Please look at the attached schematic and the Fairchild mosfet datasheet attached.
Normally, the mosfet would be tied directly to ground and the Picaxe would have a full 5V logic swing to control the mosfet. However, I want to measure the current flowing through the circuit, and I need the measurement to be referenced to ground, so I’ve put a current limiting/measuring resistor that drops exactly 2.0 volts at 13 milliamps of current (which is my selected maximum) between the source of the mosfet and ground. Thus, the gate of the mosfet is also referenced to a voltage above ground so the Picaxe has reduced congtrol of the mosfet gate.
Feedback to the Picaxe via an ADC pin allows the Picaxe to maintain the current at the level specified.
I have selected a mosfet (Fairchild BS270, datasheet attached) which I believe is a good choice to use in this situation where available gate swing is much less than 5V. Could someone confirm that my choice looks right? Or if I’m missing an important consideration and this won’t work, better to know it now before I go off on an unworkable tangent. Thanks in advance.
Normally, the mosfet would be tied directly to ground and the Picaxe would have a full 5V logic swing to control the mosfet. However, I want to measure the current flowing through the circuit, and I need the measurement to be referenced to ground, so I’ve put a current limiting/measuring resistor that drops exactly 2.0 volts at 13 milliamps of current (which is my selected maximum) between the source of the mosfet and ground. Thus, the gate of the mosfet is also referenced to a voltage above ground so the Picaxe has reduced congtrol of the mosfet gate.
Feedback to the Picaxe via an ADC pin allows the Picaxe to maintain the current at the level specified.
I have selected a mosfet (Fairchild BS270, datasheet attached) which I believe is a good choice to use in this situation where available gate swing is much less than 5V. Could someone confirm that my choice looks right? Or if I’m missing an important consideration and this won’t work, better to know it now before I go off on an unworkable tangent. Thanks in advance.