How to get power from PC to your circuits
Copyright Tomi Engdahl 1997
Serial port was never designed to supply any current to external circuits. Despite of this many manufacturers have designed many circuit which take power from serial port signal lines. With different methods you can realistically get only about 10 mA current at 5V voltage from the serial port. If you take very little current you can get around 11-12V (even +-12V if you set the output pins to right states). For more details on getting power from serial port read my article Get power out of PC RS-232 port.
PC parallel port was never designed to supply any power to the equipment connected to it. There are some methods for getting power from parallel port signal lines, but using all methods the voltage drops quite much if you try to get more than 1 mA per output used. For more details read my article Powering devices from PC parallel port.
The parallel port in the printer end (36 pin centronics connector) has an option for supplying poer to devices connected to it. The centronics connector has +5V power at pin 18. The power available from this pin is usually limited to about 100 mA.
Keyboard port has +5V power output to the keyboard. There is few hundred mA current available from this port and the keyboard takes it's own part of this available power. The maximum power available is usually limited by a fuse which is connected between motherboard +5V power line and the +5V output in the keyboard connector. Usually the fuse (typically 1A) is a small component which looks like a resistor and is directly soldered to the motherboard. Some older motherboard have used a normal glass fuse or they have not had fuse at all.
There are two different PC keyboard connector models in use. The older model is 5-pin DIN connector. This connector has +5V available at pin 5 and ground at pin 4. The pin numbering is usually marked in the connectors, but here is the pin numbering of the connector then you look it at your computer back:
2 4 5 1 3The other keyboard connector is PS/2 keyboard connector which is 6-pin mini-DIN connector. This connector has +5V power at pin 1 and ground at pin 2. Here is the pin numbering when you look at the keyboard connector at the back of the computer.
4 u 6 1 . 2 3 5
PS/2 mouse port
PS/S mouse port has +5V power output for powering the mouse connected to it. You can get small amouts of power from this port for your own circuits. The computer can turn the PS/2 mouse power off for some times when it initializes the mouse. The mouse does not take much current, so the power capability of this port might not be much (I guess that you can take around 100 mA safely). The pinout is same as in PS/2 keyboard connector: +5V power is at pin 1 and ground at pin 2. The pin numbering is also the same (looking at the connector on the back of the computer):
4 u 6 1 . 2 3 5
Joystick port in PC has +5V power output which is directly connected to PC +5V power line. The advantage of this is that there is quite much power available (up to few amperes). The disadvantage is that in short circuit condition there can be over 20 A current flowing and this can easily burn PCB traces and small in your circuit or inside your PC. Here is the pinout of the joystick port:
.-----------------------. \ 8 7 6 5 4 3 2 1 / \ 9 10 11 12 13 14 15 / ~~~~~~~~~~~~~~~~~~~You can get +5V power from pins 1 and 9. The ground can be found from pins 4 and 5.
Note: Some new PCI soundcards can supply 3.3V instead of standar +5V in the joystick connector. According some articles in the news DIAMOND MM SONIC IMPACT S70 has a 3.3volt game port and it is listed in their readme file for the product. I can just wonder what kind of incompability problems this can cause to existing joystick products and joystick port MIDI interfaces which are designed to use that standard +5V power from normal joystick port.
VESA DDC compliant monitor connector
If your computer graphics card has plug-and-play monitor support, then your computer has a VESA DDC monitor connector. VESA DDC standard defined that monitor connector pin 9 can be used as optional +5V 300 mA power supply from graphics card to monitor (the VGA connector ground is at pin 5). The standard does not define that there must be +5V supply at that pin. If you are making small circuit for just yoour on computer, you might want to check this option.
Remember that the VESA DDC standard demand the devices which take power from th graphics card use special VESA DDC modified version of DB15HD connector which guarantees the correct plug-in sequence of power ground and signal pins.
Power from USB connector
USB connector is designed to feed power up to 100 mA +5V power to the devices connected to it. The power is fed though Vbus and GND pins. The host USB pert can supply up to 500 mA at voltage at least 4.75 V (for the HUB to be able to power 4 port HUB which can supply up 100 mA at voltage of at least 4.40V). The USB port does not need to supply more than 100 mA unless the device connected to it requests more current. UL overcurrent-prevention requirement says that no USB port ever pass more than 5A onto any port (short circuit protection to avoid damage to equipments and wiring).
How to get power from inside PC
Disk drive connector
Normal floppy disk and hard disk drives use normally +12V and +5V power supplies, so spare disk drive power connector is very useful if you need those voltages for your own projects which you fit inside PC (for example extra fan for better cooling). Typically you can take up to few amperes of power form the disk drive power connector (remember not to exceed the total power rating of the PC power supply when you start taking extra power for your circuits).
Here is the power connector pinout when you look the CEE-type connector on the drive back:
_______ / \ | 1 2 3 4 | |_________| Pin Function 1 +5 V 2 Ground 3 Ground 4 +12 V
Motherboard power connector
Standard AT-style motherboard gets power from the power supply through 12 pin power connector which is typically built from two 6 pin parts. The power connector has four different voltages available. The positive +5V power has lots of current available (typically over 10A), +12V has few amperes. The negative power supplies can supply typically quite limited power (-12V few hundred mA and -5 even less).
