Transistor vs MOSFET - Page 1
Forum section: electronics / beginners

author: littlebill
author_rank: Regular Contributor
timestamp: April 19, 2015, 03:42:33 am
content: 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

author: Dave
author_rank: Super Contributor
timestamp: April 19, 2015, 04:10:03 am
content: A MOSFETisa transistor.

author: littlebill
author_rank: Regular Contributor
timestamp: April 19, 2015, 04:15:20 am
content: Ok bjt. Better?

author: w2aew
author_rank: Super Contributor
timestamp: April 19, 2015, 04:15:48 am
content: Quote from: littlebill on April 19, 2015, 03:42:33 amFirst 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.

author: Whales
author_rank: Super Contributor
timestamp: April 19, 2015, 04:17:49 am
content: Quote from: littlebill on April 19, 2015, 03:42:33 amThe purpose you are hinting at is power switching, but there are many other things you can use transistors (of all types) for.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2015, 07:13:57 am
content: 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

author: mathias
author_rank: Regular Contributor
timestamp: April 19, 2015, 07:20:20 am
content: 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.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 19, 2015, 11:50:33 am
content: Dear AllI'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...

author: Anand
author_rank: Contributor
timestamp: April 19, 2015, 12:01:56 pm
content: Quote from: MMDuino on April 19, 2015, 11:50:33 amYou should make a new post and give us a schematic so we can provide a solution.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 19, 2015, 12:43:04 pm
content: Dear AnandI'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.

author: littlebill
author_rank: Regular Contributor
timestamp: April 19, 2015, 12:47:47 pm
content: 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 expensiveis 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

author: Zero999
author_rank: Super Contributor
timestamp: April 19, 2015, 12:55:08 pm
content: Quote from: MMDuino on April 19, 2015, 12:43:04 pmThere are numerous things wrong with that circuit but please start your own thread, rather than derailing someone else's.Quote from: littlebill on April 19, 2015, 12:47:47 pmAt 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.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 19, 2015, 01:06:17 pm
content: Hero999As 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...

author: Zero999
author_rank: Super Contributor
timestamp: April 19, 2015, 01:13:39 pm
content: Quote from: MMDuino on April 19, 2015, 01:06:17 pmI have not problem with your post. Start a new thread and I'll happily respond to it.

author: Kjelt
author_rank: Super Contributor
timestamp: April 19, 2015, 01:21:15 pm
content: 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.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 19, 2015, 01:41:15 pm
content: Dear AllSorry for misunderstanding by me, i will make new thread about my problem...

author: Simon
author_rank: Unknown
timestamp: April 19, 2015, 05:13:46 pm
content: Quote from: MMDuino on April 19, 2015, 01:06:17 pmActually 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

author: Kjelt
author_rank: Super Contributor
timestamp: April 19, 2015, 06:16:56 pm
content: Simon, I think judging by his last post and opening a new topic that MMDuino already got the point

author: Simon
author_rank: Unknown
timestamp: April 19, 2015, 07:16:34 pm
content: eventually yes, i hadn't seen the last post but I still would not like to see responses like that one.

author: rickey1990
author_rank: Regular Contributor
timestamp: April 21, 2015, 09:44:29 pm
content: 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 equivalentFor 1A, use a TIP31 or equivalent.For 3A - 5A use a BD679 or equivalent with heatsinkFor 5A to 10A use TIP3055 with heatsink

author: Howardlong
author_rank: Super Contributor
timestamp: April 21, 2015, 10:04:36 pm
content: 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.