Those FETs are not ideal, you really need logic level FETS. While the will do some switching with only a 5V gate they are not fully on.
I hope the reset line you want to pull is not on the same processor doing the switching as that will not work, the data sheet says so.
Grumpy_Mike:
Those FETs are not ideal, you really need logic level FETS. While the will do some switching with only a 5V gate they are not fully on.
Do you mean none of the 4 MOSFETs could be MMBF170 ? Do you advice "logic level" FETs for LEDs and commands, for one of the two problems ? What SMD MOSFET would you advice for my schematic ?
General question: why those FET will not be "fully on", the datasheet says that the Gate Threshold Voltage is (min 0.8v) < 2.1v < (max 3v) ?
Grumpy_Mike:
I hope the reset line you want to pull is not on the same processor doing the switching as that will not work, the data sheet says so.
Those are not the same processor: I want the SIM900 module to be resetable (using NRESET_CMD) by the ATmega.
the datasheet says that the Gate Threshold Voltage is (min 0.8v) < 2.1v < (max 3v) ?
The gate threshold is the point where the FET starts to conduct, not the point when it is fully turned on. Therefore with only 5V the resistance of the FET will be 3R when conducting 800mA. See figure 2 in the data sheet. This may be good for you but the FET will be dissipating more power than it would with a higher gate voltage. I suppose for just turning on a single LED it will do. You can also see from figure 2 that 10V urns on the FET fully. It depends on the reset line if a few ohms to ground makes a diffrence, however on reflection I suspect it will not.
Therefore conclusion - yes in the limited sense you are using it you will get away with less than an ideal LED.
As to a resistance, when the FET turns on it is effectively charging up a very small capacitance which appeared like a short, thus exceeding the maximum current rating of the pin. Normally the way round this is to put a 100R resistor in series. However, again as you are not doing rapid repeated switching then you will be fine connecting it directly.
General question: why those FET will not be "fully on", the datasheet says that the Gate Threshold Voltage is (min 0.8v) < 2.1v < (max 3v) ?
Because Gate Threshold Voltage is a often missunderstood parameter. It's not the gate voltage where the device is fully saturated on, but rather the gate voltage where the source/drain just starts to conduct current, perhaps just milliamps of flow.
There are graph drawings in most mosfet datasheet showing the actual source/drain current value Vs applied gate/source voltage applied. So while those mosfets you have selected might work on low current circuits it really is best to select the mosfet for the gate voltage it will be working with. So for arduino +5vdc digital output pins one is always better to select a LOGIC LEVEL mosfet that will insure full saturation current flow up to the devices maximum rated current flow with +5vdc applied to the gate.
Grumpy_Mike:
the datasheet says that the Gate Threshold Voltage is (min 0.8v) < 2.1v < (max 3v) ?
Therefore conclusion - yes in the limited sense you are using it you will get away with less than an ideal LED.
As to a resistance, when the FET turns on it is effectively charging up a very small capacitance which appeared like a short, thus exceeding the maximum current rating of the pin. Normally the way round this is to put a 100R resistor in series. However, again as you are not doing rapid repeated switching then you will be fine connecting it directly.
Grumpy Mike, I think I understood. Do you think BSS131 will match better my need ? I attached the datasheet below.
About the resistor, I modified my schematic and joined it also to this message. Does it match you meaning (I also added a 10k resistor to avoid floating gate) ?
Yes those circuits look fine. While the new FET is a logic level one the on resistance is higher than your original FET so both will do but the first is better.
Grumpy_Mike:
Yes those circuits look fine. While the new FET is a logic level one the on resistance is higher than your original FET so both will do but the first is better.
Would you have a logic level MOSFET that would match my need to advice to me ?
