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[Daisy Hughlett]

Ihave been trying to use raspberry pi to use as a switch to turn on a motor. However I realized that I could not use PN2222 as the current limitation IIUC is less than a few hundred mA, much less than 1.5A. So I have been trying to get a new transistor but have been very lost on reading the datasheet. After much research, I think I have the basics, but wanted to double check from the experts here whether what I am doing is not going to blow anything up.

You are quite correct in your calculations, the 4V Vce is important, it means you need a 16V supply or else your motor will only see 8V and run slow and under power. This lost power will heat up the transistor at 4V x 1.5A = 6W so require a heat sink. The current will likely be a bit lower as the motor is getting less than 12V but not neat 480mA

To get the full current even with the 16V supply the low hfe means that you need to find more gain and this is usually done with a logic buffer to give you the extra drive or an extra transistor to amplify the logic output.

As others have suggested a logic drive MOSFET is a strong contender as long as you are not trying to switch it at high speeds to control the motor speed, this will cause heating in the MOSFET if you do not make use of more elaborate gate drive control.

While starting out in an effort to minimise the risk of having motor voltage reach your controller (and for general galvanic isolation for a lot of reasons) I would recommend a relay as well. The relay you can usually drive with a single transistor and it would be selected to drive the motor with a safe margin.

Remember that your motor starting current may be much higher than the rated running current and if your transistor or relay contact are rated too low you may have regular failures. A 5A rating would be a happy margin.

EDIT:

There are also darlington transistors available and these can be used but will have the same high Vce saturation voltage. Your load of 12V and 1.5A is often these days handled with MOSFET or relay when using microcontrollers.

I was about to post a question asking for a schematic critique, although while checking things I've noticed a few things on the transistor datasheet that strike me as being a little strange. The datasheet in question is for an NPN, TO-92. The second page states the electrical characteristics, and they're making things a little harder to understand how transistors actually work.

The Vceo and Vces are marked down as minimum values, which would seem to imply that there must be a minimum voltage between the collector and emitter of 45V to 50V depending on the total current passing through the collector. Should these two values be marked as maximum rather than minimum?

Similarly, the Emitter Base Voltage, as I understand is the maximum voltage that can exist between the emitter and base, assuming that the emitter ended up with a positive voltage in reference to the base voltage. So again, should this be marked as maximum rather than minimum?

Finally, the DC Current Gain has a minimum of 100, but this appears for an Ic of 100mA. Am I able to assume that I'll always have a current gain of at least 100, irrelevant of Ic? If not, how am I to know the minimum current gain if I'm not passing in 100mA?

EDIT:After looking at a different transistor, I've found the 2N5551 from Fairchild Semiconductors, and the respective datasheet. It's far easier to understand, as the parameter names are far less ambiguous. For example Collector-Emitter Breakdown Voltage etc, the keyword being breakdown in the name. Also, they also provide a spice model and plenty of graphs. Seems there's a large difference in the quality of datasheets.

It's saying that the transistor can be operated up to 45V. This has to be specified as a minimum value because if it was specified as a maximum value then you wouldn't know the lower limit that might cause it to fail.

On the other hand if the car manufacturer specified acceleration from 0 to 60mph as 4 seconds, you'd want that to be a maximum value i.e. you can always guarantee to do 0 to 60 in 4 seconds (max). Think about it.

Regarding hFE, use a better transistor that has a full spec. The spec you linked doesn't even have a part number other than the generic (but suspicious) name "T0-92". Here's what the current gain for the BC847A transistor looks like in its data sheet: -

Usually the columns min/typical/max on datasheet refer to the spread of the mentioned parameter among a manufacturing batch. This means that, for example, Vceo is guaranteed to be at least 45V for any specimen in the batch (at the given conditions).

