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Author Topic: [Question] Why do not use FET to active coil  (Read 3436 times)
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vantusaonho
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« on: September 17, 2012, 04:56:56 16:56 »

Hi everybody!
I have a question as: why do not we use a FET to active a coil (Relay coil)?
As normal, we use a transitor to do that but I wonder why don't use a FET
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borberk
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« Reply #1 on: September 17, 2012, 05:11:25 17:11 »

What kind of FET you have in mind?
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metal
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« Reply #2 on: September 17, 2012, 05:19:17 17:19 »

Why do you want to use FET any way?
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Sideshow Bob
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« Reply #3 on: September 17, 2012, 05:35:02 17:35 »

I guess it many reasons. Like cost, and also availability. BJTs are cheap and common, traditionaly FETs are more uncommon. FETs are voltage controlled and BJTs are current controlled. Many FETs need around 10 volt to be turned fully on. That may be unpractical in a 5 volt system. But almost any common 5 volt logic. Will be able to source the base current needed to switch on a common transistor. In order to control most basic relays
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« Reply #4 on: September 17, 2012, 09:00:43 21:00 »

Whatever you choose don't forget to add the protective 'flyback' diode across the coil so you don't kill the FET or transistor...
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solutions
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« Reply #5 on: September 18, 2012, 03:07:34 03:07 »

Because bipolar guys live on myths and are bent on using a current controlled device on a voltage output. We had a thread on this - the guy spent MONTHS trying to make a bipolar work.

You CAN get logic threshold FETS now, typically a couple of volts to turn on, not 10V, which is fine for a coil. The 10V devices are more in the dozens of amps stuff, which is a pretty HUGE coil :-)

As was said, you should clamp the spike you get from the relay coil with a diode, but even then, some FETs don't need one because their intrinsic body diode is robust enough to do the job (we had another thread on that one as well). That diode can make the difference in the economics argument between FETs and bipolar.

IMO, running anything of modest power off of a logic gate BEGS for a FET and use of a bipolar is a silly waste of board real estate and a couple of $0.003 resistors (emitter degeneration, base current limiter)...

All assumes a DC relay coil, of course.
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metal
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« Reply #6 on: September 18, 2012, 08:38:23 08:38 »

This page will answer your question: http://www.learnabout-electronics.org/fet_03.php Think about it, you need to utilize a voltage divider in order to make Vg lower than Vs such as using a voltage divider on the Vs, or use -ve voltage on Vg when Vs = 0, this is not considered feasible solution looking at BJTs which don't require such conditions.




You CAN get logic threshold FETS now, typically a couple of volts to turn on, not 10V, which is fine for a coil. The 10V devices are more in the dozens of amps stuff, which is a pretty HUGE coil :-)

Solutions, designers use MOSFETs for amplifier output and FETs for input, so the FETs don't even see the large coil. Also, FETs and MOSFETs are usually used as direct switching devices, even for AC, but require another configuration for AC, of course.
« Last Edit: September 18, 2012, 08:57:48 08:57 by metal » Logged

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Sideshow Bob
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« Reply #7 on: September 18, 2012, 09:36:22 09:36 »

@vantusaonho
It could be interesting to know why you are asking. The answers you have got so far. Is then looking at them not very good at all. The answer from Solutions is quite bombastic, but very much based on half-truths. And my own answer is kind of in the same alley. But the bootom line. It is nothing wrong in using a FET to activate a relay. For the hobbyist a "logic level" fet may be hard to get depending on which part in the world you live. And hence it would be easy and cheaper to use a transistor.    
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solutions
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« Reply #8 on: September 18, 2012, 11:31:30 11:31 »

^^^ Hey - you started it 'Bob  ;-)

Metal - you are correct, BUT what you describe is a depletion mode FET. An enhancement mode device has a positive Vt, and will pinch off at about a volt or so and has full turnon at around 2-3V. FET is a transistor type, MOSFET is an implementation - sorry if I confused things by omitting "MOS".

example: http://www.irf.com/whats-new/nr060209.html
« Last Edit: September 18, 2012, 11:43:20 11:43 by solutions » Logged
metal
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« Reply #9 on: September 18, 2012, 11:46:59 11:46 »

