Calculating Base resistance value Of BC109 Transistor as a switch

[dsk2858]

Hi,
I am planing to use BC109 as a switch to control supply voltage of a sensor through a MCU.

The sensor specifications 5VDC and <10mA.

The Base input voltage is 3.3VDC from the micro-controller.

My question are?
1.How to calculate the Base resistance voltage.
2.Whether only a resistance added in the base is sufficient to control the sensor or should i added any other resistance.
3.what about the leakage current although there is no Base voltage applied would it be there or not ,if suppose i observe leakage current how should i troubleshoot it.

 

[enjunear]

You need to stop thinking about the transistor as a resistor... it acts like a diode, so best to think of it as one.

Here are your steps to analyze the circuit:
Need Ic = 10 mA (max)
Most BJT's need 0.7V across the base-emitter junction to turn on.

Looking at hFE (DC current gain) on the spec sheet, you'll see that a BC109 has a minimum hFE of 200. We know that Ic = Beta*Ib in the linear region (Beta is equivalent to hFE), so Ib needs to be at least 0.010 A/200 = 50 uA.

Now, if you know that the MCU will output 3.3V, and the base of the transistor will be 0.7V above ground (assuming the emitter is grounded), then you will have to drop 3.3-0.7 = 2.6V across a series resistor going into the base pin.

Now we know V and I for the base resistor, so we can use Ohms Law to find resistance. V=I*R... 2.6=50e-6*R, so R=52 kohms. This is the largest resistor that you should use (if you make R bigger, then the base current will decrease, which in turn decreases the collector current, which would starve the sensor of current). I would try using a 20k, 10k, or 5k resistor, to make sure that you drive the transistor into saturation (Vce ~0.2V), which makes it act like a switch (low voltage drop from collector to emitter).

As for leakage current, you shouldn't see any if your MCU can pull the base of transistor low and hold it there.

 

[dsk2858]

@enjunear

Thank you for your brief explanation . could you please also explain how to test the transistor with a multimeter.

 

[enjunear]

Some multimeters have transistor test ports, like this (see blue circle in bottom-left corner).

You will need more than just a single multimeter to test most parameters of a transistor, since it needs some signal on the base pin to make the collector-emitter path operate. It's a current-controlled switch, effectively, so it needs an input. Your best bet is to build a simple circuit, power it up, then measure the device's operating characteristics (voltage drop, currents, etc) with your multimeter.

What characteristics do you want to test?

 

[dsk2858]

Hi,
I tested the transistor it is working properly.

Here i attached the circuit diagram of what i had designed on a doted PCB board.

The BC109 transistors are being controlled by the MCU with 3.3v as input.

when the transistor is turned on i could measure 5v at the 1 and 2 terminals as indicated in the circuit diagram.

But when i turned OFF (Cut off 3.3v from MCU) transistor and i again measured the voltage across 1 and 2 terminals of the connector then i found fluctuations in the voltage from 3.3 to 4.9v. then i measured the base voltage i found it as 0v the MCU could control its voltage.
I even turned OFF entire power supply to the board and turned ON still it was same.

could you please help wit the problem.

 

[enjunear]

Why are you turning off the MCU and not just putting it into an idle state, and letting it keep the output pins driven to 0V?

If you are removing all power from the MCU, then it's outputs are not being held down at 0V... they are just floating, which is allowing enough energy to leak onto the base (from the energized collector, through the package parasitics) to turn it on. You need to drive the base low to keep it from turning on.

By putting your voltmeter on the base of the transistors, you provided a path to ground (there is a few megaohm resistor inside the voltmeter), so it could show 0V when probed, but float back up when you remove the meter's load.

If you have to power down the MCU, then I'd try using a pull-down resistor on the base circuitry to bleed off any energy that leaks in. Pick something on the order of 50k-100k, and put it on the node labeled "+3.3V from MCU". If you put it on the other side (connected directly to the transistor) it'll form a voltage divider with the 4.5k resistor and decrease the bias point voltage of transistor you're trying to keep in saturation.

 

[dsk2858]

Why are you turning off the MCU and not just putting it into an idle state, and letting it keep the output pins driven to 0V?

I mean that after driving the pins to 0v i could measure fluctuating voltage across the collector so then i turned off the MCU power and checked again at time also the the pins where to 0v but still i could measure voltage.

At last i could trouble shoot the problem there was a soldering mistake due to which the transistor got damaged.
when i replaced it with newer one then i could measure very little voltage across collector when i drive the pins to 0V.

thank you for ur detailed explanations and trouble shooting tips, and spending your valuable time for me.

thanking you again