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[SOLVED] PNP Transistor to control Tx module not working

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Junior Member level 3
Aug 26, 2011
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Hi Everyone,

I have a Tx module on 433 Mhz. I want to use this module in a project, however I want maximum battery power, so I would like to be able to turn off Vdd to prevent Tx module quiescent current. The project runs on 2xAA batteries

I am using an MCU output to turn on an NPN, which then turns on the PNP as it's collector load.

The problem is that when the PNP is on, the voltage is there, all seems ok, but the Tx module will not work properly. I have used a 10K from the MCU to NPN and again a 10K from the PNP base to NPN collector.

I have removed the PNP and shorted the collector and emitter connections with a wire and all works perfectly!!

Do I need a special type of transistor to be able to effectively switch on/off an Rf load?

I have tried variants of PNP transistors but they are cheapy 2n3906 equivalents pretty much.

Any help would be much appreciated :)

Show use the specification for the 433MHz module and which MCU are you using.

If it's an AM module, typically the data input is the bias supply to the oscillator so it doesn't draw current if the input data is held low when idle. For FM modules it may be different, I use FM modules powered directly from the IO pin of PIC MCU, the pin can output ~20mA but the module only uses about 5mA.


Your ~20mA may only be average and not peak.

To support 100mA peak current, Assume hFE drops to 10% or Ic/Ib=10 for rated Vce(sat)

Since 1st NPN stage does not saturate , assume hFE=100
2nd stage PNP saturates assume hFE=10 worst case

Thus to support PNP output = 100mA peak, Ib2=10, Ib1=100uA
For 0.2V=Vce2 R load ~29 Ohm for 100mA peak
Rb series to PNP= 220 Ohm
Rb series to NPN= 22K

Not ratios 22k / 220R/ 29R ratios are 100, 8

As Vbat increases Vbe can be nelected then, and it approaches 100,10 for R ratios with respect to Load worst case min dynamic R

You might not have scoped the Vout when it was at peak current.

If not 100mA or 20mA but something in between, then calc worst case Rload and use the ratios of 100,8 x Rload for Rb1 Rb2


electronics has to do with voltage and currents.
These are the most important values we need to know.
Supply voltage, Tx supply current.

It seems you have about 200uA base drive current. Maybe too low.

Maybe you have wrong schematic, or wrong port setup.

Provide all data so we can verify your circuit.

Last edited:

Hi All,

Thanks for all the quick answers :D I will take your advice SunnySkyGuy and give the transistors a little more current and see if that does the trick :) The module I am using is here:,31

Yes I know it is a cheapy chinese one but I have them lying around and want to use them :D

The MCU I am using is a PIC16F723, so it can supply enough current to the base of the NPN if required. I'll post my results tomorrow. Once again many thanks :D :D

- - - Updated - - -

One other thing, notice they haven't quoted a quiescent current? When I have one connected to a circuit it drains the battery when left idle, that's why I decided to toggle the power using the transistors.

One other thing, notice they haven't quoted a quiescent current? When I have one connected to a circuit it drains the battery when left idle, that's why I decided to toggle the power using the transistors.

Before making things complicated, try measuring the current again with the DATA pin tied to ground, my experience of these kinds of modules is the data signal provides the bias to the oscillator stage and without it the module draws virtually nothing. It is quite possible the DATA pin has a pull-up resistor on the module so it would remain in transmit condition if the pin was just left disconnected. If the current does drop to near zero, tweak your software so the serial data line driving the module is held low when not in use.


Hi All,

I have changed the npn base resistor to 4.7k and the pnp base resistor to 1k, everything works fine and the range is as expected.

Many thanks for everyone's help :D

It seems so stupid now I know what's wrong!!! More base current = more output power :D

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