Abstract:
There is provided a load detector for determining whether a load is connected to an AC-AC power supply. The power supply comprises a transformer having a primary winding and a secondary winding, the primary winding being coupleable to an AC voltage supply via a switch, and the secondary winding being coupleable to a load. The load detector comprises a signal generator for generating a signal; a sensor for detecting the signal, the sensor being arranged to detect the signal if a load is coupled to the secondary winding and to not detect the signal if a load is not coupled to the secondary winding; and switch control circuitry coupled to the sensor and being arranged to keep the switch closed if the sensor is detecting the signal and to keep the switch open if the sensor is not detecting the signal. There is also provided an AC-AC power supply comprising such a load detector.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a load detector for determining whether a load is connected to an AC-AC power supply and to an AC-AC power supply comprising such a load detector.  
       BACKGROUND OF THE INVENTION  
       [0002]     External power supply adaptors usually have two modes of operation: an active mode (in which the input of the power supply adaptor is connected to an AC power supply and the output is connected to a load) and a no-load mode (in which the input of the power supply adaptor is still connected to the AC supply, but no load is connected at the output). An example of an AC-DC external power supply adaptor is a charger for a mobile telephone. The charger is in active mode (to charge up the telephone) when the telephone is placed in the cradle for charging and is in no-load mode when the telephone is not in the cradle. An example of an AC-AC external power supply adaptor is a speaker for a personal computer (PC). When the PC speaker is switched on, it is in active mode and, when the speaker is switched off, this is equivalent to disconnecting the load, so the speaker is in no-load mode. Other examples can, of course, be envisaged. In active mode, the external power supply adaptor should ideally supply power to the load at high efficiency and, in no-load mode, minimal power should be expended—ideally just enough for the adaptor to switch back to active mode when a load is connected.  
         [0003]     One known way to achieve low power consumption during no-load mode is to use a switching mode power supply (SMPS). However, an SMPS has drawbacks: a lot of switching noise is generated, the implementation can be costly and there may also be some limitations on the power consumption of an SMPS during no-load mode, especially if the load requires high power during active mode.  
         [0004]     Another power supply design, which is simpler and less costly, is a linear power supply. An AC-DC linear power supply comprises a rectifier and filter capacitor on the secondary side of a transformer, whereas in an AC-AC linear power supply, the rectifier and capacitor are moved over to the load itself. However, in either case, because the AC power supply is still connected to the primary winding of the transformer, even when no load is connected at the output, there is still high power consumption during no-load mode. This problem has been partially solved by adopting a standby mode in which, when no load is connected on the secondary side of the transformer, the AC power supply is disconnected from the primary side. Of course, this means that some sort of load detector is required to determine whether a load is connected and to switch between active and standby modes appropriately.  
         [0005]     In an AC-DC linear power supply, the load detector can be rather simple and various load detectors have been developed, one of which is described in U.S. Pat. No. 5,624,305. This is because, firstly, it is easier to measure and monitor conditions in DC and to detect any relevant changes due to the presence or absence of a load. Further, the load detection circuit needs some power, in the form of DC, to function. This is readily available for the DC case but not for the AC case. Finally, for the AC-AC case, the load detection circuitry will have to be coupled to the secondary winding of the transformer. The secondary winding tends to present a closed circuit to whatever circuitry that is implemented and is a short circuit for DC and low frequencies. For the AC-DC case, however, the filter capacitor decouples the power supply from the load and so a load detection circuit can be placed in between.            
         [0006]     Although an AC-DC linear power supply can mean a rather simple load detector, an AC-DC linear power supply does have the disadvantage that the efficiency during active mode can be quite low because of the presence of the rectifier.  
         [0007]     Thus, an AC-AC power supply may be preferred. However, in an AC-AC power supply, the load detector cannot be so straightforward, because the power being supplied to the load is AC i.e. fluctuating between zero and a maximum, so it is much more difficult to determine whether or not a load is connected. One way to detect a load for the AC case is to detect the AC current drawn by the load using a current sense transformer, which translates a current flow to a voltage signal. However, as the frequency of the AC power source is low (typically 50 or 60 Hz), such transformers tend to be bulky and costly. Also, for light loads that do not draw much power, the current sense transformer will have to be made quite sensitive, by increasing the number of turns in the transformer windings. Further, when the load is not constant, this operation of the current sense transformer will be even more complicated.  