In other words, if you buy 100 such BJTs and determine their Vceo you'll discover that each one has a different value, but all the values will be at least 45V. Could they also list the maximum value? Sure! But that won't be useful in designing a product: who cares if one out of 100 BJTs has a Vceo of 60V? The designer need to know what's the guaranteed minimum value so that its design doesn't break when a random sample is placed on the PCB!

The confusion stems from the use of the words "minimum/maximum". Here you use it twice, with different meaning: Vceo as reported is the minimum maximum Vce value, i.e. the minimum value (among the batch) that you can get for the maximum allowable Vce voltage.

On the other hand, Icbo is reported as a maximum value, because that is the maximum guaranteed value among any sample in the batch. Why not reporting the minimum? Again, the relevant parameter for reliable design here is the maximum: Icbo is a leakage current, ideally you would want it to be zero! So when you design a product the worst case scenario is when Icbo is big, hence you need a guaranteed maximum value.

As for hFE, in most design it is useful to have a guaranteed minimum value, because most of the time you use negative feedback circuits which work well as long as the gain is at least a specified value. Here they report also the typical value, since this gives you an idea of the parameter spread among the batch. In the specific case you have min 100 and typical 400, so you could guess that for 100 samples of that BJT most of them will have an hFE around 400, with a minimum of 100 guaranteed and, by symmetry probably you won't have samples with hFE greater than 700=400+(400-100).

Imagine you (using a curve tracer) increase the collector voltage (with zero base current) until the collector current is 1mA. That voltage will be a minimum of 45V. Similarly for Vces, collector current of 0.1mA and emitter current of zero.

As far as current gain goes- the short answer is that you don't know for sure. You cannot assume the gain is >100 for other currents. You can look at the typical graph of gain vs. current and make some reasonable assumptions of what the minimum gain is for other currents, but there is no guarantee (so make conservative assumptions if you want a reliable design). If you are at a bit less than 100mA usually the gain will be similar. At 50nA or at 500mA you're likely to see quite different behavior.

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Most MOSFET manufacturers used to follow this organization. An 8-page datasheet is kind of nice, because if you have favorite components, you can print them out as 2-up double-sided documents on two pages of paper, and put them in a three-ring binder. Call me old-school, but I can read a datasheet more easily on paper than on a computer screen.

The last important thing about a specification is the set of conditions under which that specification is valid. These are the qualifications for the numerical spec. For example, the RDS(on) limit of 0.04 Ω is not guaranteed unless you comply with the conditions of the spec: The junction temperature TJ must be 25C (this is typical for most specs), the gate-to-source voltage must be 10V, and the drain current must be 28A. If you conduct 29A into the device, all bets are off; same if the junction temperature is 30C, or if the gate-to-source voltage is 9.5V. In all three of these cases, on-resistance will be higher.

These show all sorts of useful information, but what you need to remember is that unless otherwise stated, these are always TYPICAL. They do not represent any kind of a specification. You can only use them for learning about general qualitative behavior. If you need anything definitive, you cannot rely on them. They may represent the mean value of a large number of sample devices. They may represent the measurement of only one particular sample, the one that Test Engineer Bob happened to have on his desk the day his boss said they needed characterization data for the datasheet.

The trick is to learn when to rely on information in the datasheet, and how to use it for what purposes. Essentially, unless a number is part of a minimum or maximum specification, you cannot use it without doing your own independent validation.

Thank you for useful article. Can you please reply what will happen when I apply voltige much above of threshhold but smaller of source-drain voltage to the gate of a mosfet and bipolar transistor? In first case I assume it break isolation of a mosfet. Bipolar transistor have insted of it pn-junction. How much voltage can I apply without risk of breaking it to this transistors? Can you say? Thanks in advance.

For the IRPF260N, for example, Vgs (Gate-to-Source Voltage) has a maximum range of 20V. This is pretty typical for most MOSFETs, however it can vary from part to part. (I had to search hard to find one that wasn't 20V max; the Rohm RE1C002UN is an example rated for 8V.)

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