[Question] Why do not use FET to active coil

[zac]

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.    

I haven't had false triggering or linear effects with mosfets driving relays, but couldn't one just add a pulldown resistor (such as a 10K between gate and source) to prevent that?  I would think this could only be a problem if the drive source is high impedance. 

[LithiumOverdosE]

Ever tried working with microcontrollers and MOSFETs that they drive in extremely electrically noisy environment?  

G-S pull-down is a standard thing whenever I use MOSFETs as switches. I don't even think about it anymore.

However, one can do all the filtering and signal conditioning but what's the point of forcing the use of MOSFET if BJT can solve all of that in such cases? KISS

[zac]

Ever tried working with microcontrollers and MOSFETs that they drive in extremely electrically noisy environment? 

G-S pull-down is a standard thing whenever I use MOSFETs as switches. I don't even think about it anymore.

However, one can do all the filtering and signal conditioning but what's the point of forcing the use of MOSFET if BJT can solve all of that in such cases? KISS

I worked on stuff in automotive environments that were pretty noisy.  One problem with bipolar transistors is the power consumption is much higher than with MOSFETS (which is almost zero).  This is mainly a problem with battery powered equipment.  My driver designs used mostly bipolar transistors 20 years ago, but I mostly use MOSFETs now for things like relay drivers.  One problem in the past was the MOSFETS needed higher than 3.3V or 5V to saturate so a separate bias supply would be needed (or needed to use a MOSFET driver chip with an internal charge pump), but this limitation has been mostly overcome with the logic level MOSFETs that have been available since around 2000.   

[LithiumOverdosE]

I worked with some RF pulsed power stuff. Really hard to control induced spikes and MOSFETs did fire on their own a few times. I did solve it with carefully placed ground planes, shielding and transient suppression but in the end I simply felt more comfortable with BJTs. 

I do agree with advantages of FET technologies in low power circuits but it only goes to illustrate the point I tried to make. It all depends on application and there is no universal rule. I use BJTs because I very rarely had anything to do with battery operated devices and higher power consumption was never a concern. I almost exclusively use MOSFETs and IGBTs for power switching and I don't recall when I last used BJT in that role. 

[bigtoy]

I've used both BJT & MOSFET. They both work. Circuits are similar but not identical. Often the reason for choosing one vs the other is based upon what other components are already on the board - it's nice to limit the bill of materials when possible.

[optikon]

I worked with some RF pulsed power stuff. Really hard to control induced spikes and MOSFETs did fire on their own a few times. I did solve it with carefully placed ground planes, shielding and transient suppression but in the end I simply felt more comfortable with BJTs. 

I do agree with advantages of FET technologies in low power circuits but it only goes to illustrate the point I tried to make. It all depends on application and there is no universal rule. I use BJTs because I very rarely had anything to do with battery operated devices and higher power consumption was never a concern. I almost exclusively use MOSFETs and IGBTs for power switching and I don't recall when I last used BJT in that role. 

Put a pull down resistor on the G-S of those FETs, thats pretty tough to get RF to reach threshold V to turn on. I'm guessing when you had problems, you didnt have a low impedance gate on the fets.

[LithiumOverdosE]

Put a pull down resistor on the G-S of those FETs, thats pretty tough to get RF to reach threshold V to turn on.

I already stated that I do that as a standard procedure.

G-S pull-down is a standard thing whenever I use MOSFETs as switches. I don't even think about it anymore.

[zab]

1 mosfet and igbt nodoubt create problem when testing with input control components removed. Ie gat circuitry open while others pins alive. may burn some time.so pull down resistor save you from such problems

[Vineyards]

Relay driving is a copy-paste unit really. I spend most of my energy for the core functions of a circuit. What do you expect to gain beyond what you effortlessly achieve using small signal transistors? FETs are actually like black magic. It is hard to match one to another and their specs may signicantly change with temperature. Bipolar transistors meanwhile are pretty bulletproof.

[Gallymimu]

We used MOSFETs and IGBTs in about the noisiest environment I can think of (vacuum deposition plasma arcing).  We used pull downs snubbers and filter caps, especially on signals coming into and out of the control boards.  Never had any problems with mosfet false triggering, usually a 1k pull down was fine.

We had more problems with CPLDs having spurious behaviors than issues with our switches.

We used the MOSFETs for driving relays, low power PWM for fans, LEDs etc.  We did use BJTs though for things like class B amp stages on opamp outputs and a few other places where linear region control of a mosfet is a huge PITA

[LithiumOverdosE]

This one was electronics for automatic control of a large Tesla transformer (not SSTC). For the reasons I still cannot fathom a decision was made (not by me) to locate electronics enclosure right between spark gaps and heavy duty primary. I suspect that extreme induced transients from primary and close vicinity of spark gaps made large enough spikes to trigger small MOSFETs. I didn't bother to investigate further due to dead line. I simply modified circuit for BJT and problem was solved. After that I simply use BJT in all critical places where it is convenient to use them just as a precaution.

[Gallymimu]

This one was electronics for automatic control of a large Tesla transformer (not SSTC). For the reasons I still cannot fathom a decision was made (not by me) to locate electronics enclosure right between spark gaps and heavy duty primary. I suspect that extreme induced transients from primary and close vicinity of spark gaps made large enough spikes to trigger small MOSFETs. I didn't bother to investigate further due to dead line. I simply modified circuit for BJT and problem was solved. After that I simply use BJT in all critical places where it is convenient to use them just as a precaution.

HA, wow that's one heck of a good EMI test for sure!

[optikon]

This one was electronics for automatic control of a large Tesla transformer (not SSTC). For the reasons I still cannot fathom a decision was made (not by me) to locate electronics enclosure right between spark gaps and heavy duty primary. I suspect that extreme induced transients from primary and close vicinity of spark gaps made large enough spikes to trigger small MOSFETs. I didn't bother to investigate further due to dead line. I simply modified circuit for BJT and problem was solved. After that I simply use BJT in all critical places where it is convenient to use them just as a precaution.

It may have been capacitive coupling to the spark gaps. With enough dV/dt even *fempto* Farads might couple enough charge the gates to switch them on momentarily. I assume a crazy scheme like this was at least enclosed in a grounded electrostatic shield.

As far as low level analog "noise" /offset  goes, BJT's are more susceptible to RF rectification than are JFet or MOSFET's. However, as far as a switching application goes, BJT are more immune due to the lower input impedance (they require uA to turn on).

Guess it depends on the app!

[EHHS1979]

Both MOSFETS and transistors are used to activate relay coils.  It is a preference thing, can be application based.  When driving relays with either MOSFETS or transistors, place a diode across either the relay coil or the MOSFET to protect the semiconductor device from voltage/current induced when de-energizing the coil.

I use devices like a DRDC3105 to drive relays, small, surface mount, SOT23 or SOT26 package, has the diode built in, to drive 3.3 or 5 VDC relays.