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What is the best way to get a 5V Supply from PC's serialport

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Bus Master

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Dec 9, 2001
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Hi there,

any body knows the best way to get a 5V Supply from PC's serial port. I need only about 3 to 4 mA at 5V to supply some interface ICs.

I've turned the line DTR ON (Pin no. 4 at the 9 pins version), this gave me a 12V on this pin, and this was quite enough to turn on a 78L05 voltage regulator, which supplied the requested 5V.( Note that you have to connect the DTR line to the I/P pin of the 78L05 IC through a forward diode. This is important because the DTR pin supplies a -12V when low, which may destroy the 78L05).

The strange thing is: I've tried the circuit on a laptop, and i had no supply voltage. this was because the DTR pin gives about 7.3V when high, which was not adequate to turn ON the regulator. May be this was an accepted voltage level for the RS232 signals. ( Hi = signal >3V, Low = <-3V). Also, in Handheld devices, when it comes to power consumtion, this is so vital argument and needs to be minimized.

My question is: How to achieve a stable design for this situation, i need my circuit to work on Desktops ,Laptops and any IBM Compatible with a standard RS232 port.

Thanks in advance,


I would look at a DC-DC converter with a wide input voltage range. There are a bunch out there. One of them can be seen at:


M@xim and Line@r also make similar converters.

7805 output 5v and 7ma

Notorious problem with notebooks serial ports, due to its poor internal power supply for RS232 ports. Try to use some low drop linear stabiliser, it should work even with 7 volts on input.

Also use some shottky diode (BAT46) to minimize forward drop voltage on it.

5v the best

Hi, like HMX said, it's important first to do not use a standard 5v regulator like 7805, because they have a very large quiescient current, for example a 7805 has a 5mA current consuption. The RS232 standard specifies that a each pin has to be in +-3v an could be able to supply up to 20mA. Obiously, not many computers do this task. If your laptop can supply for about 7mA (per example), 5mA will go on 7805 an the rest for yous application, if your design has a current consuption higher than 3mA, output voltage will decrease rapidly under +-3v. So you have two options, use a DC-DC converter to step down to 5V, or to use a LDO, Linear regulator with Low Dropout, this devices can work with input voltage many similar to output voltage. And it's important if to use an input schoktkey diode. Since they have very low forward voltage at low forward current. If you are going to use only RX or TS of you PC, you can get extra power making an Logic OR with two diodes from DTR an RTS:

DTR --------->!------
|--------- POWER
RTS --------->!------


78l05 efficiency


You can use an MAX205 serial booster, but this one needs an
external 5V supply. I also have some problems with the serial
port of my laptop (Compaq Presario 1200).
What kind of circuit do you want to drive from the serial port?


max603 czy 7805

For this purpose i used a rail-to-rail low power opamp like the AD820 and a low power voltage reference like the LT1004-1.2 with 1Meg bias resistor. The Opamp is providing the +5V up to 15mA with only 5.8V input voltage, using feedback resistors of 3.8Meg and 1.2Meg.
This solution provides a ultralow power / lowdrop voltage regulator with about 600uA quiescent current.

A few years ago I had a similar problem. The simplest solution was :
A series drop resistor (around 100-150R) and a 3.3v zener in series with an led to stabilise it to around 5V, (the led as a power on indicator), and a 100-220m as a reservior cap.


There are some very good LDO regs out there that draw little or no current, check out AAT!
Schotky diode all the unused outputs from the pc into the LDO and turn them on.

a smps is good choice

hi,in this condition,a high efficiency smps is a good choice.somechips from maxim and linear tech works i remember,they have some article about get a 5v or 3.3v from serial port use a stepdown chip,it is easy.try to visit :
**broken link removed**
to find fact,any low quiescent current high efficiecy stepdown converter can do the job well.
good luck!

Re: What is the best way to get a 5V Supply from PC's serial

I'm always against such solutions as taking power from serial port. It is not used in preffesional equipment. Sometimes it happens that voltage at the serial port pins is in -3..3V range so it's useless. In addition, taking power from them may cause transmission errors. It is much better to use an external power source.

Re: What is the best way to get a 5V Supply from PC's serial

Please have a look on the following website
**broken link removed**

It explains a circuit capabling of delivering stable 5V out of serial port

Re: What is the best way to get a 5V Supply from PC's serial

These note take from Oreilly book: Design Embedded Hardware
Many RS-232C signals go unused and can supply a moderate amount of current
(nominally 50 mA, but it can vary and, as always, you should check the specific device to which you are interfacing). If your embedded system requires less than this for its total current draw, you can use an RS-232C control signal for power.

For instance, the RTS (Request To Send) or DTR (Data Terminal Request) signals may not be used in many RS-232C applications. Either can be used as the power input to a voltage regulator and thereby provide the system with power. The host computer therefore uses RTS of its serial port as the power control for the embedded system. Under software, the host sends RTS high, and the embedded system is powered up. Send RTS low, and the embedded system is switched off. The catch to all this is to ensure that your embedded system's current draw is low enough so that it can be powered by RTS. The advantage of this technique is that you require no external power supply for your embedded system. It works, as if by magic, whenever plugged into a serial port. The other catch is that you can't then use that RS-232C control signal for its original purpose. It must turn on and stay on to provide your computer with power.

Explaination for the schematic below:

Note the diode, D1. Since RTS will be a
negative voltage (as low as -15V) when low, some protection is required for the voltage regulator, since it is not designed to have its input taken below zero volts. The diode can be any garden-variety power diode, such as a 1N4004, and will conduct only when RTS is positive. The voltage regulator (MAX604) converts the voltage from RTS to a supply of 3.3V for the embedded system. If we required a supply of 5V, we'd simply use a MAX603 instead. The circuit would otherwise be the same. The output of the regulator is smoothed by the capacitor C5, and a power-on LED is provided to show us when we have power. The MAX3232 sits between the RS-232C port and the processor, levelshifting the serial transmissions from the processor's logic levels to RS-232C and vice versa.

Hope theses note will help.

You can use several output pins of your rs232 if you will connect them via diodes to one capasitor assuming if those piins are supplied from different sources . If they are suppied from one charge pump like dc-dc converter - it wont make sence and only one output pin will be enough . In addition to that you can get 5v from:
- mouse connector
- keyboard connector
- usb port
latter is most powerful up to 0.6 -1 A (check usb spec fpr exact info)

If efficiency is not the case for rs232 supplied source you can use zener diode and resistor as it was suggested in one of the above posts .

Re: What is the best way to get a 5V Supply from PC's serial

Is there a way for getting -5 from PC?
and what is the best way for converting +5 to -5 (without pc)?

Re: What is the best way to get a 5V Supply from PC's serial

verilog_crack said:
Is there a way for getting -5 from PC?
and what is the best way for converting +5 to -5 (without pc)?

RS232 has bipolar output .Although standard sets min level at somewhere +- 3 V
PC output voltage is always more than 5 V . You can get both polarities without convertor .

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