I'm trying to choose the best values of resistors given the stock I have so that I can deliver sufficient power and current to an HM-TRP radio module.

In this circuit, I used the 100K resistor instead of the 8051 microcontroller because I'm trying to simulate the microcontroller impedance when the port is not set low in software. If I set the port low in software, then the left-most transistor emitter is grounded.

The main problem I have here is that when I used a simulator to measure current, Its registering between 197mA and 203mA, yet I want to provide the HM-TRP radio module enough current so it can function.

The problem with those current values to me is when I calculate wattage, it (according to the datasheet I found) exceeds the maximum value by approximately 25mW.

  • I can understand that if I exceed the wattage by a large amount, I might get a melted transistor, but what about by a small amount? Does it matter?
  • And what if the wattage is exceeded for under 100ms at a time?

For my circuit, the 15 ohm resistor is 1W, and the other resistor is 1/4 watt. Both are carbon film 5% tolerance. The transistors are both PN2222 and the 3.3V supply is fed from 5V (that directly feeds the 100K) through an LM1117-3.3 regulator.


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    \$\begingroup\$ 100mS is a measurement of conductance. I'm guessing you meant 100ms, a measure of time. \$\endgroup\$
    – Hearth
    Feb 8 '19 at 3:22
  • \$\begingroup\$ Yes I mean 100 milliseconds \$\endgroup\$ Feb 8 '19 at 4:05
  • \$\begingroup\$ Before I say any more, are you trying to programmatically control the 3.3 V power supply rail to your module using a 5 V I/O pin from your 8051 so that you can only power up the module for shorter periods of time to avoid wasting power when you don't need to? And is your attempt above trying to use a BJT as a shunt to turn it off and otherwise it gets its power through the 15 Ohm resistor? Or am I just completely confused about this? \$\endgroup\$
    – jonk
    Feb 8 '19 at 4:53
  • \$\begingroup\$ You're right. except that I need to turn the (radio) module power off then on programmatically via the GPIO pin in order for the module to spit out its initialization string. It does it no other time. So my idea is to make the transistor with 15 ohm resistor a short to ground if the GPIO value is logic high. \$\endgroup\$ Feb 8 '19 at 6:20

Your requirement to simply switch 3.3V and let it draw up to 120mA for Tx=20dBm.

The circuit you have shown will not do this.

You could drive the base with 10% of IC to achieve the rated Vce(sat) with a PNP, but then you drop 3.3 by thus much so it not so good a solution to reduce the Tx power as using a power logic level PFET with an RdsOn of 0.1 Ohm

You could also use a fixed 3.3V regulator instead that has an enable signal.

  • \$\begingroup\$ But do I really need that much current? The circuit I'm making is only meant to auto-setup the radio module, not transmit data or receive data to/from the air. \$\endgroup\$ Feb 8 '19 at 8:12
  • \$\begingroup\$ You only voltage with PNP switch and input to 2 to 3.3 V \$\endgroup\$ Feb 8 '19 at 8:17

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