fet vs bipolar transistors...

I see more and more people using mosfets for jobs not required...

So how (when designing) do you decide to step up to a fet from a regular transistor?

it's been a long time :wink: XD

using mosfets for jobs not required...

examples ?

someone posted that (about a fuse)

and I was thinking, why a fet?...

cjdelphi:
and I was thinking, why a fet?...

Ignorance?

To the untrained mind FETs are newer and more powerful, therefore "better!"

cjdelphi:
someone posted that (about a fuse)

and I was thinking, why a fet?...

agreed, but even if he ha used a BJT, I would have thought "why a transistor? " - Just to light a led when fuse is broken ?
I'd better use a PNP transistor, with a low resistor between emittor and base ( calculated so that the transistor is ON when the current is just below the fuse limit - example 0,22 Ohm 3W would switch on at about 2,7A ) , and a Led + resistor between collector and GND .
This way, I would be warned before the fuse breaks :slight_smile:

fungus:

cjdelphi:
and I was thinking, why a fet?...

Ignorance?

To the untrained mind FETs are newer and more powerful, therefore "better!"

FETs are far better for power switching, for other uses their inherently higher
bandwidth can be a deciding factor (FETs are majority carrier devices, BJTs are
minority carrier devices and recombination time makes them inherently slower,
all things being equal). For a lot of analog uses BJTs are easier to design with
I think (Vbe much more predictable than Vthr), and they still are important there.

FETs aren't new any more, they've been mainstream for 35 years or so!

alnath:
Just to light a led when fuse is broken ?

That was the request. Light an LED when a fuse is blown.

That circuit has a number of deficiencies (for instance with blown fuse and no load the
gate floats). It is also rather vulnerable as there is no protection on the gate from
stray voltage transients. A BJT circuit with a PNP high-side switch for the LED would
be a better choice here perhaps. The simple scheme of LED+resistor across the fuse
isn't unreasonable.

Might do some 'research'

https://www.google.com/search?q=fuse+blown+indicator&espv=210&es_sm=93&source=lnms&tbm=isch&sa=X&ei=Yd6jUuWmKMKg2QXgtYDwCA&ved=0CAkQ_AUoAQ&biw=999&bih=802

MarkT:
For a lot of analog uses BJTs are easier to design with

For a lot of digital uses, too. BJTs are much easier to drive than MOSFETs.

MarkT:
FETs aren't new any more, they've been mainstream for 35 years or so!

Yes, but they're newer.

This one is simple

retrolefty:
This one is simple

http://www.talkingelectronics.com/projects/200TrCcts/images/BlownFuse.gif

yep, that's why I said :

......... even if he had used a BJT, I would have thought "why a transistor? " - Just to light a led when fuse is broken ?

:wink:

But.. it's constantly powering 2 leds (until the fuse blows), am I really considering This with a bjt?..... well yes I am lol... I'll submit my entry soon :slight_smile:

cjdelphi:
http://www.talkingelectronics.com/projects/200TrCcts/images/BlownFuse.gif

But.. it's constantly powering 2 leds (until the fuse blows), am I really considering This with a bjt?..... well yes I am lol... I'll submit my entry soon :slight_smile:

nope, the red one won't light until the fuse blows
as long as the fuse is OK, only the green led is ON , and if the fuse blows, the red led is ON, the green one is OFF

Is that the purpose of the diode in series with the red led? To prevent red from taking the green led's juice?

sort of : it rises the voltage needed to light up the red LED

As long as the fuse is OK, the voltage between +12V and Leds cathodes is limited by the green Led

Well without the diode they both do light up...

The diode lowers the voltage and the green comes on, theres still some current across the red led even when green is on (very low current though)

But it's simple and it works...l

actually, I would add one or even two 1N4148, because Red Led voltage is often lower than green ones.
If the forward voltage of the red led is, say 2.0V , and if it is 2,8V for the green Led (which is common), then we'll need more 1N4148 if we don't want the red led to be on while the fuse is OK
But, below about 1,8V, no current flows through the Led. (look at the datasheet, for example :http://www.everlight.com/datasheets/333-2SDRT-S530-A3_datasheet.pdf :wink: )

Edit : OK, the Led in the link above is a special one - I should have pointed more basic led datasheet, like this one https://www.sparkfun.com/datasheets/Components/LED/COM-09590-YSL-R531R3D-D2.pdf Anyway, the forward voltage is still between 1,8 and 2,2 . If the green one has the same, no problem, but....

alnath:
actually, I would add one or even two 1N4148, because Red Led voltage is often lower than green ones.

Correct, "modern" green LEDs typically have a Vf of > 3.0V. Green LEDs with older chemistry had a lower Vf much closer to red's.

You can still get low-Vf green LEDs, you just have to be careful and check the specifications. And they're usually not as bright.

It's wonderful how the discussion always wanders so far off topic. The topic isn't meant to be "critique my circuit", but "why choose a FET over a BJT?".

I'm going to try and drag it back on track and throw in my tuppence to the original question.

My personal rules of thumb, in ascending order of weight:

  1. A BJT has a fixed voltage drop regardless of current, a FET has variable voltage drop relative to current - if the voltage drop of the BJT is less than the expected voltage drop of a FET at the expected current, then a BJT is a good candidate for selection.

  2. Small FETs generally handle larger currents than the equivalent size BJTs, so FETs often win out on that arguement.

  3. Then there is the economy of scale factor - if you already have lots of FETs or BTJs in other parts of your design, and it doesn't matter which you use for another part of the design, then choosing the same part as you use elsewhere can reduce the component cost of your circuit (5 identical FETs cost less than 4 FETs and 1 BJT)

And that's not getting into anything technical like bandwidth / switching speed, current gain, power dissipation, etc. If it's just general low-speed switching, where the actual specifics of the device have little or no importance on the application, then you don't need to worry overly much about which you choose - FET or BJT - so just go with what you have or what you fancy. In those situations theres rarely any real reason to select one over the other, except personal choice.