[Axl_Rose]

I wanna build something like this: Serial port control of power switch - Bolis but I'm not all that up on the small electronics stuff. So I need a few questions answered.

1. Why does he need 2 relays? Wouldn't it make sense to get one Solid State relay, instead of a SSR that switches on a Mechanical relay?

2. If I were to want to put in a fuse could I use a automotive fuse? The switch the guy built in the link is only 7 amps, and I know car fuses go higher than that(unless the are rated in mAmps)

3. If I were to scavenge a diode off of something...how would I know if it is big enough?

4. Is there any revisions that should be made?

[Drakkon]

1) the solid state relay cant handle the current that the mechanical relay can...i assume he used the two for this reason. 1 to handle the switch for something more powerful and the other to switch the power coming from the port.

2) with a larger automotive fuse your going to be having more resistance and remembering our old equation V=IR the more resistance you got the more voltage in your circuit so you'd have to adapt across the board.

3) again advise against this, if you get to large a diode would cause possibly a higher voltage, allow to much current through, etc. test it with a multimeter - how to test a diode: http://www.kilowattclassroom.com/Archive/AN0001.pdf

4) if you've never done anything like this before i'd avoid making any revisions, changing resistance here and there can throw voltages off across the circuit and possibly fry the whole thing.

[Axl_Rose]

Originally Posted by Drakkon

1) the solid state relay cant handle the current that the mechanical relay can...i assume he used the two for this reason. 1 to handle the switch for something more powerful and the other to switch the power coming from the port.

2) with a larger automotive fuse your going to be having more resistance and remembering our old equation V=IR the more resistance you got the more voltage in your circuit so you'd have to adapt across the board.

3) again advise against this, if you get to large a diode would cause possibly a higher voltage, allow to much current through, etc. test it with a multimeter - how to test a diode: http://www.kilowattclassroom.com/Archive/AN0001.pdf

4) if you've never done anything like this before i'd avoid making any revisions, changing resistance here and there can throw voltages off across the circuit and possibly fry the whole thing.

Ok question concerning number 1. The SSR has 4 terminals correct? 2 for the input voltage(for switching). And then 2(one in, one out) for the lines you are actually switching. So the the line that gets switches comes in and goes back out again into the Mechanical Relay.

So what I'm trying to get at here is that, ultimately all the power has to run through the SSR anyways correct? So wouldn't the SSR be a Current bottleneck, thus making the mechanical relay useless?

[hans5849]

im not a engineer, but this is what i came up with for a schematic of how i would do it.



You can choose a power source for the relay. You might use a SCR instead of a Transistor for switching, or a FET of some sort. I think a P-Channel Enhancement Mode MOSFET would be a perfect choice because there is no chance of current traveling back to the computer. And you wouldn't have to worry about what level the power has to be in order to get the circuit working, just as long as the mosfet can handle being triggered by +4V to +20V. But FETs are ESD so you have to take precautions for static electricity.

It would be nice if he would of given a schematic so i could know what was going on.

[Axl_Rose]

I have no idea what you are talkin about hans lol.

[hans5849]

SCR= Silicon Controlled Rectifier, their used for switching, they can handle high current loads.

MOSFET= UM, yea. Its like a transistor (its actually what they were trying to make when they came up with the transistor), except with a transistor current flows thru the Collector, Emitter, and Base; with a MOSFET it flows from Source to Drain and the Gate (base) is just used to turn it on or off.

[Axl_Rose]

So a MOSFETcan be used for switchin just like Relay?

In your schematic are the 2 "squiggles" in the middle resistors? If so what are they for? I don't remeber how to read schematics so your drawing is very confusing to me.

Also as SSR's go...wouldn't this be sufficient? G3NA-210B-DC5-24 5-24VDC in and it has a 10A output rating.

And if someone could reply to my 2nd post about the bottleneck that would be great. I would like to know for learning purposes.

[hans5849]

In my schematic for turning on the relay it goes Transistor, Resistor, Inductor (really the inductor is part of the relay assembly).

Not really switching like as in the relay, but switching as in on and off. A mofset could be used in place of the transistor.

I don't know what a SSR is.

[MovingTarget]

he said rectifier....

[Axl_Rose]

SSR is Solid State Relay

[Axl_Rose]

So would the relay i posted work or what?

[hans5849]

theres another way you could do it, just i don't feel like drawing it right now.