A little help on how to switch transistors

[Vince Tangcalagan]

Hi guys! I am currently making a project using an arduino. In this project, I will be using transistors to multiplex 7 segment displays. I know a little about transistors. But I am having a problem reading the datasheet. I'd like to use the transistor (specifically 2n2222a) as a switch but I don't know how to choose which Beta to use from the datasheet. I know that in order to saturate the transistor IB > ICsat/Beta, but how should I know beta from this datasheet:


Which beta should I use to base my calculations? Thanks!

 

[kam1787]

what frequency will you switch at?
what current ?
what Voltage?

 

[chuckey]

Work out your Ic, then you can find out what the B is. divide by 20% and use this value. it is unlikely that you will over dissipate the transistor driving a seven segment display (Iseg = 20 mA?), so providing Vce sat is less then a volt or so it should be good to go.
Frank

 

[FvM]

You see that the saturation voltage specifications are made with forced Ic/Ib = 10 ratio. Providing 1/10 of the intended Ic is a safe value if you don't need to be tight with base current, otherwise chose a higher value which is clearly below minimum B for the respective operation point.

 

[Audioguru]

I doubt the LED/transistor current is only 20mA because they are multiplexed. Multiplexing causes PWM dimming so the peak current must be pretty high to make enough average current.

As you can see from the specification for saturation, beta is not used when a transistor is used as a switch. Beta is used when a transistor is a linear amplifier with PLENTY of collector to emitter voltage (the datasheet shows at least 10V which is far from saturation).

 

[asdf44]

So a quick way is to just take the lowest one, in this case 35. If you want to switch 35mA provide at least 1mA of base current and you'll be ok.

For your application I'm guessing that's fine. In practice a 1k-4.7k resistor from the arduino to the base will provide this easily.

If you're switching more current than that you may have to put a little more thought into it and logic level FET may be easier in that case.

 

[Audioguru]

So a quick way is to just take the lowest one, in this case 35. If you want to switch 35mA provide at least 1mA of base current and you'll be ok.
For your application I'm guessing that's fine.

No.
Then only some transistors will switch fairly well and other transistors will switch poorly with a high saturation voltage loss. Don't you want every circuit to work properly?? Or only some of them?

All the transistors are guaranteed to saturate at the maximum loss shown on the datasheet when the base current is 1/10th the collector current (not 1/35th).

 

[asdf44]

No.
Then only some transistors will switch fairly well and other transistors will switch poorly with a high saturation voltage loss. Don't you want every circuit to work properly?? Or only some of them?

All the transistors are guaranteed to saturate at the maximum loss shown on the datasheet when the base current is 1/10th the collector current (not 1/35th).

Specifically you're talking about "Collector-Emitter Saturation Voltage" which is specified for 0.3V at IC = 150 mA, IB = 15 mA and 1V at IC = 500,mA IB = 50 mA. Which are both specifying IB which is 1/10th of IC. Correct?

Fair enough. The point is that the target is to provide enough base current to guarantee that your load current can be supplied. Plus some margin. The numbers you cited are a better indicator of that margin.

 

[Audioguru]

The 2N2222A has a maximum allowed collector current of 800mA. I think it works poorly at only 500mA when its base current is 50mA and its maximum collector-emitter saturation voltage loss is a whopping 1V.