       SUMMARY OF THE INVENTION  
       [0008]     According to a first aspect of the invention, there is provided a load detector for determining whether a load is connected to an AC-AC power supply, the power supply comprising a transformer having a primary winding and a secondary winding, the primary winding being coupleable to an AC voltage supply via a switch, and the secondary winding being coupleable to a load, the load detector comprising: 
        a signal generator for generating a signal;     a sensor for detecting the signal, the sensor being arranged to detect the signal if a load is coupled to the secondary winding and to not detect the signal if a load is not connected to the secondary winding; and     switch control circuitry coupled to the sensor and being arranged to keep the switch closed if the sensor is detecting the signal and to keep the switch open if the sensor is not detecting the signal.        
 
         [0012]     Thus, the load detector is arranged to determine whether a load is connected to the secondary winding of the power supply, and to open and close the switch between the AC voltage supply and the primary winding of the power supply appropriately. Thus, when a load is connected so that the sensor is detecting the signal, the load detector keeps the switch between the primary winding and the AC voltage supply closed, so that the AC voltage supply can deliver power to the load. However, when a load is not connected so that the sensor is not detecting the signal, the load detector keeps the switch between the primary winding and the AC voltage supply open.  
         [0013]     The signal generator is preferably connectable across the secondary winding of the transformer of the AC-AC power supply.  
         [0014]     Preferably, when the signal generator is connected across the secondary winding and a load is coupled to the secondary winding, a closed path is formed from the signal generator back to the signal generator via the load and the sensor. Because a closed path is formed via the load and the sensor, the signal generated by the signal generator can be detected by the sensor. Thus, the presence of the load, which results in the closed circuit, means that the switch control circuitry of the load detector keeps the switch on the primary side of the AC-AC power supply closed.  
         [0015]     Preferably, when the signal generator is connected across the secondary winding and no load is connected to the output nodes, no closed path is formed from the signal generator back to the signal generator. Because no closed path is formed, the signal generated by the signal generator cannot be detected by the sensor. Thus, when no closed path is formed, the switch control circuitry of the load detector keeps the switch on the primary side of the AC-AC power supply open.  
         [0016]     In one preferred embodiment, the signal generator is arranged to generate a pulsed signal. This is advantageous because a pulsed signal comprises high frequency content. The signal generator may generate a pulsed signal by repeatedly charging and discharging a capacitor, thus providing a pulsed voltage at an output node.  
         [0017]     The sensor may comprise a transformer for locating between the secondary winding of the AC-AC power supply and an output node for a load. The primary winding of the transformer may form part of the connection between the secondary winding and the load output node. The secondary winding may be connected to the circuitry for controlling the switch.  
         [0018]     The switch may comprise a relay. In that case, the switch control circuitry may be coupled to the relay such that, when the sensor is detecting a signal, current flows through the coil of the relay, closing the switch between the AC power supply and the primary winding, and, when the sensor is not detecting a signal, no current flows through the coil of the relay, and the switch between the AC power supply and the primary winding remains open.  
         [0019]     According to a second aspect of the invention, there is provided an AC-AC power supply for a load, the power supply comprising: 
        a transformer comprising a primary winding and a secondary winding, the primary winding being coupleable to an AC voltage supply via a switch, and the secondary winding being coupled to output nodes for a load, via a load detector, the load detector comprising:     a signal generator for generating a signal;     a sensor for detecting the signal, the sensor being arranged to detect the signal if a load is connected to the output nodes and to not detect the signal if a load is not connected to the output nodes; and     switch control circuitry coupled to the sensor and being arranged to keep the switch closed if the sensor is detecting the signal and to keep the switch open if the sensor is not detecting the signal.        
 
         [0024]     Thus, the load detector in the power supply is arranged to determine whether or not a load is connected to the secondary winding of the power supply, and to open and close the switch on the primary side appropriately. When a load is connected and the sensor is detecting the signal, the switch between the primary winding and the AC voltage supply is kept closed so that the AC voltage supply can deliver power to the load. Then, the power supply is in active mode. However, when a load is not connected, and the sensor is not detecting the signal, the switch between the primary winding and the AC voltage supply is kept open. Then, the power supply is in no-load mode.  
         [0025]     In one embodiment, the signal generator is connected across the secondary winding. In that embodiment, the power supply is preferably arranged such that, when a load is connected to the output nodes, a closed path is formed from the signal generator back to the signal generator via the load and the sensor. Because a closed path is formed via the load and the sensor, the signal can be detected by the sensor. Thus, the presence of a load, which results in the closed circuit, means that the circuitry keeps the switch on the primary side closed. In that embodiment, the power supply is also preferably arranged such that, when no load is connected to the output nodes, no closed path is formed from the signal generator back to the signal generator. Because no closed path is formed, the signal cannot be detected by the sensor. Thus, when there is no load connected at the output nodes so that no closed path is formed, the circuitry keeps the switch on the primary side open.  
