Much thanks goes to Tomi Engdahl who wrote and has the following information copywritted. If you would like to contact him a link to his site has been posted below.
Introduction:
PC parallel port was never designed to provide any output power to divices connected to it. it was only designed for connecting printer to your PC. But in this weird PC business everything changes. Suddenly the parallel port have become a port where you can connect very wide selection of devices like software protection dongles, removable disks, modems, network adapters, printer sharing units, parallel port extenders,
Powering low voltage logic:
If you design your circuit to reliably operate at voltages from 3 to 4.5V you have no problem in getting right voltage from the parallel port. You have plenty of marginal for voltage drops in diodes.
Generating +5V from power available from parallel port:
If your circuit absolutely needs stable +5V power source, you have no other choice than to use a DC/DC converter to generate stable +5V voltage from the power it can take from the parallel port. Usually ready-made DC/DC converter modules are not suitable for this because they are designed mostly for bigger loads. What you are looking for is a very small current switching power supply which can supply +5V output from input voltage range of 2.4-4.5V and uses the power economically. Suitable components are those small single-chip miniature switches designed for small battery powered devices.
Directly taking the power from parallel port to power 5V logic
One method often used for powering small circuits is to use the power from data output pins directly to power the external IC. If you have one IC which takes less than 1 mA current, you can easily power it directly from one data output pin. You get around 4.5-5V to the IC very easily in this way just by using one data output pin. You can expect reliably draw about 0.5 mA from each data line which take total of 4 mA current.
If you need a little more power, you have to take the power from many data pins of the parallel port. Directly connecting all data output pins you wish to use to the +5V power rail of the circuit is not a good idea because this will cause problems. If you accidentally pull down one of those data pins you use for supplying the power, it will short the other outputs to ground which will overload the parallel port output chip. Also if you have parallel port datapins directly tied together, the port will likely fail in power-on shelf-test done every time the computer is started.
The more sophisticated way of getting the port from many data pins is to use diodes which enable only the current from the parallel port to come to the circuit, not in the other way. Those diodes will cause voltage drop (around 0.5V in silicon diodes at those current), which is what you want because you have already a slightly low voltage for your circuit. The voltage drop in the diodes can be minimized using schottky type diodes (voltage drop of around 0.2-0.3V). You might also consider using germanium diodes, but those have higher internal resistance and they are getting rare.
Very typical circuit which takes power from PC parallel port data lines through diodes is the telecard reader circuit presented in What you need to know about electronics telecards document. If you parallel the 8 data lines, you can reliably draw about 0.5 mA each. Due to the diode voltage drop in power paralleling circuit, you would be lucky to get 4-5 mA at 4.5V. There are 5 control outputs from which you can reliably draw 1 mA each. That comes to around 9 mA. If you can hold all outputs high, up to 50mA seems possible but the voltage would sag a lot. If you use switcher for generating the operation voltage for your circuit, then you can take out the maximum amount of power from printer port in this way.
In general, the more current and the higher the voltage your circuit requires the less likely it is to work on a variety of different parallel ports. Taking power directly form parallel port work well to fire up a few CMOS ICs that will work down to 3V (or less) and draw only a few mA.
How to use the power available from parallel port:
Because the power capabilities of the parallel port is very limited, the circuit you vant to connect to parallel port must be designed so that they take as little as possible power. This is possible by using small number of low power logic chips and running then at low clock frequencies.
Link to Site:
http://www.tkk.fi/Misc/Electronics/circuits/lptpower.html
Introduction:
PC parallel port was never designed to provide any output power to divices connected to it. it was only designed for connecting printer to your PC. But in this weird PC business everything changes. Suddenly the parallel port have become a port where you can connect very wide selection of devices like software protection dongles, removable disks, modems, network adapters, printer sharing units, parallel port extenders,
Code:
<= in DB25 Cent Name of
=> out pin pin Signal
------ ---- ---- --------
=> 1 1 -Strobe
=> 2 2 Data 0
=> 3 3 Data 1
=> 4 4 Data 2
=> 5 5 Data 3
=> 6 6 Data 4
=> 7 7 Data 5
=> 8 8 Data 6
=> 9 9 Data 7
<= 10 10 -Ack
<= 11 11 +Busy
<= 12 12 +PaperEnd
<= 13 13 +SelectIn
=> 14 14 -AutoFd
<= 15 32 -Error
=> 16 31 -Init
=> 17 36 -Select
== 18-25 19-30, Ground
33,17,16
Powering low voltage logic:
If you design your circuit to reliably operate at voltages from 3 to 4.5V you have no problem in getting right voltage from the parallel port. You have plenty of marginal for voltage drops in diodes.
Generating +5V from power available from parallel port:
If your circuit absolutely needs stable +5V power source, you have no other choice than to use a DC/DC converter to generate stable +5V voltage from the power it can take from the parallel port. Usually ready-made DC/DC converter modules are not suitable for this because they are designed mostly for bigger loads. What you are looking for is a very small current switching power supply which can supply +5V output from input voltage range of 2.4-4.5V and uses the power economically. Suitable components are those small single-chip miniature switches designed for small battery powered devices.
Directly taking the power from parallel port to power 5V logic
One method often used for powering small circuits is to use the power from data output pins directly to power the external IC. If you have one IC which takes less than 1 mA current, you can easily power it directly from one data output pin. You get around 4.5-5V to the IC very easily in this way just by using one data output pin. You can expect reliably draw about 0.5 mA from each data line which take total of 4 mA current.
If you need a little more power, you have to take the power from many data pins of the parallel port. Directly connecting all data output pins you wish to use to the +5V power rail of the circuit is not a good idea because this will cause problems. If you accidentally pull down one of those data pins you use for supplying the power, it will short the other outputs to ground which will overload the parallel port output chip. Also if you have parallel port datapins directly tied together, the port will likely fail in power-on shelf-test done every time the computer is started.
The more sophisticated way of getting the port from many data pins is to use diodes which enable only the current from the parallel port to come to the circuit, not in the other way. Those diodes will cause voltage drop (around 0.5V in silicon diodes at those current), which is what you want because you have already a slightly low voltage for your circuit. The voltage drop in the diodes can be minimized using schottky type diodes (voltage drop of around 0.2-0.3V). You might also consider using germanium diodes, but those have higher internal resistance and they are getting rare.
Very typical circuit which takes power from PC parallel port data lines through diodes is the telecard reader circuit presented in What you need to know about electronics telecards document. If you parallel the 8 data lines, you can reliably draw about 0.5 mA each. Due to the diode voltage drop in power paralleling circuit, you would be lucky to get 4-5 mA at 4.5V. There are 5 control outputs from which you can reliably draw 1 mA each. That comes to around 9 mA. If you can hold all outputs high, up to 50mA seems possible but the voltage would sag a lot. If you use switcher for generating the operation voltage for your circuit, then you can take out the maximum amount of power from printer port in this way.
In general, the more current and the higher the voltage your circuit requires the less likely it is to work on a variety of different parallel ports. Taking power directly form parallel port work well to fire up a few CMOS ICs that will work down to 3V (or less) and draw only a few mA.
How to use the power available from parallel port:
Because the power capabilities of the parallel port is very limited, the circuit you vant to connect to parallel port must be designed so that they take as little as possible power. This is possible by using small number of low power logic chips and running then at low clock frequencies.
Link to Site:
http://www.tkk.fi/Misc/Electronics/circuits/lptpower.html
Last edited: