Transistor Switch
Transistor Switch
(OP)
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?
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?





RE: Transistor Switch
TTFN
RE: Transistor Switch
RE: Transistor Switch
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.
RE: Transistor Switch
TTFN
RE: Transistor Switch
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Transistor Switch
TTFN
RE: Transistor Switch
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.)