Wednesday, November 12, 2014

PC Power Saver

This circuit is designed to help minimise the  quiescent power consumption of PCs and  notebooks, using just our old friend the 555  timer and a relay as the main components. The  circuit itself dissipates around 0.5 W in operation (that is, when the connected PC is on);  when switched off (with the relay not energised) the total power draw is precisely zero. A prerequisite for the circuit is a PC or note book with a USB or PS/2 keyboard socket that  is powered only when the PC is on. The power saver can be used to switch PCs  or even whole multi-way extension leads. The unit can be built  into  an  ordinary  mains  adaptor (which must have an earth  pin!) as the photograph of the  author‘s prototype shows. The  PC is plugged in to the socket  at the output of the power saver  unit, and an extra connection  is made to the control input of  the unit from a PS/2 (keyboard or mouse) socket or USB port. Only  the 5 V supply line of the interface is used.
 
  
PC Power Saver Image

When button S1 on the power saver  is pressed the unit turns on, and the  monostable formed by the 555 timer is  triggered via the network composed by  R4 and C7. This drives relay RE1, whose contacts close. The connected PC is now tentatively powered up via the relay for a period  determined  by  P1  (approximately in the range from 5 s to 10 s). If, during this interval, the PC fails to indicate  that it is alive by supplying 5 V from its USB or  PS/2 connector (that is, if you do not switch  it on), the monostable period will expire, the  relay will drop out and any connected device  will be powered down. No further current will  be drawn from the supply, and, of course, it  will not be possible to turn the PC on. When-ever you want to turn the PC on, you must  always press the button on the power saver  shortly beforehand. 

If, however, 5 V is delivered by the PC to the  input of optocoupler IC2 before the monostable times out (which will be the case if the  PC is switched on during that period), the  transistor in the optocoupler will conduct  and discharge capacitor C6. The monostable  will now remain triggered and the relay will  remain energised until the PC is switched off  and power disappears from its USB or PS/2  interface. Then, after the monostable time  period expires, the relay will drop out and the  power saver will disconnect itself from the mains. There is no need to switch anything  else off: just shut down the system and the  power saver will take care of the rest.
 
Circuit diagram :
PC Power Saver Circuit Diagram
 
It is also  possible to leave the machine as it updates its  software, and the power saver will do its job  shortly after the machine shuts down. Power for the unit itself is obtained using a  simple supply circuit based around a miniature transformer. Alternatively, a 12 V mains  adaptor can be used, as long as a relay with a  12 V coil voltage is used for RE1. In his proto-type the author used a relay with a 24 V coil  connected as shown directly to the positive  side of reservoir capacitor C2, the 555 being  powered from 12 V regulated from that sup-ply using R1 and D1. A fixed resistor can of  course be used in place of P1 if desired. If the  adjustment range of P1 is not sufficient (for  example if the PC powers up very slowly) the  monostable period can be increased by using  a larger capacitor at C6.  The relay must have at least two normally-open (or changeover) contacts rated at at  least 8 A. The contact in parallel with S1 is used to supply power to the device  itself, and the other contact carries  all the current for the connected  PC  or  for  the  ex tension  lead  to  which  the  PC  and  peripherals  are  connected. 

Pushbutton S1 must be rated for 230 VAC  (US: 120 VAC) operation: this is no place to  make economies. The coil current for the relay  flows through LED D5, which must therefore  be a 20 mA type. If a low-current LED is used,  a 120 Ω resistor can be connected in parallel with it to carry the remaining current.  The Fujitsu FTR-F1CL024R relay used in the  author’s prototype has a rated coil current of  16.7 mA. Optocoupler IC2 provides isolation between  the circuit and the PC, and is protected from  reverse polarity connection by diode D4. The power saver should be built into an insulated enclosure and great care should be  taken to ensure that there is proper isolation  between components and wires carrying the  mains voltage and the other parts of the circuit. In particular, the connection to the PC  and associated components (R6, C5, D4 and  IC2) should be carefully arranged with at least  a 6 mm gap between them and any part of  the circuit at mains potential.

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