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polarity protection mosfet selection

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MrDEB

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am looking to add some polarity protection to my Mexican Train project but getting correct component? Having very little experience with mosfets I am using this as example
http://digital-diy.com/general-elec...ser-look-at-the-p-channel-mosfet-circuit.html
then using this as component

https://www.jameco.com/Jameco/Products/ProdDS/889760.pdf



this is a simple 4.5volt (3 AA batteries) powered circuit. Searched for a smaller inexpensive battery to use but the 3 AA holder is best I found but it is really to tall but it works.
 

crutschow

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That MOSFET should work fine.

If you want a smaller battery package you could use three AAA's.
If course you'd have to replace the batteries more often.
 

MrDEB

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Battery life hopefully will no be an issue
ten 10K pull up resistors = .0045ma
10 leds w/330 ohm resistors aprox =13ma if all ten are on which is pretty rare.
AA batteries = 2000mah 1 game lasts 4 hours. on average only 3-4 leds are on.
On my revised board I have the mosfet and utilizing the HLVD feature.
On my proto type build I measure no more than 25ma w/ all the leds on but have three different colored leds.
 

crutschow

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the data sheet says vgs = -.4 min -.6 typ. -1.5 max
how to determine if this component will work?? as per this example
http://digital-diy.com/general-elec...ser-look-at-the-p-channel-mosfet-circuit.html
For using a MOSFET as a switch it's not Vgs(th) that's critical it's the Vgs for a given ON resistance.
The data sheet for the P-MOSFET you selected shows the ON resistance as:
upload_2017-3-11_1-24-22.png
You can see that the ON resistance is quite small for a Vgs as low as -1.8V, so it will be fine with the 4.5V of 3 AA batteries.
 

shortbus=

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Wouldn't a regular diode work better? Just a hobbyist but never heard of a mosfet for polarity protection.
 

JonSea

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Read the link MrDEB posted twice, by yours truly. It explains the advantages, namely, very little voltage drop compared to a diode, even a Shotkey.
 

shortbus=

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Didn't read the link, thought it was just for a certain mosfet. Never saw one used like that before. Thanks
 
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