the right transistor??  - Page 1
Forum section: electronics / beginners

author: aduul
author_rank: Newbie
timestamp: September 04, 2013, 06:22:32 am
content: is there any method to know the correct transistor for my switching applicationssuch as feeding a relay or motor dependently on a micro-controller signal at the base of transistorwhat is the value Hfe and how can I choose the correct value??how can i determine the right base resistor??how can i avoid the overheating of a transistor??please help me coz i think this is the most important block in electronics design

author: Skimask
author_rank: Super Contributor
timestamp: September 04, 2013, 07:09:28 am
content: Uh oh...Google's broke again...

author: aduul
author_rank: Newbie
timestamp: September 04, 2013, 12:55:36 pm
content: ??

author: onlooker
author_rank: Frequent Contributor
timestamp: September 04, 2013, 01:33:23 pm
content: I would say:1). Show you have done your homework (e.g. google, schematics) and show what you know, instead of showing the appearance of laziness.2). Ask pointed questions.3). Your questions are too general and showing that you need to prepare yourself on more basic things than transistors.

author: mariush
author_rank: Super Contributor
timestamp: September 04, 2013, 01:36:17 pm
content: Maybe this will teach you something and answer at least a part of the questions:

author: PA4TIM
author_rank: Super Contributor
timestamp: September 04, 2013, 01:38:54 pm
content: Buy " the art of electronics" and you find your answers.All the things you want to know are also in the datasheets. Things like powerdissipation, hFE, current, voltage etc.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: September 04, 2013, 02:01:27 pm
content: Picking a correct transistor isn't very easy, since there is a large number of available variables, such as cost, availability, different types of performance specifications and so on.What I do instead is try not to pick the correct transistor, but one that will just work and then try to improve the choice if it is necessary to do so (keep in mind that none of the stuff I do is professional).So what you should do is get familiar with the basic specifications of transistors in general and then get the minimal performance your circuit needs. After that, try to find a transistor that will fulfill what you need.For example, let's say that you're trying to switch a motor whose current is 500 mA and nominal voltage 12 V from a microcontroller.Let's see what we can get just from those simple specifications:We know that when the motor isn't working, we need a switch that will be able to block 12 V indefinitely.Next, we know that the switch will need to pass 500 mA through it.After that, we remember that motor is inductive and that we need something that will survive the high voltages generated when motor's power supply is switched off. So we thing a little bit about that. We could get a transistor that will be rated to survive that or we could use a freewheeling diode to solve the issue.Next, we know that transistor needs to be controller using a microcontroller. That microcontroller will have some output voltage and current ratings. We need a transistor that can be operated by that voltage or current.Now this is already a lot of useful information and it will eliminate some easily obtainable components from the list of potential candidates for your use.After that, we research a bit about transistor types and see what type would be most appropriate using what we know and the list of components we can easily get. At this point, it would be a good idea to start reading datasheets for potential candidates. To do that you need to know where to start. What I do is determine how much I'd like to spend for the component and see if any components in the price range are suitable. If they are not, I expend the price range. This is also where experience comes into play. You need to be comfortable reading datasheets and know how to extract useful information from them. You gain that ability by practice.Now let's say that we want to use a bipolar junction transistor. We know that the transistor should be capable of passing at least 500 mA. Then we see what our microcontroller can source. let's say that it can source 10 mA from a pin. To get the current gain we divide the input current by base current and get 50. That means that we'll need a transistor of current gain that is at least 50. Now in order to avoid any problems with manufacturer tolerances and to make sure that the transistor is fully saturated, we should pick a larger current gain than 50. A 100 would be great, if we can find it. Also we need to know how to calculate the base resistor for the transistor in order to set the current to needed value.When we get the minimum gain figure, we need to look further in the datasheet to see how much heat would the transistor generate when running at 500 mA and see if the transistor would survive that. If it won't we look or one in larger case to which we can attach a heatsink or we look for a different type of transistor that will dissipate less heat. For example, a field-effect transistor may dissipate less heat if properly selected.The story goes on and on and on and is quite complicated. For example, I wrote all this under assumption that you only want to turn the motor on and off. If you wanted to use transistor for speed control using pulse-width modulation, that would complicate issue even more, since you'd need a transistor that is fast enough for the frequency you will be using.Now, since this is a bit complicated, one thing some people do is to get partial specifications that will hopefully prevent component from burning and then hope for the best. For example, if get a transistor that can dissipate large amounts of heat, you can try "experimenting" with it to see if it will work correctly.Basically it's you who needs to decide when you got a component that's working good enough.

author: xrunner
author_rank: Super Contributor
timestamp: September 04, 2013, 02:06:49 pm
content: Quote from: aduul on September 04, 2013, 06:22:32 amYou could build large parts of a college course around those questions. It's really too much to answer in a single forum post. There are thousands of types of transistors, yet for a lot of small switching applications such as a relay you can usually use one of a few common types. Watch that video for starters. Really, there are many web sites that fully cover your question far better than any single forum or posts could do.Here's a link that might help you though -http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

author: olsenn
author_rank: Frequent Contributor
timestamp: September 04, 2013, 02:27:59 pm
content: QuoteAssuming an NPN BJT is used:1) Hfe (sometimes known as Beta) isn't really chosen, it is an inherant property of the transistor you are using. However, despite what theory may teach you, this value is by no means constant. It highly depends on your operating current. Of course you can pick your transistor based on this value, but don't trust the datasheet. Essentially, this value is the small-signal current-gain. It is calculated as (delta I_CE)/(delta I_BE).2) Typically, the base-emitter junction drop is around 0.7 volts. Therefore, the base-emitter current (I_BE) will be (V_BE - 0.7)/R. Make sure the base-emitter current doesn't exceed the maximum the transistor can handle. Pick the resistor such that I_BE multiplied by Hfe is the collector-emitter current (I_CE) that you desire. Also make sure this current doesn't exceed the max for the device.3) In addition to the advise in part two, remember that the collector-emitter junction will have a voltage drop of around 0.2 volts in saturation mode and may be calculated otherwise. Whatever I_CE multiplied by V_CE is, that is the power that the transistor is dissipating. All of that power is dissipated in the form of heat. Remember, that even if the datasheet says the transistor you are using can handle (example) 10 watts of power, you still need to look at the maximum temperature as well! You may need a large heatsync or alternative cooling to safely handle that amount of power.

author: free_electron
author_rank: Super Contributor
timestamp: September 04, 2013, 02:56:17 pm
content: find out first what is the maximum voltage you need to be able to holdfind out what is the maximum current you need to runfigure out how much power you will burn off in the transistordecide npn or pnpgo to digikeysearch transistor and filter npn or pnp , select everything for your max current and max voltage and all higher currents and votages. that will narrow it down to a few thousand transistors.then look at power dissipation and pick the package.that will shave off some more.if it is for saturate operation point i simply then pick a transistor that can handle double the current and 1.5 time the max operating voltage. sort by price , whamo. doneif its for linear operation then you need to calculate pmax and pick a transsitor that can handle that pmax . find correct package and heatsink if required.

author: jasonbrent
author_rank: Regular Contributor
timestamp: September 05, 2013, 03:11:56 am
content: Quote from: mariush on September 04, 2013, 01:36:17 pmThank you for this video... as a noob, this was very useful!-jbl