SCRs and MOSFETs
IamSmooth, Wed Sept 06 2006, 01:24AMI understand some of the differences with the two devices. I guess what I don't appreciate yet is how one decides which one to use for an application.
Re: SCRs and MOSFETs
EDY19, Wed Sept 06 2006, 03:08AM
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!
EDY19, Wed Sept 06 2006, 03:08AM
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!
Re: SCRs and MOSFETs
Quantum Singularity, Wed Sept 06 2006, 03:36AM
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.
Quantum Singularity, Wed Sept 06 2006, 03:36AM
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.
Re: SCRs and MOSFETs
IamSmooth, Wed Sept 06 2006, 11:43AM
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?
IamSmooth, Wed Sept 06 2006, 11:43AM
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?
Re: SCRs and MOSFETs
uzzors2k, Wed Sept 06 2006, 12:04PM
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.
Well thats just my 0.12 NOK...
uzzors2k, Wed Sept 06 2006, 12:04PM
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.
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.
Well thats just my 0.12 NOK...
Re: SCRs and MOSFETs
IamSmooth, Wed Sept 06 2006, 12:47PM
How does one deal with the gate capacitance so it is not an issue? Ensure good grounding or current path when the Gate is suppose to be off?
IamSmooth, Wed Sept 06 2006, 12:47PM
uzzors wrote ...
This is due to gate capacitance, and is a major issue when switching MOSFETs.
This is due to gate capacitance, and is a major issue when switching MOSFETs.
How does one deal with the gate capacitance so it is not an issue? Ensure good grounding or current path when the Gate is suppose to be off?
Re: SCRs and MOSFETs
uzzors2k, Wed Sept 06 2006, 05:58PM
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.
uzzors2k, Wed Sept 06 2006, 05:58PM
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.
Re: SCRs and MOSFETs
Steve Conner, Thu Sept 07 2006, 10:27AM
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.
Steve Conner, Thu Sept 07 2006, 10:27AM
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.
Re: SCRs and MOSFETs
Marko, Thu Sept 07 2006, 11:58AM
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.
Marko, Thu Sept 07 2006, 11:58AM
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.
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.
Re: SCRs and MOSFETs
Simon, Sun Sept 10 2006, 01:19AM
One of my first CGs had a sensitive gate SCR with gate left floating. I learnt my lesson after a few unintended firings.
Simon, Sun Sept 10 2006, 01:19AM
uzzors wrote ...
Either switch the gate off hard by using a negative signal during off time, or a pull down resistor.
A pull down resistor of maybe 1M is worth having anyway in a lot of apps.Either switch the gate off hard by using a negative signal during off time, or a pull down resistor.
One of my first CGs had a sensitive gate SCR with gate left floating. I learnt my lesson after a few unintended firings.
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