How to pick the right transistor?

snipped-for-privacy@gmail.com wrote in news:1155358462.361995.161140 @i3g2000cwc.googlegroups.com:

Picking transistors and values for components connected to them is an advanced electronics skill. Some people go to college and get an engineering degree to learn this.

Best for you to start by building circuits already designed by someone else, using schematics you find in books or magazines or by Googling the Web. (LOTS of them are available! You can find a circuit to do just about anything you can think of.)

Once you've built some circuits and find they don't work right off, and then got them working the way they're supposed to, you can start understanding how these circuits work and why the components have the values they do. You'll see why a certain transistor works well in a partcular kind of circuit, and can try using it in a similar circuit.

And you can ask some intelligent questions about circuit design here.

My particular situation is that I need to find a transistor that can drive a 220mA load with a very high current gain, I was thinking maybe a darlington pair, but there is just too many too choose from.

OK you have one paramater (current) and have some idea what gain you need, presumably you also know what voltage it has to stand off - then you'll just have to pick one of the hundreds that will work. As likely as not, there won't be one transistor that will work, but many. There probably won't be one that is clearly the best, unless your application is tough.

What is the application?

Don't rule out mosfets. They are voltage operated rather than current driven - hi input impedance translates to high gain since they take pico amperes to turn on. (but many require about four volts of bias to turn on if that is a concern) Very efficient in lots of applications.

A mosfet that can switch 60 amps may have an "on" resistance of a few hundredths of an ohm so it won't drop much voltage. The trade off comes when you need to switch high voltage - then the "on" resistance for a HV part is higher. At some point it makes more sense to use a bipolar or an insulated gate bipolar (best of both worlds for some applications)

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These are a few of the popular transistors for high gain NPN in small signal low voltage applications . . . Popular= common and low cost

Check out the 2N7000

It is a handy mosfet that can directly replace an NPN transistor in lots of applications - with lots of gain. TO92 case

Takes about 4 volts to turn on with a few microamps of current - pulsed operation with 2 amps of drain current and it takes 8 volts to turn on

\\ (propaganda)

(data sheet)

From the propaganda: They can be used in most applications requiring up to 400mA DC and can deliver pulsed currents up to 2A. These products are particularly suited for low voltage, low current applications such as small servo motor control, power MOSFET gate drivers, and other switching applications.

and they are very inexpensive

The other old standby for high gain NPN is the MPSA13 darlington

DC gain of 5,000+ collector current of 500 ma

inexpensive and very common TO92 case

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Thanks default, I will look at the links you offerd. You mentioned using MOSFETs, I may not be able to use these because I need to drive two different base(gate) with less than 25mA from a PIC Output signal that also goes through a voltage drop (due to a diode) to about 3.8v. So the MOSFETS, without a transistor amplifier, are out of the question.

In hobby circles, generally a small selection are used. People pick what they are seeing in the other articles in the hobby magazines, and those types are reinforced by what the hobby outlets carry. So somehow decades ago, someone started using a 2N2222, and the hobby outlets decided to carry it (it being a recognizable device), which fueled it's use in further projects.

So traditionally if someone was starting from scratch rather than copying an existing circuit, they'd go through the relatively short list of devices that could be bought easily, checking the specs if they didn't immediately notice one that stood out for their purpose, until they found something suitable. Sure, there were loads of other devices available, but maybe not in small quantities or without requiring lots of gyrations.

I suppose that has changed with places like Mouser and Digikey, relatively accessible to the hobbyist but a fairly full line catalog.

Michael

I would be looking for a transistor with an advertised maximum current

3 to 10 times higher than such a requirement, since most transistors hit their peak current gain at 1/3 to 1/10th of maximum rated Ic (collector current). But if you need highest gain, you might look at transistors specifically designed with this characteristic, like the ones made by Zetex. You don't mention whether you need PNP or NPN, the highest collector voltage the device must withstand, whether you are switching 220 mA or linearly amplifying at that current, the signal frequency (or switching speed), how important low on state voltage drop is (if switching). But if you are switching, remember that darlingtons always drop at least 1.2 volts when on, whereas simple transistors might saturate down to a small fraction of a volt when on, lowering the on state losses.

Here are a pair of random Zetex data sheets to give you some idea what you might find from them:

a curve you should think about (for switching) are VCE(sat) (collector to emitter saturation voltage, with a specified ratio between collector current and base current up to 50/1 or a base current of 4.4 mA for your load). Both these transistors keep the saturation voltage well below .1 volt at 220 mA and 4.4 mA base current. If you can tolerate a little more saturation voltage, you could use less base current.

