LPT anomalies "help"

Thread Starter

Bluebirdiran

Joined Feb 5, 2010
62
Hi Mike
I have done it for all 3 parallel ports at my disposal.
1- The normal LPT on my workroom PC
58mA @4.45v
2- The second LPT port on the smae PC
54mA @5.2v
3- The actual PC that is connected to my CNC router
50mA @3.3v
I know that only the third one is of interest but the other 2 may be of help for comparison purposes.

Cheers
Bluebird
 

Thread Starter

Bluebirdiran

Joined Feb 5, 2010
62
Pls. take note that the voltage drops to below .4v while measuring the current, ie under load. I have measured this using two DMMs.
Bluebird
 

MikeML

Joined Oct 2, 2009
5,444
Ok, the PP can source enough current to turn on the Emitter inside the opto-coupler. As suggested, you do need to limit the current. If the open circuit high voltage is ~4.5, and the forward emitter voltage is ~1.2V, and you want to get ~15mA into the emitter, then the resistor should be ~ 3.3/15m = 220Ω, which is what the Sarge said.

Most opto couplers have a current transfer ratio of between 0.5 and 2, so the collector current in the opto could be as little as 7mA or as much as 30mA. I would design the relay switching transistor to switch reliably at only 7mA from the opto, unless you have a guaranteed opto-coupler CTR of 1 or more.
 

Thread Starter

Bluebirdiran

Joined Feb 5, 2010
62
The Opto's data is as follows:
Input forward voltage 1.15v typical, I(f)=10mA
CTR=50% min
The PP's voltage to ba taken at 5.2v from the second port just installed in the CNC PC.
Using the above data, pls. tell me how exactly the transistoe should be setup.
Thanks
Bluebird
 

SgtWookie

Joined Jul 17, 2007
22,230
It's fine just the way it is.

With the new 3.6k base resistor and the extra pull-up resistor, you only need the opto to sink around 3mA. If you're putting 10mA through your IR emitter with a CTR of 50% worst case, you still have a couple mA to spare.
 

Thread Starter

Bluebirdiran

Joined Feb 5, 2010
62
Thank you,
I have just replaced the 2n3906 with a S8050 out of curiosity (I have quite a numer of these) and I just wondered if the circuit would work with these. The relay latches as soon as I turn on the 12v supply. It dosn't even let go if I take out the optocoupler from its socket. Also without the opto, when the supply is turned on, the relay latches. The transistor is ok, I have tested it.

It would be nice to know why this is happening.
Cheers
Bluebird
 

SgtWookie

Joined Jul 17, 2007
22,230
Does it work when you put the 2N3906 back in?

I don't see why the S3050 would not work as well for your particular application.
 

Thread Starter

Bluebirdiran

Joined Feb 5, 2010
62
I have loads of 2N3906 transistors too. So I wouldn't want to waste your time on this matter. I just thought there may be a simple explanation to this which I wanted to know too.

Thanks anyway, I have enjoyed our discussions all along.

Thanks again

Bluebird
 
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