         [0026]     The signal generator may be arranged to generate a pulsed signal. This is advantageous because a pulsed signal comprises high frequency content. If the signal generator is connected across the secondary winding, a pulsed signal is particularly advantageous because the high frequency content of the pulsed signal will mean that the secondary winding presents a high impedance to the pulsed signal. Thus, the secondary winding will not provide a closed path for the pulsed signal from and to the signal generator, which could mean that the sensor accidentally detects the signal even when no load is connected at the output nodes. The signal generator may generate a pulsed signal by repeatedly charging and discharging a capacitor, thus providing a pulsed voltage at an output node.  
         [0027]     The sensor may comprise a transformer between the secondary winding and one of the output nodes. The primary winding of the transformer may form part of the connection between the secondary winding and the output node. The secondary winding may be connected to the circuitry for controlling the switch.  
         [0028]     The switch may comprise a relay. In that case, the switch control circuitry may be coupled to the relay such that, when the sensor is detecting a signal, current flows through the coil of the relay, closing the switch between the AC power supply and the primary winding and, when the sensor is not detecting a signal, no current flows through the coil of the relay, and the switch between the AC power supply and the primary winding remains open.  
         [0029]     In a first embodiment, the power supply further comprises a standby power supply for supplying power to the signal generator when no load is connected to the output nodes. Thus, when a load is connected to the output nodes, power for the signal generator is supplied by the AC voltage supply and, when no load is connected to the output nodes, power for the signal generator is supplied by the standby power supply. The standby power supply is preferably connectable to the AC power supply.  
         [0030]     In a second embodiment, the power supply further comprises a capacitor across the switch. In this second embodiment, when the switch is closed, the AC power supply is connected directly to the primary winding, bypassing the capacitor, and, when the switch is open, the AC power supply is connected to the primary winding via the capacitor. Thus, when the switch is open (i.e. no load is connected to the output nodes on the secondary side), power is still delivered to the secondary side, but the amount of power can be controlled by suitable choice of the value of the capacitor.  
         [0031]     In the second embodiment, the power supply may further comprise a connection from the secondary winding to the signal generator, via a rectifier, for supplying DC power to the signal generator when no load is connected to the output nodes.  
         [0032]     According to a third aspect of the invention, there is provided a method for detecting whether a load is connected to an AC-AC power supply, the power supply comprising a transformer having a primary winding and a secondary winding, the primary winding being coupleable to an AC voltage supply via a switch, and the secondary winding being coupleable to a load, the method comprising the steps of: 
        generating a signal on the secondary side of the transformer;     if a load is coupled to the secondary winding, detecting the signal and, in response to the detected signal, keeping the switch between the primary winding and the AC voltage supply closed;     if no load is coupled to the secondary winding, not detecting the signal and, in response to no detected signal, keeping the switch between the primary winding and the AC voltage supply open.        
 
         [0036]     Features described in relation to one aspect of the invention may also be applicable to other aspects of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, of which:  
         [0038]      FIG. 1  shows a first embodiment of the invention;  
         [0039]      FIG. 2  shows a second embodiment of the invention;  
         [0040]      FIG. 3  shows one possible circuit implementation of the embodiment of  FIG. 2 ;  
         [0041]      FIG. 4  is a plot of the voltage at node  313  with respect to time, for the arrangement shown in  FIG. 3 ; and  
         [0042]      FIG. 5  is a plot of the voltage at node  315  with respect to time, for the arrangement shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0043]      FIG. 1  is a diagram of a first embodiment of the invention. Referring to  FIG. 1 , AC-AC linear power supply  101  comprises a transformer X 1 . The primary winding X 1   a  of the transformer X 1  is connectable to the AC power supply  103  at nodes  105  and  107 , via a switch  109 . The AC power supply may be any AC voltage at any frequency e.g. 110VAC, 120VAC, 230VAC or 240VAC at 50 or 60 Hz. The secondary winding X 1   b  of the transformer X 1  is connectable to a load  201  (shown disconnected in  FIG. 1 ) at nodes  111  and  113  (normally via a cable and connector) via load detector  301 . The AC-AC linear power supply  101  also includes a standby power supply  115 .  
         [0044]     The switch  109 , between primary winding X 1   a  and AC power supply  103 , is for switching on and off the AC power supply  103  to the transformer X 1 . The switch  109  may be any suitable type of switch for example a relay or an optocoupler. Switch  109  is controlled by control  307  (to be described below) in load detector  301 .  
         [0045]     The load detector  301 , between secondary winding X 1   b  and nodes  111  and  113 , comprises pulse generator  303 , sensor  305  and control  307 . The pulse generator  303  is connected across the secondary winding X 1   b  of transformer X 1  at nodes  309  and  311 . Sensor  305  is connected to the line between one side of the secondary winding X 1   b  and the output node  113 . As already mentioned, control  307  controls switch  109 . The control  307  receives an input from sensor  305 . The control is arranged to keep the switch  109  closed only if a load is present. If no load is connected to nodes  111  and  113 , the switch  109  is open.  
         [0046]     The load  201  typically comprises a rectifier  203  and a filter capacitor  205  to convert the AC voltage to a DC voltage for the load R L .  
         [0047]     Operation of the arrangement of  FIG. 1  will now be described.  
         [0048]     Consider a first stage, when the AC-AC power supply  101  is connected to the AC input  103  at nodes  105  and  107  but there is no load connected on the secondary side of the circuit to nodes  111  and  113 . Since there is no load connected, we are in standby or no-load mode. At this stage, switch  109  is open so standby power supply is providing power for the pulse generator  303  and the control  307 . Pulse generator  303  receives power from standby power supply  115  and starts to send a pulsed signal through node  309  to check for the presence of a load at nodes  111  and  113 . Since, at this stage, no load is connected to nodes  111 ,  113 , the circuit is open, so no return path is provided for the pulsed signal so no signal is picked up by sensor  305 .  
         [0049]     Then, in a second stage, a load (like  201  for example) is connected at nodes  111  and  113 . The pulse generator  303  is still sending its pulsed signal to node  309 , but now there is a load at nodes  111  and  113  so the circuit is closed. Thus, the load  201  provides the return path for the pulse from  309  to  311 , via rectifier  203  and capacitor  205 . Therefore, a signal is picked up by sensor  305 . Once sensor  305  detects the pulsed signal indicating that a load is present at nodes  111  and  113 , it sends a signal to control  307 , which then closes switch  109 . Thus, primary winding X 1   a  of the transformer X 1  is now connected to the AC power supply  103  so that the AC power supply  103  can deliver power to the load at nodes  111 ,  113 . Thus, we are now in active mode.  
         [0050]     Then, in a third stage, the load  201  is again disconnected from nodes  111 ,  113 . Because the circuit is now open again, the pulsed signal is no longer picked up by sensor  305 . Once sensor  305  no longer detects the pulsed signal (indicating that the load has been disconnected), it sends a signal to control  307  to open the switch  109 . Once switch  109  is open, primary side X 1   a  of transformer X 1  is no longer connected to the AC power supply  103 . This returns the AC-AC power supply to standby mode once again, with standby power supply  115  supplying power for the circuit.  
         [0051]     The standby power supply  115  is connected to the AC power supply before the switch  109 . Thus, even when switch  109  is open, the standby power supply is still connected to the AC power supply so as to be able to provide power to the pulse generator  303  and to the control  307 . When the AC-AC power supply is in standby mode, the standby mode power supply  115  should preferably deliver just enough power for load detector  301  and switch  109  to function properly. This minimizes the power consumption during standby mode.  
         [0052]     A pulsed signal is used to check for the presence of a load at nodes  111  and  113  because it has high frequency content. When a load is connected to nodes  111  and  113 , the secondary winding X 1   b  of the transformer X 1 , which is an inductor, will be seen as high impedance to the pulsed signal from pulse generator  303 , whereas the load  201  will be seen as low impedance to the pulsed signal. Thus, most of the pulsed signal from pulse generator  303  via node  309  will pass through the load  201  and return to the pulse generator  303  via node  311 , so that the sensor  305  will detect the signal.  
         [0053]      FIG. 2  is a diagram of a second embodiment of the invention. The arrangement of  FIG. 2  is very similar to that of  FIG. 1 . The only difference is the way in which power is supplied to the load detector  301  and to the switch  109 . As in  FIG. 1 , AC-AC linear power supply  101 ′ comprises a transformer X 1 . The primary winding X 1   a  of the transformer X 1  is connectable to the AC power supply  103  at nodes  105  and  107 , via a switch  109 . In the  FIG. 2  arrangement, there is also a capacitor  115  across switch  109 . Once again, the AC power supply may be any AC voltage at any frequency. The secondary winding X 1   b  of the transformer X 1  is connectable to a load  201  (shown disconnected in  FIG. 2 ) at nodes  111  and  113 , via load detector  301 . The AC-AC linear power supply of  FIG. 2  also includes a rectifier  117  and filter capacitor  119  connected across the secondary winding X 1   b , via resistors  121  and  123 .  