Solutions, we are talking about JFET, not HEXFET MOSFET. I can remember an example now, 2SK170 is a FET transistor. All of us know that we can use MOSFETs to control a relay and the N-Channel MOSFETs need +ve voltage, can't you remember the well known BS170, the depletion version.
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Sideshow Bob
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« Reply #10 on: September 18, 2012, 12:06:15 12:06 »

^^^ Hey - you started it 'Bob  ;-)

Metal - you are correct, BUT what you describe is a depletion mode FET. An enhancement mode device has a positive Vt, and will pinch off at about a volt or so and has full turnon at around 2-3V. FET is a transistor type, MOSFET is an implementation - sorry if I confused things by omitting "MOS".

example: http://www.irf.com/whats-new/nr060209.html
That is correct. But those are almost without exception meant for power application. And to use such device to switch most common relays will be grossly overkill.  And also quite expensive. Not to mention board real estate. For those power FETs. A search at my component supplier gave me however only one component that could be interesting in such a setting. The MMFTN170. You also say that you do not any kind resistor at the gate, compared to the BJT base resistor. Well that is sort of true if you only want to switch a relay now and then. But if you move somewhat up in switching frequency. You will find that driving a FET correct. Need careful design. Just Google mosfet gate driver design
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vantusaonho
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« Reply #11 on: September 18, 2012, 02:56:44 14:56 »

@vantusaonho
It could be interesting to know why you are asking.   
I asked that thing because a stepper motor driver can be made by FET of Transitor. In stepper motor, there are 2 or more coils, so that control stepper motor is to control its coil.
I think you are right when said here
Quote
I guess it many reasons. Like cost, and also availability. BJTs are cheap and common, traditionaly FETs are more uncommon. FETs are voltage controlled and BJTs are current controlled. Many FETs need around 10 volt to be turned fully on. That may be unpractical in a 5 volt system. But almost any common 5 volt logic. Will be able to source the base current needed to switch on a common transistor. In order to control most basic relays
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GunMage
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« Reply #12 on: September 18, 2012, 03:42:37 15:42 »

I asked that thing because a stepper motor driver can be made by FET of Transitor. In stepper motor, there are 2 or more coils, so that control stepper motor is to control its coil.
I think you are right when said here

Why not use a stepper motor driver ?? We are working on a stepper motor driver board now and are using a dual full bridge. Thing works great. It is around a $7.00 chip though.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002293.pdf
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Gallymimu
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« Reply #13 on: September 18, 2012, 07:11:23 19:11 »

Allegro makes very nice stepper drivers for about half the cost (A4988 for instance).

I use FETs for driving relay coils all the time.  There isn't a problem.  Most MOSFETs today can be driven with 5V or even 3.3V logic though they may not come to as low of an Rdson unless they are specified for logic level.  FETs are so good these days I can't imagine a relay could that would need a very low Rdson.
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Ichan
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« Reply #14 on: September 18, 2012, 08:46:47 20:46 »

How complicated you all guys..  Grin

Question:
Quote
why do not we use a FET to active a coil (Relay coil)?

Answer:
we do not use fet to drive relays because it costs several times of the bjt counterparts, while bjt is enough for that purpose.

-ichan
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solutions
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« Reply #15 on: September 18, 2012, 10:01:09 22:01 »

LoL - $0.11 in qty 100...a major fortune for them thar MOSFETs

http://www.digikey.com/product-detail/en/2N7002WT1G/2N7002WT1GOSCT-ND/1967031
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GunMage
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« Reply #16 on: September 19, 2012, 03:40:12 15:40 »

Allegro makes very nice stepper drivers for about half the cost (A4988 for instance).

I use FETs for driving relay coils all the time.  There isn't a problem.  Most MOSFETs today can be driven with 5V or even 3.3V logic though they may not come to as low of an Rdson unless they are specified for logic level.  FETs are so good these days I can't imagine a relay could that would need a very low Rdson.

