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Registered Member #798
Joined: Tue May 22 2007, 07:15PM
Location:
Posts: 1
i have a quick question. why and when would you use a SCR over a BJT. I don't mean using bjts to create a SCR, but why would you want the SCR component. I see alot of switching diagrams that use an SCR where it looks like a NPN transistor would do the job, and other schematics which are similar and use NPN's. So why and when. Thanks
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
You are right, an SCR could sometimes be replaced with a BJT, but they are inherently different devices, each having their place. SCRs are good at low frequency work, and can be made to conduct *enourmous* currents (up to thousands of amps). And they have a gate, rather than a base, to control them. Its really just terminology, but driving a gate of an SCR to turn the device ON is easier than driving the base of a transistor, this holds particularly at high currents where base drive can become quite difficult! So, SCRs have lower losses (which is why they can handle more current typically) and are easier to switch on. But, their main drawback is that they cant be made to turn off a current (with exception of the GTO SCR). So this is why they are popular for AC line control, where the AC current naturally reverses direction (putting the SCR into a blocking state) where it can be shut down.
An SCR could never replace a BJT in say an audio amplifier. In this case the BJT must be made to change the current flow rapidly, and you must also have more control of the current rather than ON/OFF.
As you become more familiar with solid state electronics, you will surely run into the MOSFET, and IGBT. These devices are similar to eachother, and somewhat like a BJT, but again, they have particular properties that make them more or less useful for specific applications. The use is so widespread for all of these various switches, thats it would be exhausting to cover all of their uses.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
So, SCRs have lower losses (which is why they can handle more current typically) and are easier to switch on. But, their main drawback is that they cant be made to turn off a current (with exception of the GTO SCR). So this is why they are popular for AC line control, where the AC current naturally reverses direction (putting the SCR into a blocking state) where it can be shut down.
Just to add up on it, normal SCR's can turn off a current with little external help, and in past they were actually popular for simple PWM control, usually of large motors, where they never work higher than HZ range and with high DC voltages.
I would say that their real drawback is, that they are reaaally slow, slowest of all silicon switches, but most robust and cost effective. Today they are mostly getting replaced by IGBT's.
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SCR vs BJT
