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Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Well, Mosfets are used where fast switching times are neccessary, and being required to turn on and off very quickly, while most SCRs are used in pulse applications where the whole charge of a capacitor bank is discharged or when the frequency isn't too great. There are WAY more differences, but these are the most important two in my opinion. A search on google will explain a lot!
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Your application generaly dictates what you use since they arent all that interchangable. You might see both used in coilguns though, depends on the specifics there. I have used some scr's in my coilgun because all I am interested in is turning it completely on and letting a cap discharge through it. They also require very little supporting circuitry like a switch, resistor, and battery is all mine use to trigger. Plus one can get pretty high rated scr's for cheap (like used ones on ebay capable of pulsing several kA, often found for under $10 US each). In my project there is no need to turn it back off, something a typical scr circuit cant do.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
I connected a SCR and MOSFET to the same setup. I have an LED going from Vc to the Anode. I have a 1k resistor going from the Source/K to GND. I have one end of a 150 ohm resistor (R2) connected to the Gate and the other end I can toggle between +5v/float/GND.
Assume that Vc is always +5v and never dissipates.
MOSFET: THe LED goes on when I connect the other end of R2 to +5v. When I disconnect R2 the LED stays lit for a few seconds and goes off. If I immediately connect the end of R2 to GND after disconnecting from +5v the LED goes off.
SCR: The LED lights when R2 is connected to +5v. It goes off when I disconnect from +5v and it floats OR is connected to GND.
Is this difference in behavior of any consideration for circuit design?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
IamSmooth wrote ...
MOSFET: THe LED goes on when I connect the other end of R2 to +5v. When I disconnect R2 the LED stays lit for a few seconds and goes off. If I immediately connect the end of R2 to GND after disconnecting from +5v the LED goes off.
This is due to gate capacitance, and is a major issue when switching MOSFETs.
MOSFETs act as switches which you can switch voltage with in almost any way you want.
SCRs are a switch which will latch and won't turn off until current stops flowing through it. Then it opens again if the gate voltage is removed.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Either switch the gate off hard by using a negative signal during off time, or a pull down resistor.
Try putting a 1k resistor from gate to source on your breadboard setup, and the LED should turn off immediately after power is removed from the gate. This isn't really enough when dealing with high frequency though.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The SCR should stay on forever after triggering. I guess it only goes out because the current through it isn't enough to latch it. Try removing the 1k resistor and replacing the LED with something that draws more current, like a flashlight bulb, to see the true way a SCR works.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Stove Cenner wrote ...
The SCR should stay on forever after triggering. I guess it only goes out because the current through it isn't enough to latch it. Try removing the 1k resistor and replacing the LED with something that draws more current, like a flashlight bulb, to see the true way a SCR works.
Minimum holding current is usually defined in SCR's datasheet.
SCR has little common with mosfet's actually; it is a PNPN wich resemble two complementary transistors with their bases and collectors interconnected.
When we give a small current pulse to NPN one it opens the upper PNP bjt, wich in return keeps current flowing trough NPNs base and device keeps conducting.
MOSFET controls current by changing ''passable area'' for current by electric field on the gate capacitance. WIth DC at gate mosfet consumes no current. There are some differences between Jfet's, DMOS, enchacement mosfets etc. but that would be a sort of simplest explanation.
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SCRs and MOSFETs
