Transistor Switch

[321Mark]

Is there a way to calculate the ideal resistances required to properly design a simple transistor switch? Let's take an NPN low side switch with the emitter grounded. The circuit has an input resistor (Rin) from an external input and a resistor from the base to ground (Rb) to hold the transistor OFF if the external drive signal is open circuit. What is the proper method to choose the values for the resistors.

For an example, lets use a 2N2222A with a minumum beta of 50 and we want to fully saturate the transistor to drive a relay requiring 500mA (5 volt relay)
The input driving the transistor can be a CMOS HC output that is either 0 or 5 volts. Now there are many ratios for Rin and Rb that will work (within reason), but is there an ideal set of values that are optimal?

 

[IRstuff]

I'm not sure what you're asking. You pick a WWC beta, like 25, which means you need a minimum of 20 mA of base current. Add in the current needed by the base shunt. Subtract out Vbe from Vcc and calculate the input resistor.

TTFN

 

[321Mark]

I've heard of a method for selecting the shunt resistor. I had something to do with the leakage current of the transistor. Anyone have any information on this?

 

[Comcokid]

I frequently use this when the signal to the base goes through a connector and I want to make sure the transistor is off even in the presence of strong EM fields.

I generally just pick values from previous experience. Sometimes, I will select the base series resistor and base shunt resistor values as a voltage divider to require an approximate minimum input voltage to make the transistor switch.

 

[IRstuff]

Unless you've got a crappy transistor, its Icbes should be minimal and rarely enters the considerations. Nonetheless, that is one approach. You can choose the resistor to sink 0.1 mA at Vbeon, which would call for a 70 K base resistor.

TTFN

 

[Skogsgurra]

That consideration (transistor leakage) was common some 50 years ago when germanium transistors were en vogue. The base to ground transistor then usually was taken to a positive voltage (most germanium transistors were PNP with a negative Vce). You should not need to worry about that with silicon transistors. As IR said: "Unless you have a crappy transistor"

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

Oops, that should have been 7K

TTFN

 

[jimkirk]

There are so many different issues and parameters that one might want to optimize, that there really is no single ideal value.

For example you would need different values depending on whether you need to minimize the power dissipation in your circuit versus minimize the switching time of the transistor.

You'd need different values depending on the specific transistor type (say a 2N2222 versus a 2N3055), temperature range your circuit need to operate over, et cetera.

On the other hand, for many applications, it really doesn't make a whole lot of difference. In IRstuff's example with a 7K resistor sinking 100 uA just in case the transistor was exceptionally leaky. If you also used a bunch of 4.7K resistors elsewhere in your design, why specify yet another value in your bill of materials? Does the extra 50 uA really matter? (If it does, your design is probably marginal anyway.)