I have used the Allegro stepper drivers. We typically have issue with them in our application. Hence the ST part. However as a general stepper driver they are good chips.
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Ichan
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« Reply #17 on: September 22, 2012, 12:49:08 12:49 »

LoL - $0.11 in qty 100...a major fortune for them thar MOSFETs

Ha, I didn't know that - so the price of BC547 is just on par with 2N7000... well at Digikey.

But in my daily works bjt for this purpose is as cheap as dirt, more then $0.04 won't fit the budget.

-ichan
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solutions
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« Reply #18 on: September 22, 2012, 06:49:32 18:49 »

I know you guys didn't know that, which is why I've been trying to pry your eyes open.

By the time you f with biasing and deal with the insertion costs of the two (at least) extra resistors, the cost is the same or less for a FET, Mr Cheap
 Tongue

Don't forget, assembly cost dominates on "low cost" components. And we all know Digikey prices are HIGH, which is why I used them as my example.

Above all, FETs don't have thermal runaway problems if you're dealing with modest to high power levels. For huge power, we have the IGBT, which is a FET/Bipolar hybrid...again, you can drive some of those with logic levels directly.

I've given up on bipolars almost entirely now. Dinosaurs.
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bigtoy
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« Reply #19 on: September 22, 2012, 08:04:31 20:04 »

I've used FETs to drive relay coils. Agreed it can cost a few pennies more than a BJT, but sometimes that high-impedance gate is just what the good doctor ordered. Oh, and I should edit, for some low-voltage applications, sometimes it's hard to accept 0.6 - 1.0V drop across a BJT, so a low-impedance FET can be helpful in that way too.
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Gallymimu
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« Reply #20 on: September 23, 2012, 04:51:00 16:51 »

I have used the Allegro stepper drivers. We typically have issue with them in our application. Hence the ST part. However as a general stepper driver they are good chips.

Can you share what issues you had with the Allegro?  I've got a design using them right now but it hasn't be put into production yet.
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GunMage
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« Reply #21 on: September 24, 2012, 03:39:55 15:39 »

Can you share what issues you had with the Allegro?  I've got a design using them right now but it hasn't be put into production yet.
My issue is the enviornment I subject my components to. My PCB's are typically submerged in oil and see very high pressures (thousands of PSI).
You are not going to have anything to worry about unless you put your circuits into extremely hostile enviornments like we do.

**EDIT - Spelling. We need spell check **
« Last Edit: September 25, 2012, 07:09:51 19:09 by GunMage » Logged
Ramnish
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« Reply #22 on: September 25, 2012, 07:57:41 07:57 »

In electronics IMO, keep the circuit as well as components as simple as possible. Never try and do an overkill. For driving a relay use of a simple transistor is the way to go unless you need strict logic level control, but for that too the final switching should be achieved through a simple switching transistor. Moreover for relay switching, the switching speed is not a critical factor. This is just my way of thinking.

- Ramnish
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haXudon
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« Reply #23 on: September 06, 2013, 10:56:34 22:56 »

i've always preferred using FET for my designs than BJTs for the simple fact that it's easier and i don't need to mess around with drive currents.

just a head's up when you want to use an FET or any transistor to drive a coil or motor, make sure you implement a snubber or clamping network. this allows for faster switching and better control of the peak reverse voltage. also a diode across the drain and source is a good idea for driving inductive loads.
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LithiumOverdosE
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« Reply #24 on: September 07, 2013, 01:09:11 13:09 »

I learned many years ago (the hard way) that using any kind of voltage controlled switches (and devices in general) in noisy environments is a very bad idea and should be avoided if not absolutely necessary. Their comparatively high input impedance makes them ideal candidates for all sorts of problems with EMI/RFI signals, ranging from brief unexpected turn on and turn off to brief excursions into linear region. Usually one can indeed solve that by all sorts of filtering, EMI/RFI shielding, carefully optimised PCB design, negative gate bias voltages etc. However it all adds to the circuit complexity and cost.

Or one could simply use current controlled switches like BJTs and significantly reduce complexity and cost of the circuit while making it less prone to unpredictable behaviour in noisy environments. Even higher gain Darlington's are less prone to problems compared to voltage controlled switches.

So the choice of the switching element really depends on the particular application and circuit in which semiconductor switch is to be used.  Wink  
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