Issues with powering SIM800L via mosfet (IRF540N)

Hey everybody I’ve got som issues making my SIM800L module to power up correctly through a mosfet.

Parts:

  • 2x LifePO4 batteries in serie (6.4V 1800mAh)
  • 1x LM2596 DC Converter (Converts to 5V and able to peak at 3A)
  • 1x SIM800L (Might has peaks of 2A)
  • 1x IRF540N Mosfet (N-channel enhancement)

See attachment for the setup.

Whenever I use these batteries, converter (Converts to 5V) and SIM800L directly without the mosfet it will power up nicely and connects itself to the network. I am able to do communication as expected.

But when I am adding the mosfet (which I wanna use to completely turn off the SIM800L) it stops working accordingly. I get power issues. Quick blinking of the LED and never connecting to the network.

I expect that the SIM800 is not drawing enough current to startup the way it is supposed to be. But according to the datasheet of the IRF540N it should be able to draw 10A at 5V. So I am a little bit puzzled why it is not working.

I added a cap. (10uF, Vcc to GND) right next to the SIM800L but that didn’t make any difference.

Am I using the wrong mosfet for this kind of setup, or do I miss some theoretical knowledge and parts that inhibit this setup from working? Is the given continuous drain current not as straight away as I think it is?

Datasheet mosfet: https://www.infineon.com/dgdl/irf540n.pdf?fileId=5546d462533600a4015355e396cb199f

Other question:

I have a cheap bench power supply (CK3005DK, no specs online :S) that can supply 5A / 30V. But running at 5V it can only draw a limited amount of current (.7A if I was not mistaken). I was under the impression that a power supply could be limited to the voltage and draw more current if needed, but I guess I can’t get it higher then a certain amperage with a voltage limit of 5V. Is this correct?

Which power supply would be a good alternative to get something that can adjust both voltage and current without being related to each other so I can test 5V and let it draw 2A when needed. I don’t want to depend on my batteries for testing my prototype.

Kind regards,

Roy

I am interested in what you are doing this since this is similar to what I am planning this weekend.

The SIM800L and the like can be powered directly with a single Li-Io cell. The voltage should be no greater than 4.4 V which at maximum charge will give you 4.1 to 4.2 volts. You have got away with applying 5 V but it really is higher than it should be according to the datasheet..

I believe that the SIM800L breakout board already includes capacitors across the supply pins so adding another 10 microF would not make a difference.

You do not say how you are signalling the MOSFET. Do you have a gate resistor and how did you decide on its value?

I am interested in why you chose this particular MOSFET. Is it not a bit of a hammer to crack a nut and isn't the gain going to be pretty poor at 2A? I do not profess to be anything but a novice on power transistors by the way.

I expect you have overcome this issue by now so I would be interested to hear how you accomplished supply switching of the SIM800L.

Don't use that IRF540 mosfet. It's not a logic level type, and needs 10volt at the gate to fully switch on. Leo..