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However, if your load current was more like 100mA, a 3V gate-source voltage might only "lose" 0.5volts across the device.Free Download SoftPerfect NetWorx 6 full version standalone offline installer for Windows, it is a powerful tool that helps you objectively evaluate your bandwidth situation. Try finding a device that shows the same graph with gate voltages down to 3V and look for no more than 100mV lost across the FET when 1A is flowing.Ī BS170 FET is going to be worse if you want 1A: -Īt 1A you'll need at least 6V to drop maybe 1.8V. Clearly if you could supply a gate drive voltage of (say) 10V then at 1A drain current the volt drop will be about 100mV. As you can see, with 1A flowing you are losing 1V across the MOSFET and this isn't what you should be aiming for. I've marked in red what the volt drop across the MOSFET is at a current of 1A flowing when the gate-source voltage is 4.5V - they don't specify any less than 4.5V so this is also a big clue that you should look for a device with much smaller Vgs(threshold). This means at 3.3V it may be only just starting to turn the MOSFET on fully:.
The IRF530 is a 100V rated device and it has quite a high gate-source threshold voltage (2V min to 4V max). Try 10kohm or 100kohm.ĮDIT NOTE - originally the question showed an IRF530 in the circuitģ.3V may be a little too lightweight for the the MOSFET you've shown in the diagram. The gate-source pins are effectively a small capacitor with very little ability to drain away charge / voltage fed to them when the switch is closed. Also you need to put a resistor across the gate and source to leak away charge once the switch has open circuited. You should have inserted the N channel mosfet in the ground wire to the load with source to ground and drain to the positive rail connected load. In this particular case it's appropriate to use a p channel mosfet switching the positive rail to the load and to do this requires an NPN BJT to interface between the gpio and the mosfet's gate. In short, look at the graph of drain current versus drain-source voltage and choose a device that has the lowest volt drop for the gate voltage you are able to provide. Is this schematic angood way to go? It is a problem that GPIO provides 3.3V and maximal V_GS is 3V for N-FET and (minus) 2.4V for P-FET?įollowing advice from Dave Tweed it is apparent that switching a USB load via the ground rail is not good so, what remains of this answer is all about choosing a mosfet and checking the appropriate graphs in the spec for the device. I did some more search, and I found a schematic by bretth on, which seems to do what I want, using a P-MOSFET instead of N-MOSFET. I can buy a new one (even couple of them, they're really cheap), but I would prefer to know what to do before I experiment again or cut some more wires. I think I demaged the transistor I bought. The official gate voltage is 3V and I supplied 3.3V to it.According to a scheme on RPi's elinux page, I should have cut the GND wire instead of the PWR one.I'm not sure that the two grounds are actually connected.(I use a speaker in the schematic to represent my USB device, and a switch to represent a pin of the GPIO.) Simulate this circuit – Schematic created using CircuitLab I cut the power wire of the USB cable, and connected the MOSFET to it in the following way: I need to get 500mA to power a GSM dongle the voltage drop should preferrably be minimal since USB is quite strict on voltage). On the RaspberryPi.SE it was suggested that one can use MOSFET BS170 to switch a USB device on and off by a GPIO pin (3.3V).
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