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Powering microcontroller ciruit by rs232 port

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sachinkp21587

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dear all, i am trying to implement the following to make a serial port powered circuit.
SERIAL POWER TEST - 1.png

in my microcontroller circuit i have a 16x2 lcd with backlight off, a max232 and a AT89S8253 and a few keys.

now with external power this is taking only 14mA current.

As far as i have read on internet rs232 provides 45 to 50mA at the most.
So, i made the above circuit and connected my microcontroller circuit where the 5V drops to 3.2 volts DC.
i tried to measure the current provided by the port by help of multimeter but i failed as multimeter is showing absolutely nothing. even when i disconnected my load and used a led in place of it to measure the current, thn event it didnt work. i noticed one thing that when i am connecting led without the help of ammeter led glows well, but when ammeter comes in picture neither the led glows nor the multimeter shows any current reading even in microAmps.

Please suggest and guide me to right direction. any help is deeply appretiated

- - - Updated - - -

in the diagram shown its actually "MICROCONTROLLER CIRCUIT LOAD" and not "MICROCONTROLLER CIRCUIT POWER"
 

I dont think so as i have tried using two different multimeters. I tried with Fluke as well as HP which has a capability of showing even in microAmps.
Also i had measured the current when i powered the circuit with external power, this proves that the multimeter is ok.
 

A multimeter acts like a wire when measuring current. Since you believe the multimeter is working then it must be with how you are setting the multimeter or how you are probing. Check the multimeter settings (DC mA?) and verify the probe are plugged into the correct jacks on the multimeter (My MM has different jacks for current measurement). Where in your circuit are you inserting the multimeter? Between the DB-9, pin 4 and the bridge? Between the bridge and the regulator input? Between the regulator and the load?
 

i am connecting as follows....
pin 4 bridge --> 7805 --> led +ve + led -ve -->ammeter +ve -->ammeter -ve -->bridge -ve --> pin 5
basically led and ammeter in series.

i think the current is too low that the probe itself is not able to let the current though it.....
 

It's not likely the current is too low. Like I said, the meter acts as a wire in current mode. If the current was too low to measure, the current would still flow through the meter. One thing I see as strange is how you have the bridge connected to the circuit. The DTR signal is DC so a full wave bridge is not necessary. Your grounds don't look to be directly connected either. Try disconnecting the DB9 ground (pin 5) and the regulator ground (U1, pin 2) from the bridge and tie the 2 grounds together. You can replace the bridge with a single diode to protect the regulator from when the DTR line goes negative.
 

7805 uses at least 5mA and requires at least 2,5-3V voltage difference, RS232 port is not designed to power some circuit.

Is there possibility to use USB port ?



Best regards,
Peter
 

my aim is to basically make a multi-purpose small pluggable device that can communicate over rs232. here i dont want to go for battery as that will make the size bigger (for 9volt battery) and would create a casing problem for me as i am not a good mechanical designer of chassis :p so here i want to go for something that doesnt need any special battery or something for operation.
USB is a good choice but in my case i dont want to go for it as the device that i am communicating to is itself a server kind of a thing so dont want to waste the resources.

Is it like you people saying that its impossible to run the 14mA load by help of serial port?????

N yes i know that the regulator needs 3 to 4 volts higher input voltage. but thats what my problem is. the input voltage gets diminished when the load is connected otherwise its 12 volts actually.
 

sachinkp21587 said:
Is it like you people saying that its impossible to run the 14mA load by help of serial port?????

N yes i know that the regulator needs 3 to 4 volts higher input voltage. but thats what my problem is. the input voltage gets diminished when the load is connected otherwise its 12 volts actually.
Click to expand...


No, you can get 5V from RS232 but you need to be careful respecting RS232 port power limits and purpose.

Voltage level on RS232 is not the same on all PCs, specially laptops/notebooks have lower voltages.


First replace 7805/78L05 with some LDO/low quiescent current regulator.

See example of circuit :

**broken link removed**
**broken link removed**



Best regards,
Peter
 
i finally tried a zener diode between pin 5 and 4. it droped out to 3.3 volts.
thn i connected pin 4 and 7 together by help of diodes and thn to zener diode as shown below.
now the circuit is working fine for me.
right now m using normal LCD. now since the visiblity is not there on the screen without backlight so now i am shifting to a reflective LCD which need -5V for contrast.
hope this works too.

Still i am not sure about how much of current does RS232 port provides at max. Does anyone know anything exactly about this??
SERIAL PORT TEST - 3.png

- - - Updated - - -

Mr. spudboy u r right with the point that the current should flow but in practice it is not flowing dont know why.
2nd yes bridge is not necessary but still i have used to keep the circuit in on state when the server is powering on as the pin behaves randomly on different computers during boot time. so the circuit will act as a xor circuit or say will help in keeping the output voltage polarity constant what ever may be the voltage on DTR (i.e. +ve or -ve wrt ping 5). Also u r right that the circuits doesnt need to have common grounds if the two are not communicating. i noticed this point yesterday that i cannot use bridge as the server and my circuits need to communicate over rs232 also. if i am using bridge in that case at times the circuit will act as short circuit. like if dtr is -ve and 5 is ground (common ground) then at that time using bridge my ground will act as source for 5 volt generation, but, due to my circuit ground connected to pin 5 the circuit will get short circuited.

so now i have left the concept of using bridge and now m carrying out my tests by using diodes only.
 
Last edited:

sachinkp21587 said:
Still i am not sure about how much of current does RS232 port provides at max. Does anyone know anything exactly about this??
Click to expand...

Around 10-15mA.



sachinkp21587 said:
Also u r right that the circuits doesnt need to have common grounds if the two are not communicating. i noticed this point yesterday that i cannot use bridge as the server and my circuits need to communicate over rs232 also.
Click to expand...

Did you use MAX232 ?


Best regards,
Peter
 

Yes i am using max232. actually whole of my circuit is already in working state. only thing i am changing is to make it 232 powered.
 

as i told, i am not using bridge now. i am only using pins 4,5 connected through diode to a 5.1V zener diode, and the pin 5 as it to circuit ground.
now this 5v and ground is used everywhere.
since the pin 5 voltage is the reference voltage this will remain constant and through a vb program i'll keep DTR and RTS pins in high state(Physical States).
So when my software starts, it will power on the hardware and will keep the circuit on and communicating.

Till now i have tested everything including the Reflective Lcd. and now m heading towards max232 test besides powering it through serial port only.
 

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