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Old 01-13-2012, 03:10 AM
spelcaztr spelcaztr is offline
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Default Substituting SCRs for Relays

HI guys. I am having a bit of problem with my project. There seem to be too many relays in it which makes it inefficient. Anyone here has any idea how to replace relay with an SCR? I heard that SCRs have a latching action which i would need to build my circuit.

I have attached a picture of how the circuit should act. I want a circuit that would latch the 1st branch while unlatching the other and vice versa. My problem is that i can't seem to unlatch an SCR in DC mode. Anyone got an idea?

Any idea would be appreciated. Thanks in advance.

PS: DPDTs are not available in my area so substituting 2 SPDTs would really be costly
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Old 01-13-2012, 03:30 AM
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MrChips MrChips is offline
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You cannot use an SCR in a DC circuit. In order to turn off an SCR, you have to reduce the current to zero. This happens in an AC circuit when the sine wave crosses zero.
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Old 01-13-2012, 11:10 AM
DickCappels DickCappels is offline
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What are you trying to do? It is not clear what function your relays perform and what the load (if there is one) looks like. Do you really want to power 6V relays from 12V?
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Old 01-13-2012, 07:55 PM
spelcaztr spelcaztr is offline
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Actually, the first string should power a relay to cut a certain supply voltage for a split second same as the second string. Don't want anyone to copy my project so i won't post anymore than this (took me days to convert my idea into a circuit)XD and i am not asking for the actual solution but at least any idea.

I read some topics about SCRs in DC mode so i really think that is still possible. i read something about using a capacitor force the SCR to reverse bias. As well as using another SCRs for reverse bias the SCR. and as far as i know, SCRs can be reversed biased with the same method as a diode. I hav'nt confirmed this theory yet so i am humbly asking for an idea on how to do this. ^^,

Anyways, I appreciate your feedback. Thanks a lot. Maybe it was actually impossible..?
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Old 01-13-2012, 08:20 PM
mcgyvr mcgyvr is offline
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A suitable replacement for relays "could" be transistors/mosfets/SSR/optoisolators.
Why are you considering a relay as "inefficient" in your device?
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Old 01-13-2012, 09:07 PM
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ifixit ifixit is offline
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Hi spelcaztr,

Here is a circuit idea that would allow you to have a latching SPDT relay using an SCR. It leaves the SPDT contacts available for other purposes.

Shorting out the SCR with the transistor reduces the SCR current to zero, so it unlatches, and then when the PB is released the relay is deactivated.

This circuit idea has not been fully designed or tested, but I have in the past used the method of shorting SCRs to unlatch SCRs.

Regards,
Ifixit
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  #7  
Old 01-13-2012, 09:22 PM
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BillB3857 BillB3857 is offline
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Check out the sketch in post #19 of this thread....http://forum.allaboutcircuits.com/sh...?t=7478&page=2

That is a simplified drawing of a single phase of a 20HP industrial inverter. Any method of reducing the current through the SCR to a level below the minimum holding current will work. A second SCR in series with a capacitor can provide a shunt current path to do just that. When the 2nd SCR fires, the charge current of the cap will shunt current around the primary SCR until the cap charges. A very high value resistor in parallel with the cap will discharge it
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Old 01-14-2012, 02:22 PM
spelcaztr spelcaztr is offline
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thanks for the ideas guys. will look up to this. My previous design involves 5-7 relays depending on the availability of a DPDT relay. I assume relays are inefficient as it consumes too much power (6v min) to be switched while modern CMOS uses 5v max (or lower for high speed switching purposes) and my project requires fast switching which is greatly hurdled by the use of a relay. ^_^ Or is my logic fraud? I guess i did not learn much during my logic circuit days. XD

@ifixit,
thanks for the idea. Shorting the SCR IS the best way to unlatch it. I modded your circuit a little and it worked.
I consider anyone who designs using a PNP transistor as a godlike designer.

@BillB3857,
i didn't really understood what the diagram mean. Sorry. We were poorly taught about Thyristors. Maybe someday i could understand it. Thanks for the reference though.

@mcgyvr,
i dont think transistors could replace a relay as it has no ability to "latch" Don't know about the other two though.. I guess i got to study them too. Thanks!

Last edited by spelcaztr; 01-14-2012 at 03:16 PM. Reason: proper thanking
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Old 01-14-2012, 03:19 PM
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MrChips MrChips is offline
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What you are attempting to do most likely can be solved with a bunch of digital components. Most certainly a single MCU chip will do the job. No relays. No SCRs.
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Worksheet Latch circuits
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