Also, look at the current gain curves versus collector voltage (Hfe vs. Ic) to see that the peak gain is well below rated maximum collector current.

Digikey carries these and many more, and their search engine can be used to narrow down the choices by polarity, collector voltage and current ratings and package, before you dive into data sheets.

You choose a popular one !

Easily recognised from the stock levels held by distributors like Mouser etc

Graham

At 25mA, the PIC outputs probably won't be 4.5V (assuming a 5V supply, about which I'm also not sure.) My guess is that they will be about

25mA*70ohms or 1.75V less than your supply voltage before they meet your diode. (Actual impedance will vary some and you need to be aware that you cannot drive all pins at the same 25mA, as there is another spec that provides an absolute maximum [don't get too close to it] that limits the total port output.) Or were you thinking that it was a diode there in the PIC to explain the 3.8V output?

Anyway, I think we understand the following:

Drive signal: PIC output Number of circuits: 2 Mode of operation: switch (unless I assume far too much here) Load current: 220mA Load impendance: unknown, but may have significant L, if relay Load voltage rail: unknown PIC Voltage rail: ??, but either 4.5V (calculated from your assumed diode drop and 3.8V mentioned) or else 5V (blindly assumed as "normal")

Could you expand the above to provide more precise information? A highly inductive or highly capacitive load may require other caveats as compared to a purely resistive load. Also, are you planning on connecting the load between ground and the 'switch circuit' or between some power rail and the 'switch circuit'?

Assuming the above doesn't stray too far from your situation, one note right now is that you should want to keep your PIC drive current as low as is reasonable, while being able to reliably drive the switch circuit. I'd target _much_ less than 25mA as reasonable, here. Something on the order of just a few mA. I don't like the idea of just seeing the maximum spec of 25mA and merrily planning on using that as an option.

Jon

Not trying to push mosfets - but I find they can do some interesting things at times and save stages of amplification - you have 25 ma of drive available from the pic, but because the mosfet only needs a microamp it may work because the voltage out will be higher for lower current loading.

As some respondants already pointed out - you tend (should) use what everyone else uses. It is proven, it is in good supply, and is cheap

- and the manufacturers get better at making them - the stated spec might be 60 volts, for instance, but you may find they all can go to better than 100 volts and keep working..

Some good ones to use if you can:

2N3906, TO92 PNP 2N3904, TO92 NPN 2N2222 . . . 2N2222A, TO18 metal, PN2222, TO92 NPN 2N2102, TO5 metal NPN 2N2905, TO5 metal PNP 2N3055, ancient and ubiquitous for cheap NPN TO3 metal power device with a host of work alikes with better specs . . . 2N3771, 2N3772, 2N3773 2N7000 mosfet, TO92 MPSA13, darlington, TO92 NPN TIPXXX TO220 - and other cases - large selection - both darlingtons and ordinary bipolar power devices Both Sexes IRFXXX TO220 ditto

It does look like TO5 and TO3 transistors have/are gone/going out of favor for cheaper plastic parts Like the TO137, TO147, and TO220 etc.. When in doubt buy the cheapest part that will work or see what others use.

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Thanks for replying. Using transistors, I was only planning on using at most 6mA of the 25mA. I dont have the actual spec sheet of the PIC, the guys I bought it from put a dot over the P/N of it. However they didnt say 25mA Max, they just said 25mA, so maybe it is a nominal output. The loads I am driving with the transistors arent large, probobly no more than a relay load, around 150mA. However I also wanted to drive a MOSFET from one of the transistors to turn on/off a 556 PWM circuit. I was thinking using the 2N4401, someone on another group suggested it to me. The thing is that I am not totally sure because I am a little weary about the math behind choosing a transistor.

I bet you are not the first person in the world to want to interface a PIC. Have you looked for circuits that already do what you want to do?

snipped-for-privacy@gmail.com wrote: (snip)

Lets take a look at the data sheet:

The collector saturation region graph on 6 shows that the collector saturation voltage falls below .6 volts for a base current of about a milliamp, and the DC gain curves show the gain falling only a little between 100 and 200 mA, so you can expect this sort of saturation voltage at 200 mA with about 2 mA of base current. If you can afford

4 or 5 mA of base current, you should have some margin for a low gain transistor.