1. Power Good (output from PS which tells that output voltages are OK) 2. +5Vdc 3. +12Vdc 4. -12Vdc 5. Ground 6. Ground 7. Ground 8. Ground 9. -5Vdc 10. +5Vdc 11. +5Vdc 12. +5Vdc
Power from ISA bus
ISA BUS connector provides +5V, +12V, -5V and -12V to power the cards connected to it. If you are building a circuit which plugs inside your PC, you might consider using the ISA connector as place to put your circuit even it it were not connected to the data bus at all. Building the circuit as ISA card gives an easy access to many voltages and provides mechanically quite secure way to fit your circuit inside the PC.
Here is ISA bus pinout with the port pins marked with bold text.
AT case back panel A1: *CHKCHK B1: GND A2: SD7 B2: *RESDRV A3: SD6 B3: +5 A4: SD5 B4: IRQ2 A5: SD4 B5: -5 A6: SD3 B6: DRQ2 A7: SD2 B7: -12 A8: SD1 B8: *NOWS [A] A9: SD0 B9: +12 A10: CHRDY B10: GND A11: AEN B11: *SMWTC A12: SA19 B12: *SMRDC A13: SA18 B13: *IOWC A14: SA17 B14: *IORC A15: SA16 B15: *DAK3 A16: SA15 B16: DRQ3 A17: SA14 B17: *DAK1 A18: SA13 B18: DRQ1 A19: SA12 B19: *REFRESH A20: SA11 B20: BCLK A21: SA10 B21: IRQ7 A22: SA9 B22: IRQ6 A23: SA8 B23: IRQ5 A24: SA7 B24: IRQ4 A25: SA6 B25: IRQ3 A26: SA5 B26: *DAK2 A27: SA4 B27: TC A28: SA3 B28: BALE A29: SA2 B29: +5 A30: SA1 B30: OSC A31: SA0 B31: GND
Filtering PC bus power
PC power available on ISA bus is very noisy particularly if the bus is very loaded. If you plan to put some other circuit that straight TTL logic to a card which you connect to ISA bus you should use some kind of filtering (especially if you are planning to build audio circuits).
Analog Devices suggests a filter circuit in their 1992 Amplifier Application Guide for filtering a 5V (digital) supply for use in single-supply analog applications:
-------/==\------*-------*------*------ +5 (filtered) \__/ | | | 2 turns --- --- --- around /-\ /-\ --- Fair-Rite | | | #2677006301 | | | ferrite bead | | | -------/==\------*-------*------*------ +5 return (filtered) \__/ 100uF 10-22uF 0.1uF elec. tant. cer.The circuit will give quite rather dramatic reduction of high-speed switching transients (according the source) for loads up to 100mA. For much higher loads you'll have to be careful not to saturate the ferrites. The circuit work just as well for dual-supply with appropriate modifications.
Other methods for getting very clean power is to get a higher voltage from the ISA bus and then filtering it to clean lower voltage using linear regulators and filtering capacitors. Typically soundcards use +-5..9V power supplies for their analogue circuits. That +-5..9V power is regulated using standard linear regulator ICs from +-12V power available in ISA bus connector.
Using PC power supply without other PC hardware on your projects
Well, your typical PC power supply is delivering about 200 Watts at its output, and is running at somewhere around 70-80% efficiency, which means that it needs to dissipate anywhere from 40 to 80 Watts internally. When you consider its relatively small size, you see why a fan is required.
PC power supplies usually have a minimum load requirement, which means that they won't regulate properly unless they're loaded to at least (typically) 20% of their maximum rating on the +5V output. This tends to make them unsuitable for low-power projects, unless you don't mind sticking a dummy load (large 1-ohm resistor) across the output.
A 250W supply will typically provide +12 at over 8 A. (You will need a small load on the +5 as well). With only minimal load on the +5, you may be able to push this a bit more. PC power supplies are readily available and inexpensive because they are mass products (any other power supply at same specs cost typically many times more).
Here is the pinout of standard pinout of the connector which goes to power typical PC motherboard (AT style motherboard, new ATX motherboards use different connector):
1. Power Good (output from PS which tells that output voltages are OK) 2. +5Vdc 3. +12Vdc 4. -12Vdc 5. Ground 6. Ground 7. Ground 8. Ground 9. -5Vdc 10. +5Vdc 11. +5Vdc 12. +5VdcHere is the power connector pinout when you look the CEE-type connector on the drive back:
_______ / \ | 1 2 3 4 | |_________| Pin Function 1 +5 V 2 Ground 3 Ground 4 +12 V
Typical wiring colors on PC power supply wires:
BLACK is ground RED is +5V YELLOW is +12VThose wiring colors are used in practically all power supplies. Colors for other voltages typically vary between different power supply brands. Don't blindly trust on the wire colors, because there are some power supplies which use totally different wire coloring.
- AT:n n�pp�imist�piiri by Tomi Engdahl
- CENTRONICS by Tomi Engdahl
- Powering devices from PC parallel port by Tomi Engdahl
- Get power out of PC RS-232 port by Tomi Engdahl
- Joystick port interface projects by Tomi Engdahl
- Display Data Channel Standard version 1.0
- PC/Logic-related ASCII Schematics V1.00
- The ISA bus web page by Mark Sokos
- How to use disk drive stepper-motors by Tomi Engdahl
- AT-emolevyn liittimet by Tomi Engdahl
- Designing a USB hub: basic options and issues, by Harry Inia and DIugh Dyment, EDN magazine, January 15, 1998, pages 113-129
Tomi Engdahl <email@example.com>