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Using SCR's as a two way switch

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ryangoodner

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I need to replace a standard two position (3 leg) switch with SCR's. The reason is that the standard switch is not fast enough when switching over the two circuits connected to it. Can this be done? And if so how?
 
I also have MOSFETs available if that would be a better idea? Searching around I read somthing about using two parallel SCR's to make a Solid State Relay...could this be the answer to my problem...and how would I configure the SCRs for this?
 
I also have MOSFETs available if that would be a better idea? Searching around I read somthing about using two parallel SCR's to make a Solid State Relay...could this be the answer to my problem...and how would I configure the SCRs for this?

ahhh... you're much better off using FETs. If you back to back SCRs you can experience a phenom called latchup. What you'd want would be a TRIAC (ac switch). The 4066 is a quad cmos switch in one IC. What is the voltage connected to the circuits on the switch, and how much current? Is this digital type stuff, or high current/high voltage?
 
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Well its a project for a class tommorrow and I only have different SCRs and MOSFETS....the voltages on the switch are around 0-13v and the current is 0-3 amps usually...
 
ok, well what are your part numbers on the mosfets? what is your power source, and what is your load? how do you plan to trigger your switch on and off?

mosfets will conduct in both directions, btw..they're not like bipolars, once you turn them on the current can flow either way... the p or n channel just tells you which way you have to pull the gate voltage from the source to turn them on...
 
ok, well what are your part numbers on the mosfets? what is your power source, and what is your load? how do you plan to trigger your switch on and off?

mosfets will conduct in both directions, btw..they're not like bipolars, once you turn them on the current can flow either way... the p or n channel just tells you which way you have to pull the gate voltage from the source to turn them on...

i have N channel mosfets...+ allows s-d and - closes s-d...I thought this would be alot easier than it is. I have a mechanical two position switch currently on the circuit and just need to make it solid state so that it switches faster than I can manually switch it...
 
i have N channel mosfets...+ allows s-d and - closes s-d...I thought this would be alot easier than it is. I have a mechanical two position switch currently on the circuit and just need to make it solid state so that it switches faster than I can manually switch it...

mosfet is an IRF510
 
your circuit requirements are still a little foggy... what is on the switch common? power, or ground. If it's power, that's trickey with n channels, if it's ground, that's better... here's the problem... you have to get the gate x volts above the source. On the IRF510 the gate threshold voltage is 2-4 V. If you put the transisitor on the high side to source the 3A to your circuit, this means you have to get the gate at least 4 volts above the source. The IRF has a on Rds (resistance drain to source) of .54 ohm. At 3A, that means the transistor will drop about 1.62V across it. This means the gate has to go to 2.32V above the drain. If you've got V+ connected to the drain pin, then this is your highest voltage. You would need what they call a high side driver to turn the mosfet on in this manner. You also have a bigger problem. At 1.62V x 3A, you're gonna dissipate about 5W! Or 2.5W at a 50% duty cycle. I hope you also have a heatsink.

If you put the transistor so that it completes the circuit's path to ground, this is the normal method an n channel was designed for. You'll still have the heat problem, though.
 
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