Help needed picking MOSFET for AA batteries in series - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: October 17, 2020, 08:45:45 am
content: I am looking to get a constant 3.0V - or less - out of two AA's in series. (Could also do two sets of AA in series, and put those in parallel, for a bit more headroom for output current).I figure AA in series will give me at the very most 3.3v and that is not too high above 3.0v for my concern.I don't want the AA batteries to be part of the circuit, since that seems unsafe, so I thought to add a MOSFET in between the batteries and the circuit.What i don't understand is picking the correct MOSFET based on drive voltage, obviously fully charged series AA's will be about 3.3v so it seems like that is a good drive voltage to choose, but how will I be certain that it's ideal for my situation and will continue to operate until the batteries are almost empty?what i do know is where the batteries would connect to my circuit, has a resistance of about 1.3 ohms. I see stuff like "Rds On (Max) @ Id, Vgs - 3.5Ohm @ 100mA, 4.5V" but I don't know if that means 3.5 ohm is the highest load it can handle at those specs (in which case my 1.3 ohms would be fine) or if I need to select a different part.

author: MarkF
author_rank: Super Contributor
timestamp: October 17, 2020, 09:06:05 am
content: Use the right tool for the job.It doesn't sound like you want a MOSFET at all; but, a voltage regulator.They come in many fixed voltages: 1.2V, 1.5V, 1.8V, 2.5V, 2.8V, 3.0V, 3.3V, 4.0V, 5.0V for example.Most MOSFETs are used as a switch: Fully ON or OFF.Are you trying to just switch the voltage ON and OFF?TheMCP1703series for currents less than 500mA.Also, theLM317is a variable regulator you can adjust.What is your desired voltage and current needs?

author: mzzj
author_rank: Super Contributor
timestamp: October 17, 2020, 09:09:01 am
content: 1.3 ohms load on 2x AA alkaline in series is going to operate only 15 minutes and battery voltage is not much over 1 volt per cell. Is that enough for you?

author: MarkF
author_rank: Super Contributor
timestamp: October 17, 2020, 09:22:04 am
content: Quote from: Mp3 on October 17, 2020, 08:45:45 amYou can think of RDS(ON)as the equivalent resistance of the MOSFET when it is fully turned ON.Therefore if the MOSFET is ON, it will appear like a resistor, with a value of RDS(ON), in series with your load.A nice refresher video might be helpful:

author: Mp3
author_rank: Frequent Contributor
timestamp: October 17, 2020, 10:26:57 am
content: Quote from: mzzj on October 17, 2020, 09:09:01 amHmm, not even close. I thought about using a cheap 2500mah~ 3.7v LiPo but i wanted to avoid having to add a LiPo charging circuit too.I guess i will go back to the drawing board and either use Li-ion or LiPo.

author: atmfjstc
author_rank: Regular Contributor
timestamp: October 17, 2020, 11:48:27 am
content: Just what sort of load do you have there that is 1.3 ohms? The windings of an electrical motor? For how long would you want it to run?

author: mzzj
author_rank: Super Contributor
timestamp: October 17, 2020, 01:16:23 pm
content: Quote from: Mp3 on October 17, 2020, 10:26:57 amNi-mh like AA Sanyo Eneloop would handle the load much better without hazards associated with the li-batteries.https://lygte-info.dk/review/batteries2012/Eneloop%20AA%20BK-3MCCE%201900mAh%20(White)%202019%20UK.html

author: Mp3
author_rank: Frequent Contributor
timestamp: October 17, 2020, 10:01:51 pm
content: Quote from: MarkF on October 17, 2020, 09:22:04 amThank you very much for this. Sorry for the late reply. Went to bed and forgot to hit "post".Let's say i was considering one of these batteries instead of some AA's, and they have a protection PCB built in, so i don't have to worry about the MOSFET specs so closely that i need it to regulate the battery voltage.https://www.ebay.com/itm/1000mAh-3-7V-lipo-rechargeable-Battery-503450-polymer-lithium-Li-Po-USA/224159441402What i want to do is have the MOSFET do the switching and nothing else so it sounds like it's the right tool for the job. I just don't want to have straight unregulated battery to the PCB or necessarily depend on the battery's own protection PCB.Though, I would be very happy to use rechargeable AA if that will get me at least an hour of operation.

author: Mp3
author_rank: Frequent Contributor
timestamp: October 17, 2020, 10:05:33 pm
content: Quote from: atmfjstc on October 17, 2020, 11:48:27 amYes there is a small motor involved. I am hoping to get approximately an hour of operation time. I understand that by ohms law I expect to draw anywhere from 1 to 1.5 amps off the battery, be it Li-po or Ni-mh.Is the motor only going to draw as much power as it wants or should i put a current limiting resistor in the circuit? I would be worried about maximum power from the battery getting pushed to it otherwise.