I'm out on pin one for my spindle relay (3v-15v input).  When toggled on I show 4.34v at pin one. When I connect the relay it drops to 1.43v and wont turn the relay on. I know the relay works if I jump it to 5v it turns on.  Tried it on pin 14 and 16 also, same result.  No luck.  It used to work fine.  Just became a problem in setting up a new PC.  I'm lost for words I've been banging at this for at least 3 hrs. 

sounds like the new computer puts out less current, energy star, I'd try a relaY that needs less current

just an idea?

Maybe I need a relay to turn on my relay?!?!

could be done. I don't have anything hooked up directly so I'm probally talking out of turn. could be the windings just shorted.

how much amperage is it pulling?

Problem is that the printer port outputs are not meant to handle much current. You need to have teh output from the computer go to a transistor that is sized correctly for the curent teh relay requires. Normally this is a function provided by a "break out board". It isolates the machine electrically from the computer and priovides the circuits to be able to handle the cuurent requirements of real world components like relays and such.

What is the relay switching? AC or DC and what current? there are a number of solid state realys that are able to switch higher currets, but are meant to take a low current control signal.

Repeating what has already been said but the computer port was never designed to switch anything more than about 10mA and even then there are conditions attached. You are pretty lucky you got it to work first time round without damaging the port.

One solution is to use an Opto- Coupler, most of these will handle switching currents up to 100mA (which is enough to drive small relays etc.) and I have attached a simple schematic of a typical setup.

Another solution, if you are switching ac loads, is to use a solid state relay as most of these will operate at less than 10mA input and switch ac loads with absolute minimal noise spikes.



Hope this helps.

Tweakie.

This is the relay I'm using.  CRYDOM EZ240D18 I bought it from MPJA.  Switches 240 @ 18A  using 3-5v.

Not sure what to do I guess  need to read up on Opto-couplers and breakout boards.  I certainly don't want to lose another computer.  The setup time is too much.

The SSR you are using should work just fine but there is, perhaps, a better way of driving it.

The LPT parallel port will always sink more current than it will source. Changing the polarity of the drive function (taking the LPT pin low activates the relay) is a much better approach (and kinder to the port). Something similar to this. (change the resistance to suit the SSR drive current - typically less than 10mA).

Tweakie.

Interesting?  I'm going to take a closer at this.  Keep the relay hot and then close it to ground. Thanks.

I changed the output to active low and add a 470k R (1/4 Watt) to the 5v power supply. It dropped the voltage to about 3.46v from 5.1v. It's didn't turn the relay on.  I also tried a 470 R (1/4 Watt) and had no voltage change, I didn't hook this up the the pinout . 

The SSR is rated for typical input current of 3-15v 15mA @ nominal voltage.  http://www.mpja.com/download/16596rl.pdf

E=IR is not a formula that I am very good with. I'm afraid to smoke the port?

The power supply is a standard PC ATX supply is that is a clue?

Not sure what's happening.