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RS-232 Power Question

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Hey everyone!

So i'm working on a project and I need to draw power to run a PIC 16F87x microcontroller board from an RS-232 (DB-9) cable. After extensive research, I have discovered this can be done in two ways.

1) Apply a voltage on pin 9 of the cable and then funnel this through to the board directly or a voltage regulator.
2) Attempt to draw power from pin 4 and 5 which are the DTR and DSR pins respectively via a two resistor - diode circuits in parallel to eachother.

The first option doesn't work for me so i'm attempting the second. My question is this: how much power can I get? Voltage wise. My intentions are to take the power from these pins and feed it into the input of a +5V voltage regulator. Now for this to work, I also understand I need at least 7 or 8V at the input. Can it be done?

This is my first time using a forum so if i'm vague in any way, or you need additional info, let me know :)

Thanks in advance,

Electronic Student
 
Mice used this technique. But it's only a few mA available, about 4 or 5. The real rub is many laptops and RS232 to USB dongles don't put out the full 12V so your milage may vary.
 
Hmm. See my idea was to draw power from the RS232, and then once I have a decent voltage after the voltage regulator, use nMOSFETs to increase my current. I'm trying to power a series of LEDs, 32 in total, using 8 I/O pins from the PIC. Do you think that's feasible? I know each LED needs about 20 - 25mA of power. My knowledge of MOSFETs is sketchy but I figured even a small gate current would yield a large drain current, or have I got that wrong?

I guess this is more a general electronic question!
 
Hmm. See my idea was to draw power from the RS232, and then once I have a decent voltage after the voltage regulator, use nMOSFETs to increase my current. I'm trying to power a series of LEDs, 32 in total, using 8 I/O pins from the PIC. Do you think that's feasible? I know each LED needs about 20 - 25mA of power. My knowledge of MOSFETs is sketchy but I figured even a small gate current would yield a large drain current, or have I got that wrong?

I guess this is more a general electronic question!

hi,
I guess you must be in your very first year as a EE student...

The RS232 or Parallel port cannot supply the current you are asking.

Is this project for your home personal desktop PC.?
 
Eric,

I am already aware the RS232 serial port will not supply the current I am asking, never did I say it would, my original question was how much power I could draw from the RS232 which was answered. I am now wondering if MOSFETs will do the job but I have since discovered an alternative way. Thank you for the help.
 
Eric,

I am already aware the RS232 serial port will not supply the current I am asking, never did I say it would, my original question was how much power I could draw from the RS232 which was answered. I am now wondering if MOSFETs will do the job but I have since discovered an alternative way. Thank you for the help.

hi,
You cannot produce a higher current from a given current source and still maintain the voltage of the source, thats something for nothing...:)

Is it for a one off PC project and how far away/distance is the project from the PC
 
Hey Eric,

I figured using a MOSFET may help to increase my current but if the source voltage remains the same that just won't happen (as you say). That's why my 'alternative way' was a different approach running power through pin 9 of the RS232. It's not for a PC project, basically i'm connecting an RFID reader to a microcontroller and the reader is using a RS232 connection, I guess 1 meter in length between reader and microcontroller circuit.
 
Hey Eric,

I figured using a MOSFET may help to increase my current but if the source voltage remains the same that just won't happen (as you say). That's why my 'alternative way' was a different approach running power through pin 9 of the RS232. It's not for a PC project, basically i'm connecting an RFID reader to a microcontroller and the reader is using a RS232 connection, I guess 1 meter in length between reader and microcontroller circuit.

hi,
The other way you could consider is to find an unused pin or free up a pin on the RS232 DB9 pin connector and connect it internally to the PC's 5V line.

An alternative is to use a spare hard drive power connector inside the PC, usually a few spares laying around, and wire that +5V and +12V via a 1mtr cable to your RFID.
 
Regarding the first way, I assume you're suggesting something similiar to what I mentioned regarding pin 9 of the RS232 connector. I've seen other people on the net attempting similiar projects using this pin. I guess it doesn't really matter what pin you use in the end though, as long as it's not needed by the microcontroller. I'm currently only using the RD, TD and GND pins so i'll give that a shot with one of the others. Thanks for the help.
 
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