Hello,
I need to monitor the battery level on one of my project but I would like to move away from the simple voltage divider solution since it unnecessarily drains some current. I only need to monitor the battery level every couple of minutes so a FET sounded like the ideal solution to quickly switch the fuel gauge on and off. The gate will be connected to a digital output and the source to an analog input of the Teensy.
I found this very nice blog here https://jeelabs.org/2013/05/17/zero-powe-battery-measurement and here is the interesting part:
"But there’s a catch: we need to be able to turn the N-channel MOSFET on and off, which means we need to be able to apply a voltage to its gate which is a few volts above the drain pin (the bottom one, attached to AIO). With a resistive divider of 10 + 10 kΩ on a 6V PWR line, that voltage will immediately rise to 3V, and there’s no way the DIO pin can keep the MOSFET on (it can only go up to logic “1”, i.e. 3.3V). The solution is to use a different divider ratio: say 50 + 10 kΩ. Then, a 6V PWR level leads to a 1V level on the AIO pin, i.e. on the drain of the MOSFET. With DIO set to “1”, that means the MOSFETs gate will be 2.3V above the drain – enough to keep it turned on."
I am a bit confused, I though the drain pin of an n-channel MOSFET was connected to PWR and the source pin to GND... Is it a typo ? did he mean "source pin, the bottom one attached to AIO" ? I contacted him and I am waiting for his reply.
I am having really a hard time choosing the right FET. I will use the 50k+10k divider as advised and I expect my battery voltage to go from 4.2v to 2.5v (absolute max and min). When I turn it on, the gate will see the 3.3v logic from the digital output, and the top pin sees at most 0.7V when the battery is fully charged (divider ratio 6). Does that mean that I need a FET with Vgs < 2.6V ? There aren't that many... most of them are rated 5V to 60V ! I must have gotten that wrong.
My understanding is that Vgs is the most critical characteristic when it comes to choosing a FET. I also want a very small Rds(ON) and ideally a small power dissipation if I want to go for a SMD package. Am I right ?
Thanks !
I need to monitor the battery level on one of my project but I would like to move away from the simple voltage divider solution since it unnecessarily drains some current. I only need to monitor the battery level every couple of minutes so a FET sounded like the ideal solution to quickly switch the fuel gauge on and off. The gate will be connected to a digital output and the source to an analog input of the Teensy.
I found this very nice blog here https://jeelabs.org/2013/05/17/zero-powe-battery-measurement and here is the interesting part:
"But there’s a catch: we need to be able to turn the N-channel MOSFET on and off, which means we need to be able to apply a voltage to its gate which is a few volts above the drain pin (the bottom one, attached to AIO). With a resistive divider of 10 + 10 kΩ on a 6V PWR line, that voltage will immediately rise to 3V, and there’s no way the DIO pin can keep the MOSFET on (it can only go up to logic “1”, i.e. 3.3V). The solution is to use a different divider ratio: say 50 + 10 kΩ. Then, a 6V PWR level leads to a 1V level on the AIO pin, i.e. on the drain of the MOSFET. With DIO set to “1”, that means the MOSFETs gate will be 2.3V above the drain – enough to keep it turned on."
I am a bit confused, I though the drain pin of an n-channel MOSFET was connected to PWR and the source pin to GND... Is it a typo ? did he mean "source pin, the bottom one attached to AIO" ? I contacted him and I am waiting for his reply.
I am having really a hard time choosing the right FET. I will use the 50k+10k divider as advised and I expect my battery voltage to go from 4.2v to 2.5v (absolute max and min). When I turn it on, the gate will see the 3.3v logic from the digital output, and the top pin sees at most 0.7V when the battery is fully charged (divider ratio 6). Does that mean that I need a FET with Vgs < 2.6V ? There aren't that many... most of them are rated 5V to 60V ! I must have gotten that wrong.
My understanding is that Vgs is the most critical characteristic when it comes to choosing a FET. I also want a very small Rds(ON) and ideally a small power dissipation if I want to go for a SMD package. Am I right ?
Thanks !