Transistor vs MOSFET

[littlebill]

why would i ever want to use a transistor over a MOSFET, apparently they handle tons of amperage and require almost no gate current to run, and they dont' get hot.

please help the newbie understand

[Dave]

A MOSFET is a transistor.

[littlebill]

Ok bjt. Better?

[w2aew]

why would i ever want to use a transistor over a MOSFET, apparently they handle tons of amperage and require almost no gate current to run, and they dont' get hot.

please help the newbie understand

First things first... A MOSFET is a transistor. I would imagine that your real question is about a BJT vs. a MOSFET.

For simple switch applications that your question implies, a MOSFET is often time preferable to a BJT for the reasons you stated. However, aiBJT requires less of a voltage swing at the base to turn it on/off. A BJT is often preferable in non-switching applications, such as amplifiers, etc. because of the inherently higher transconductance, ease of bias setup, etc. 

[Whales]

why would i ever want to use a transistor over a MOSFET, apparently they handle tons of amperage and require almost no gate current to run, and they dont' get hot.

The purpose you are hinting at is power switching, but there are many other things you can use transistors (of all types) for.

[T3sl4co1l]

I've used BJTs in switching, plenty of times... all a matter of knowing how to use them.  Transformers are very handy, since you can use part of the load current as drive current to solve the drive problem.

Tim

[mathias]

The inherent gain of BJTs is higher than MOSFETs (transconductance times output resistance). Besides, they also have lower noise (no 1/f for example). So for linear circuits (opamps, comparators, mixers, ...) they are preferred.

For everything digital and switching: MOSFET rules.

[MMDuino]

Dear All

I've built a (linear power supply 20V DC) the output driven by MOSFET 50N06 but i got heat problem, i dont know why there is overheating on the MOSFET even with low loads, e.g. 0.42 mA.

could anyone advice me on this issue...

[Anand]

Dear All

I've built a (linear power supply 20V DC) the output driven by MOSFET 50N06 but i got heat problem, i dont know why there is overheating on the MOSFET even with low loads, e.g. 0.42 mA.

could anyone advice me on this issue...

You should make a new post and give us a schematic so we can provide a solution.

[MMDuino]

Dear Anand

I've built a (linear power supply 20V DC) the output driven by MOSFET 50N06 but i got heat problem, i dont know why there is overheating on the MOSFET even with low loads, e.g. 0.42 mA.

[littlebill]

Sorry for my being more specific. I am talking power switching applications. I have not learned the other stuff.

It seems in a lot of tutorials. They are always using transistors to turn on relays never MOSFETS. I guess it's price at they do seem more expensive

is there any down side?  I see the higher voltage but that's a non issue with my controllers. Can they be used with pwm?  Just looking for a down side?

Can they switch ac loads? Why not use them in place of relays?

How do I determine the bleed off resistor or is 10k standard across the gate to ground

[Zero999]

Dear Anand

I've built a (linear power supply 20V DC) the output driven by MOSFET 50N06 but i got heat problem, i dont know why there is overheating on the MOSFET even with low loads, e.g. 0.42 mA.

There are numerous things wrong with that circuit but please start your own thread, rather than derailing someone else's.

Sorry for my being more specific. I am talking power switching applications. I have not learned the other stuff.

It seems in a lot of tutorials. They are always using transistors to turn on relays never MOSFETS. I guess it's price at they do seem more expensive

is there any down side?  I see the higher voltage but that's a non issue with my controllers. Can they be used with pwm?  Just looking for a down side?

Can they switch ac loads? Why not use them in place of relays?

How do I determine the bleed off resistor or is 10k standard across the gate to ground

At low power levels, BJTs are often cheaper.

BJTs are better at higher voltage and will work with PWM too.

MOSFETs can be used to switch AC loads, if two are connected back-to-back but the control voltage needs to be applied between the source and gates, which can be done capacitively or via an isolation transformer. The value of the gate-source pull-down resistor is unimportant.

Examples of an AC solid state relay. The capacitors need to be Y1-rated and the transformer needs to be approved for connecting between the mains and extra low voltage DC.

[MMDuino]

Hero999

As it seem, you are Charles Walton, and the forum here for beginners like me, so if you don't like my posts, Kindly please don't reply again on my posts...

[Zero999]

Hero999

As it seem, you are Charles Walton, and the forum here for beginners like me, so if you don't like my posts, Kindly please don't reply again on my posts...

I have not problem with your post. Start a new thread and I'll happily respond to it.

[Kjelt]

mMDuino, you are hijacking someone elses thread with your new problem.
This is not a case of not want to help but it is impolite to post a different problem in someone elses topic. And very confusing sine readers can not distinguish anymore who is answering to who.

You should open your own topic and enough people will help you.

[MMDuino]

Dear All

Sorry for misunderstanding by me, i will make new thread about my problem...

[Simon]

Hero999

As it seem, you are Charles Walton, and the forum here for beginners like me, so if you don't like my posts, Kindly please don't reply again on my posts...

Actually it works like this: if you have a question to ask you ask it by writing a new post and people can answer. Shoving your question onto another thread that is not related is not helpful for anyone and at that point I'll step in and delete your posts if I deem it neccesary! That is how it works, if you don't like it go find another forum

[Kjelt]

Simon, I think judging by his last post and opening a new topic that MMDuino already got the point 

[Simon]

eventually yes, i hadn't seen the last post but I still would not like to see responses like that one.

[rickey1990]

Heres a little trick!

The 555 will deliver 200mA to a load but the chip gets extremely hot (12v supply). The answer is to use a buffer transistor.
For 200mA, use a BC547 or equivalent.
For 500mA use a BC337 or equivalent
For 1A, use a TIP31 or equivalent.
For 3A - 5A use a BD679 or equivalent with heatsink
For 5A to 10A use TIP3055 with heatsink 

[Howardlong]

Quick couple of reasons that immediately spring to mind...

MOSFETs, particularly power MOSFETs tend to have lots of parasitic capacitance (gate/source for example can be well into the 1000pF+ department), so it takes effort to switch them as a result. This is also a reason why it's taken a while for MOSFETs to catch on in RF applications, but they're definitely here to stay now.

In addition some MOSFETs have quite high Vgs to get them to switch hard on, so driving them directly from simple logic isn't always an option.

For these reasons, there is a whole sub-industry making MOSFET gate drivers as a result for switching apppilcations.