Choosing a transistor

[Stoneww]

I'm never sure which transistor to use. I cannot seem to make sense of the datasheets as they're too intimating. So could someone please inform me when to use which transistor or when it's better to use a MOSFET & for these examples:

>3V LED

>5V Motor

I just need to know what is important in choosing a transistor so I can do this my self in the future.

Also I college I got taught that a transistor has 1 indefinite Gain where as the data sheets say that the gain differs depending on the base current. Thank you!

 

[Audioguru]

Almost every transistor has a voltage rating shown on its datasheet that is higher than 30V so thinking about 3V or 5V is silly.

A transistor used to turn on an LED or a motor must be able to provide enough current. If it needs 3A then you select a transistor rated on its datasheet for maybe 5A, not 100mA. A Mosfet is used when the current is very high and there is a high enough voltage (10V for most Mosfets) feeding its gate as shown on its datasheet.

When a transistor is used as an on-off switch then it must not be linear, instead it must saturate. Gain (beta or hFE) is only used when a transistor has plenty of collector to emitter voltage and is not saturated as shown on its datasheet. Then you look at the saturation characteristics on its datasheet where it does not talk about gain, instead it says the base current must be 1/10th of the collector current or less.

Why don't you learn about the simple spec's on a datasheet?

 

[Stoneww]

Almost every transistor has a voltage rating shown on its datasheet that is higher than 30V so thinking about 3V or 5V is silly.

A transistor used to turn on an LED or a motor must be able to provide enough current. If it needs 3A then you select a transistor rated on its datasheet for maybe 5A, not 100mA. A Mosfet is used when the current is very high and there is a high enough voltage (10V for most Mosfets) feeding its gate as shown on its datasheet.

When a transistor is used as an on-off switch then it must not be linear, instead it must saturate. Gain (beta or hFE) is only used when a transistor has plenty of collector to emitter voltage and is not saturated as shown on its datasheet. Then you look at the saturation characteristics on its datasheet where it does not talk about gain, instead it says the base current must be 1/10th of the collector current or less.

Why don't you learn about the simple spec's on a datasheet?

We just literally get taught how a basic competent works, we don't look at datasheets what so ever. And by college I mean pre-university like ages 17-18 so I assume there will be looking at datasheets at university.

 

[Audioguru]

In high school I was taught about simple electricity, not electronics. I read electronics magazines and learned many things then I designed and made many electronic devices with the details from the datasheets.

In university I was taught all the details and about the datasheets that I already knew.

 

[Stoneww]

In high school I was taught about simple electricity, not electronics. I read electronics magazines and learned many things then I designed and made many electronic devices with the details from the datasheet.
In university I was taught all the details and about the datasheets that I already knew.

oh right I better get learning all this then and start reading, do you have any suggestions on what is good to read?

 

[Harald Kapp]

You may start here. Or just enter "how to read a datasheet" in the search engine you trust to find quite a few references.
You're welcome to ask specific questions here if you encounter a point you don't understand.

Note that in order to understand a datasheet it is mandatory that you understand the operating principle described by the datasheet. It is no use discussing the value of a parameter if you don't know what the parameter signifies.

 

[Minder]

transistor or when it's better to use a MOSFET & for these examples:

>3V LED

>5V Motor

My first go-to choice is Mosfet if I can, starting with the 2n7000, this is aimed at logic level circuits as is other mosfets rated for low level devices such as micro controller etc.
Being transconductance devices, they require much less in the way of drive current over bi-polar version.
M.

 

[Audioguru]

Don't just look at the graphs on a datasheet because they show parameters that apply only to a "typical" device.
Most parameters have a range of values that is shown as minimum and maximum in the text of the datasheet because each transistor is different and you cannot buy the exact value for a parameter, you get whatever they have. So it is best to design using the "worst case" values of each parameter so that any passing transistor will work fine in your circuit.

 

[AnalogKid]

While there are zillions of transistor part numbers, most designers quickly settle down to a small number of favorites. This can be because those are what already is in the inventory at work, the favorites of a mentor, etc. Also, if you roam around the web looking at hobby site schematics, you will see the same numbers come up often in a wide variety of applications. That is because many applications have very broad requirements in terms of which transistors will or will not work for it.

Of course, there are applications where just any old part will *not* work. High-end audio circuits need transistors with very flat gain curves, bred specifically for that application; video circuits need better-than-average high frequency characteristics; RF circuits use very weird parts in strange packages optimized for gigahertz frequencies.

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