br6kbmnd78i70o40ce75jma@4ax.com> > >Yes I did. Thanks for asking. And for the circuit >suggestion. > >But I think if I have to use two devices, then I would >rather just use a P-channel mosfet for the polarity >protection. It wouldn't need any additional parts since its >gate would be tied directly to ground. > >Then the switch transistor could either be another mosfet, >or just a simple PNP switch. With either choice, I wouldn't >need the diodes or the extra resistor. >Why not use a series schottky diode? -- John Larkin Highland Technology, Inc lunatic fringe electronics
