can we derive a 5v relay with a parallel port?

[hassan jamal]

Hi,
My query is can we derive a 5 volt relay directly from a parallel port ? i think that its not a good idea coz it might be hazardous for the pc due to the flow of current from the pc to the relay coil ( plz note that at the output side of relay i want to derive an ac equipment 220V) or i should use a buffer between the relay and the parallel port or any gate like XOR so that the 5V Vcc that would be given to the IC of the buffer may derive the relay, kindly help me. THanx

[nshakeeb]

Though Some pc can drive Relay[6v low amp] directly
I also think that it will not be wise to do so

You can use buffer

But I prefer to use optocupler. I think these ICs[4N25,4N35...] are available everywhere. They give good separation between
input and output and They are not costly.

[Tomi Engdahl]

My query is can we derive a 5 volt relay directly from a parallel port ?

Some very low power relay directly to parallel port has worked for me.
I have driven for example some semiconductor relays directly from parallel port.

Plase note that the guaranteed current from parallel port is only few milliamperes (can be more on some ports). Traditional prallel ports gave pretty near to 5V output voltage, but on some new PCs the output voltage on parallel port can be just only 3.3V.

When connecting normal relay to parallel port, remeber protection diode, otherwise the inductive kickback on relay coil can cause problems to parallel port (can damage it in worst cases).

or i should use a buffer between the relay and the parallel port or any gate like XOR so that the 5V Vcc that would be given to the IC of the buffer may derive the relay, kindly help me.

I would recommend driver IC.

There are circuit plans for this at
http://www.epanorama.net/circuits/parallel_output.html

[hassan jamal]

Dear Mr. Tomi Engahl
Thank you very much for providing me the useful information regarding my question. Now after all this discussion can i conclude that it would be ok if i derive the 5V electromechanical dc relay directly from the parallel port without limiting the curent that would be flown between the port and the relay coil (providing a diode for preventing the kickback of the relay )?

[hassan jamal]

Sir,
Actually i am afraid that if i do so (that is directly derive the electromechanical relay from the parallel port) it may be possible that the relay coil may draw heavy current that is more current than the usual current rating of the parallel port, kindly help me out in removing this ambiguity.

[Tomi Engdahl]

To limit the relay coil current to low value, you need to select a 5V relay that originally limits the current to this low value that parallel port can handle. This means that the relay has high enough coil resistance.
For example 5V relay with 1000 ohms coil resistance draws 5 milliamperes (are there this low power relays I am not sure).

You can't use a relay with higher current need and some external current limiting circuit (like for example series resistor), because this does not work. The extra current limiting woudl limit the current to safe value for parallel port, but this lower current will not operate the relay that is designed to need more current to work.

[lionking]

Hi,
My query is can we derive a 5 volt relay directly from a parallel port ? i think that its not a good idea coz it might be hazardous for the pc due to the flow of current from the pc to the relay coil ( plz note that at the output side of relay i want to derive an ac equipment 220V) or i should use a buffer between the relay and the parallel port or any gate like XOR so that the 5V Vcc that would be given to the IC of the buffer may derive the relay, kindly help me. THanx

Hi there,

It is not safe to drive something from parallel port directly. I ever used my parallel port to activate a 3V bulb, then my parallel port got burnt and cost me few hundred bucks to get a new pc motherboard. Now I will use a buffer IC to isolate the paralllel port from any electrical/electronic components.

I ever visited a website www.geocities.com/bagtool/io_activex.html which provides some interfacing info and talk on how to read/write the parallel port through internet. There is a IO activex for Visual basic to download there, but it is a trial one Now I am trying to build some remote control program in vb.

dan

[Tomi Engdahl]

It is not safe to drive something from parallel port directly. I ever used my parallel port to activate a 3V bulb, then my parallel port got burnt and cost me few hundred bucks to get a new pc motherboard. Now I will use a buffer IC to isolate the paralllel port from any electrical/electronic components.

You can drive certain loads that do not take too much current dirctly from parallel port. But you need to be very careful what you drive from it.
Generally it is a good idea to use a driver IC between the load and parallel port. I have for example driven small LEDs with suitable resistors without damage to my port whatsoever directly from parallel port. Details are at http://www.epanorama.net/circuits/parallel_output.html

And other tip is not to play with the parallel port on your motherboard if there is possibility that somethign in your circuit might be wrong. Get one of those extra parallel port cards to your PC and experiment with the port on it. This kind of card cost maybe 20 dollars/euros, and is easy plus cheap to change of something goes wrong.
Some details on this kinds of cards can be found at http://www.epanorama.net/circuits/parallel_output.html

I ever visited a website www.geocities.com/bagtool/io_activex.html which provides some interfacing info and talk on how to read/write the parallel port through internet.

Looks somewhat interresting. I am not a fan of activex controls...
I prefer doing my hacks using open source software.
I have provided some examples how I have done the similar things in Linux system at http://www.epanorama.net/circuits/paral ... webcontrol

[hassan jamal]

thank you very much for giving me such a great great knowledge.

[coolvista]

If you would like to purchase a relay board, I have a few left which I can sell, please see my homepage [url]http://www.coolvista.co.uk[/url]

This is already assembled, unlike a lot of kits which you have to assemble yourself.

I Have successfully controlled this remotely from a web browser, via my linux pc, feel free to ask me questions, and I'll try as best I can to help you use your board should you buy one.