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I use a TIP32C transistor to switch the power line of a SIM800L module, but unfortunately this method isn't working. The module is powered on, but it only blinks once every second instead of every 3s. If I connect the power pin of the module to Vcc it works fine. Why?

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Velvel
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DJDB999
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    Decrease the value of R1 to 470 Ohms and tell us what happens then. 10k is too much. – Jonathan S. Feb 04 '22 at 19:58
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    Probe the input power of SIM800L when you power on the transistor. How much voltage does it give? How much is your VCC voltage? How much is your GPIO high level voltage? – Christianidis Vasileios Feb 04 '22 at 20:32
  • Is there a possibility of replacing your transistor with a voltage regulator, and controlling its enable line with the grid instead? – Colin Feb 04 '22 at 21:01
  • VCC voltage is 4V and gpio high voltage is 3,3V, but in active mode the gpio is 0V because it is a pnp transistor – DJDB999 Feb 05 '22 at 08:07

1 Answers1

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You have not told us how much is your VCC and how much voltage do you apply on the base of the transistor.

Your SIM800L is not getting enough voltage and/or current to run. The TIP32C has a dropout voltage as all transistors do, in this case 1.8V.

Vbe transistor drop voltage

If you need to power the SIM800L with 5V, you will need to have a VCC Base voltage of 5V+1.8V=6.8V and a VCC voltage of >=6.8V.

You might also need a lower-value resistor at the gate. You have a maximum current gain of 50 (lets say its 50 in your case, you can be sure how much the gain is for your current/voltage from the graphs on the datasheet).

current gain

So if your SIM800L draws 50mA, you will need to give 1mA at the input of the transistor, so if your GPIO high level is 5V, you will need a maximum value resistor of $$ R=5/0.001=5Kohms $$

Christianidis Vasileios
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    Vce sat is more relevant than Vbe here, its about 150 mV at 0.5 A – Colin Feb 04 '22 at 21:04
  • VCC is 4V and Base voltage is 3,3V – DJDB999 Feb 05 '22 at 09:04
  • What type of transistor should i use? – DJDB999 Feb 05 '22 at 09:27
  • What do you want to achieve? you have not stated what do you want to do. Why do you want to swtich on/off your device via transistor?\ – Christianidis Vasileios Feb 05 '22 at 09:35
  • I use it with esp32 for SMS sending once a day. So if i not sending SMS, i want to switch of the SIM800L. – DJDB999 Feb 05 '22 at 10:15
  • @ChristianidisVasileios You are right, i choose too large resistance. with 50 ohm its works fine. Thank you. – DJDB999 Feb 05 '22 at 13:51
  • @djdb999 with a 50 ohm base resistor your gpio is sinking about 65 mA, I don't know the ESP32, but most GPIO pins on processors are only specced to 20 mA. Try 220-470 ohms. – Colin Feb 05 '22 at 15:47
  • @ChristianidisVasileios Okay, but i want to use IRLR024N logic MOSFET instead of transistor. How can I calculate the charge time? For example with 3,3 volt and 200 ohm in Gate. This is important because the SIM800L module have 500microsec 2amp peak cureent in the beginning. In this case enough one MOSFET or I need transistor to turn on the MOSFET? – DJDB999 Feb 06 '22 at 09:54
  • @DJDB999 this is a entirely new question, make a new question to the forum. – Christianidis Vasileios Feb 06 '22 at 13:07