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Author Topic: rs-232 wire gets warm  (Read 874 times)
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I have noticed that if I run my freeduino (NKC Electronics Serial V.1)  from a 9V wall wart everything is fine but if I run 9 volts (voltmeter shows 8.71V) from a computer power supply through the Vin and Gnd, the rs232 line gets noticeably warm.  Is there a current limiting resistor that I should be running in between the power supply and the Vin on the board?  Looking at the schematic for this board, it seems that the input from the wall wart and the Vin are electrically common except for a 1N4004 diode that runs between them.  Any ideas on why this may be happening?

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What do you mean by Computer Power Supply?  I don't remember seeing 9V coming out of a PC power supply, only 12V and 5V.
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It's almost certain it is an earth problem. Try disconnecting pin 5 from X1.
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Oh geez I bet you are right.  I didn't even think of that.  I am getting the nine volts from the power supply by using the +12V and +3.3V wires.  The rs-232 is expecting a true ground and my 9V is referenced to a +3.3V ground.  I'm surprised I didn't fry my serial port or worse my entire mother board.  I'm really new to this hobby so unfortunately I will probably make many more of these mistakes.
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I am getting the nine volts from the power supply by using the +12V and +3.3V wires.
I seem to recall reading somewhere that it was unwise to do that.

--Phil.
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I seem to recall reading somewhere that it was unwise to do that.

--Phil.

Do you remember what the reasons were for that?  Like I said above....I am very new to all of this.
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I seem to recall reading somewhere that it was unwise to do that.

--Phil.

Do you remember what the reasons were for that?  Like I said above....I am very new to all of this.

The computer power supplies are constructed supposing that you will use a "+ pin" and a "gnd" to feed some equipments, boards, etc.
When you use +12 and +3.3v to create a "8.7volts power supply", the computer will supply currents different than what was supposed to work.

Another problem you can have, is when you make a mistake, connecting a +3.3v as a "relative gnd" and in some point of the circuit you can forget you did this, and connect the real gnd to the same point, creating a short-circuit on +3.3 power supply.

As you said you're a newbie, I suggest you don't do this.
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the computer will supply currents different than what was supposed to work.

That is totally wrong. There is nothing odd or different about the current sourced this way.

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connecting a +3.3v as a "relative gnd" and in some point of the circuit you can forget you did this, and connect the real gnd to the same point, creating a short-circuit on +3.3 power supply

That is totally right and was the problem here.
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That is totally wrong. There is nothing odd or different about the current sourced this way.

Mike,

I know that, in theory, we can use different voltages (12 and 3.3) to create a voltage difference (8.7) that we need.
I'm not sure if the computer power source construction allow us to work in that way, because the current will flow in opposite direction on the 3.3 pinout. I meant, I'm not sure if the computer power source 3.3 is made to work as a "negative" pole of the circuit, like the gnd pinout does.

I think it's necessary to check the schematics to be sure, no? Or am I wrong? :-/
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Probably easiest [= laziest] of all is to get an appropriate plugpack [wall-wart]...
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Cheers,

PeterV in Canberra

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because the current will flow in opposite direction on the 3.3 pinout[\quote]

Yes I understand what you are saying but I disagree. A power supply has a low output impedance and will not be affected by the current flow what ever the direction.
If there is already current being sourced by the 3.3 volt rail then the extra load will go to supply that load. The regulation will then take up the slack and throttle back the current from the load. The only way I can see difficulty is if the 3V3 rail is not regulated but relies on being a proportion of the higher voltage. Even then I can't see it being much trouble at the currents we are talking about. Powering op-amp circuits like this has been going on for years and I have done it many times myself.
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Ok. Thanks. smiley-wink
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