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Author Topic: Isolating VCC in circuit  (Read 1943 times)
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Left Coast, CA (USA)
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Ok, what am I missing here?  On the left is SFE's SPDT symbol.  On the right is their SPST symbol.   So, why can't I use the SPST switch?  Isn't it just a flip-flop between the two poles?  If the AVR is on the left, and the two poles on the right are one to the FTDI's VCC and the other to the PSU VCC?

The SPDT seems to have more poles than I need ...

What am I missing?

Your symbol drawing is trash. The one on the left is a DPDT and the one on the right is a SPDT. A SPDT (as drawn on the right symbol) is what will work for power switching on your board. The center terminal will become the board's +5vdc bus. The two other contacts will be wired to the USB's +5vdc and the external +5vdc regulators output.


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Colorado
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Well to my mind the left one has two poles, it's a DPDT and the right one is a SPDT.

You're thinking is spot on with mine.

The SFE drawings are wrong ...

Wouldn't be the first time ...
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Colorado
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Your symbol drawing is trash.

It's SFE's symbol, not mine. smiley

The one on the left is a DPDT and the one on the right is a SPDT. A SPDT (as drawn on the right symbol) is what will work for power switching on your board. The center terminal will become the board's +5vdc bus. The two other contacts will be wired to the USB's +5vdc and the external +5vdc regulators output.

Then, what I had in my schematic earlier is correct then.
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Your symbol drawing is trash.

It's SFE's symbol, not mine. smiley

Understood. But SFE means little to me, I just call them like I see them.  smiley-wink

The one on the left is a DPDT and the one on the right is a SPDT. A SPDT (as drawn on the right symbol) is what will work for power switching on your board. The center terminal will become the board's +5vdc bus. The two other contacts will be wired to the USB's +5vdc and the external +5vdc regulators output.

Then, what I had in my schematic earlier is correct then.

Perhaps, it's a long posting thread, perhaps you could link again and I would be glad to take a look at it.

Lefty
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Colorado
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Here ya go.


* Simple.png (25.34 KB, 1464x793 - viewed 12 times.)
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Here ya go.

Yep, perfect. Did you understand about using 3 pin SIP male header pins and a jumper clip instead of a physical switch? Takes less PCB space and won't be switched accidently as easily.

Lefty

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I did, but this board will be inside of an enclosure that won't be opened too often, so if I have a slide switch that can be accessed through a thin slot, that's perfect.  The only one who would be reprogramming it (for now) would be me anyway, so I'm happy with that.

Now, for all intents and purposes, a DPDT would also work, yes?  Just connect both poles to the same thing.
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Now, for all intents and purposes, a DPDT would also work, yes?  Just connect both poles to the same thing.

Yes, A DPDT is just two SPDT contact sets sandwiched together. As long as you wire only to one side or the other 3 pin rows, a DPDT can be used as a SPDT, or as two SPDT, or even as one or two SPST switch if desired. It's only confusing if you let it be.  smiley-wink


Lefty

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Hey, Confucius is my middle name. smiley

Thanks for the help!
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By the way, should those series resistors on the TX/RX lines be 1K, or 10K?  Somewhere someone told me to use 10K, but I can't remember anymore where ...  Does it make a lot of difference, such as completely rendering those lines "dead", preventing a reprogram?
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By the way, should those series resistors on the TX/RX lines be 1K, or 10K?  Somewhere someone told me to use 10K, but I can't remember anymore where ...  Does it make a lot of difference, such as completely rendering those lines "dead", preventing a reprogram?

The arduino design uses 1k isolation resistors, but I suspect 10k would might work also, but best to stick what has been proven to work over all the baudrates used on FTDI based arduino boards.

http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf

Lefty
« Last Edit: February 23, 2012, 03:03:34 am by retrolefty » Logged

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Dont forget decoupling caps BTW
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Dont forget decoupling caps BTW

Yup.  I didn't put them in the posted schematic because it was 'in it's simplest of simple form', where all I cared about was the VCC lines.  The actual schematic has a lot more stuff in it, decoupling caps from here to the moon and back is just one of those things.  *grin*
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Basically, if ONLY the external PSU is connected, I don't want (or need) the FTDI IC powered up.

So I'm probably missing something, but I have to ask:

If you don't always want to use the FTDI chip, why don't you simply put an FTDI 6pin header on the board and then use an FTDI cable (or a USB/TTL Serial adapter) when you need it?

I'm probably missing something...

Brad.

PS I like Lefty's idea of using 3 0.1" headers as your on-board switch.  Then if you want a real switch, simply wire one up with a small jumper (w/ female pins).  Of course this takes a bit more enclosure space but it's more flexible.
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I'm probably missing something...

Yes. He wants the board mounted FTDI chip to draw power only if it's actively plugged into the PCs USB buss. It's a current saving thing that some want or need and as the Seeeduino design allows. Using an external USB serial cable or module is another way of accomplishing the same thing.


Lefty
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