MOSFET vs Relay

There's lots of TTL controlled mechanical relay modules available but why not MOSFET modules? What's the pro's and cons here?
 
PRO's:
 
MOSFET
-Faster
-Less operating power
-Can use PWM
 
RELAYS
-Readily available in 1,2,4,8,16 banks
-Can switch AC
-Can have DPDT, TPDT, etc..
 
 
I use Relay modules for my webcontrol boards and use them for switching DC only. Wouldn't a MOSFET work just as well or better?

Machineman said:

There's lots of TTL controlled mechanical relay modules available but why not MOSFET modules? What's the pro's and cons here?
 
PRO's:
 
MOSFET
-Faster
-Less operating power
-Can use PWM
 
RELAYS
-Readily available in 1,2,4,8,16 banks
-Can switch AC
-Can have DPDT, TPDT, etc..
 
 
I use Relay modules for my webcontrol boards and use them for switching DC only. Wouldn't a MOSFET work just as well or better?

Click to expand...

 
MOSFET has a non-zero Rds(on), which means that they will dissipate heat. At trivial currents, it's not much. At substantial currents, it can be quite a lot.
 
Relays are just as happy driving high-side as low-side. N-channel FETs can be difficult to drive high-side without complex drivers.
 
Relay contacts can switch AC, MOSFETs need two series FETS (and more complex drives) to switch AC.
 
Relay contacts are intrinsically isolated. MOSFETs require more complex isolated drivers to achieve this.
 
Just for a start....

isolation between logic control and power by relay, that is one reason.

Depends on what you need to do.  If you need isolation there  are a number of ways to do that with MOSFETs that aren't terribly complex but it's not as simple as a relay.
 
Here is a small, isolated MOSFET output device:
 
http://www.vishay.com/docs/81646/vo14642a.pdf
 
Others may make similar parts, this is just one I am aware of.
 
Opto22 makes ac and dc modules but the ones I am familiar with I think are their older lines.