SS relay w/ NC and NO terminals

D Faust

Would the following schematic work as a solid-state relay as shown?· The ssr with NC/NO terminal that I have seen are pretty expensive.
PS- Sorry for the bad schematic, I don't have special software, so I used MS paint.

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Twisted Pair

Hello D Faust,

If your input to your transistor is a pin from a chip, it is customary to put a resistor between the pin and the base of the transistor to protect the chip. A 220 ohm resistor would be common in most cases. It is un-clear what hardware you have and what you want your circuit to do....

Twisted Pair....

D Faust

Yes, I guess I forgot the resistor, thanks.· As for a purpose, I was looking for a solid state way to expand my number of inputs for my stamp.· I plan on adding a few more optoisolators to get up to 30 i/os for my stamp.· I will probably double only four of them.· The reason that I want to go with solid state is to cut down on breakdown, and noise.· Also, If I use this concept correctly I can get direction control for a motor without a clicky relay.· In case anybody wants to know, I started thinking about this after the thread titled: "substitution" in the basic stamp forum.

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Phil Pilgrim (PhiPi)

I don't think you need the extra transistor. Here's something that should work...

-Phil

Update: My bad. My bad, bad, BAD! The original circuit is a great way to fry two optoisolators. Here's a better one. It's not perfect though. If the input is floating, both optos will be on.

'Sorry.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 8/21/2007 5:25:04 AM GMT

D Faust

GENIUS!!!··Thanks, that makes it a lot easier.··If I want to connect more can I use the outputs of those two to drive the other ones, right?· If I load any more in series it will blow out the stamp or not work, right?

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Phil Pilgrim (PhiPi)

Some genius. I hope you didn't fry any parts because of my suggestion. See my correction above.

-Phil

D Faust

It's the concept that counts.· I actually don't have any optoisolators, but I was trying to see what I could do with some. I'm not sure, but shouldn't the resitor on the line from the top optoisolator be on the +V side rather than the -V side to protect the led in the optocoupler.· If this isn't what you tried to change please explain what you did. Thanks for catching your mistake.

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Phil Pilgrim (PhiPi)

Either resistor can be in either place, just so there's one resistor in series with the active opto (on the anode or cathode side) when the input is either at Vdd level or at ground level. The reason my first idea was so bad is that there were two diodes in series across Vdd and ground, without any resistive current limiting. Instant smoke! But, again, when the input floats in the second design, there will still be some current through both optos, and they might both come on.

-Phil

D Faust

Might it still be wise to put the 220 resistor befor the pin to protect it.· With the resistors you put in the second time are you trying to limit the current to the stamp or optos.· I understand that if the stamp pin is set low and the line is pulled high that the stamp wil fry.· Is this what the resistor is for?· Thanks for all your help.


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Phil Pilgrim (PhiPi)

The resistors are there to limit the current in the optos. But they also limit current into or out of the Stamp pin, depending on whether the pin is high or low. The attached illustrates.

If, in fact, you added an additional resistor to the Stamp pin, both optos would always be on. This is because the resistor would share current with the opto that's supposed to be off.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 8/21/2007 6:47:09 PM GMT

D Faust

Okay, thanks for clearing that up.

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