         [0054]     As in the  FIG. 1  arrangement, the switch  109 , between primary winding X 1   a  and AC power supply  103 , is for connecting and disconnecting the transformer X 1  directly to the AC power supply  103 . However, in  FIG. 2 , because there is a capacitor  115  across switch  109 , when switch  109  is closed, the AC power supply  103  is connected directly to the transformer X 1 , whereas, when switch  109  is open, the AC power supply  103  is connected to transformer X 1 , but only via capacitor  115 . This will be described further below. As before, switch  109  may be any suitable type of switch, for example a relay or an optocoupler.  
         [0055]     The load detector  301 , between secondary winding X 1   b  and load  201 , of  FIG. 2  is identical to that of  FIG. 1 . That is, the load detector  301  comprises pulse generator  303 , connected across the secondary winding X 1   b  at nodes  309  and  311 , sensor  305 , connected to the line between one side of the secondary winding X 1   b  and the load  201 , and control  307 , for controlling switch  109  and receiving input from sensor  305 . As before, the control is arranged to keep the switch  109  closed only if a load is connected at nodes  111  and  113 . If no load is connected, the switch  109  is open.  
         [0056]     The load  201  may also be identical to the load in the  FIG. 1  arrangement. That is, load  201  comprises a rectifier  203  and a filter capacitor  205 , to convert the AC voltage to a DC voltage for the load, represented by R L .  
         [0057]     Operation of the arrangement of  FIG. 2  will now be described.  
         [0058]     Consider a first stage, in which the AC-AC power supply  101  is connected to AC power supply  103  at nodes  105  and  107 , and there is a load connected at nodes  111  and  113 . Since there is a load connected, we are in active mode. As in the  FIG. 1  arrangement, the pulse generator is sending its pulsed signal to node  309 . Because the circuit is closed by load  201 , the load  201  provides the return path for the pulsed signal from node  309  to node  311  via rectifier  203  and capacitor  205 . Therefore, the pulsed signal is picked up by sensor  305 , which sends a signal to control  307 , which keeps switch  109  closed. So, the AC power supply  103  is connected directly to the transformer X 1  (bypassing capacitor  115 ) so that the AC power supply  103  is providing power for the load  201  at nodes  111 ,  113 . Power for the load detector  301  and switch  109  is taken from the secondary side of the transformer X 1  after conversion to DC by rectifier  117  and filter capacitor  119 .  
         [0059]     Then, in a second stage, the load is disconnected from nodes  111  and  113 . Thus, the circuit is now open, no return path is provided for the pulsed signal from pulse generator  303  and no signal is picked up by the sensor  305 . Thus, control  307  opens switch  109 . Now, the primary winding X 1   a  of transformer X 1  is connected to the AC power supply  103  via capacitor  115 . Capacitor  115  acts as a current limiter, limiting the current, and effectively the power, to the primary side X 1   a  of transformer X 1 . Since the load  201  is disconnected, we are in standby mode and only a small amount of power is required to keep the load detector operational. The exact amount of power supplied, can be selected by appropriate choice of capacitor  115 . Ideally, the capacitor should deliver just enough power for load detector  301  and switch to function properly. Power for the load detector is still provided from the secondary side of the transformer X 1 , after conversion to DC by the rectifier  117  and filter capacitor  119 .  
         [0060]     The resistors  121  and  123  are included to provide a high impedance to the pulsed signal from pulse generator  303  and hence prevent the pulsed signal taking this path. Inductors could be used as an alternative to resistors  121 ,  123 .  
         [0061]      FIG. 3  is a diagram of the second embodiment of the invention (as previously shown in  FIG. 2 ) but with possible circuitry of the pulse generator  303 , the sensor  305 , the control  307  and the switch  109  shown. The rest of the circuit is exactly the same as shown in  FIG. 2  and will not be described further. The load  201  is not shown in  FIG. 3 . Note that the circuitry shown in  FIG. 3  is only an example of possible circuitry for the  FIG. 2  arrangement. The skilled person will appreciate that any alternative suitable circuitry could be used instead.  
         [0062]     Referring to  FIG. 3 , the circuitry of the pulse generator is shown in box  303 . The pulse generator comprises transistors Q 1  and Q 2 , resistors R 1 , R 2  and R 3 , capacitors C 1 , C 2 , C 3  and C 4  and zener diode D Z . Operation of the pulse generator is as follows.  
         [0063]     Power to the pulse generator at node  312  is DC, after the rectifier  117  and filter capacitor  119 . At the beginning of a cycle, the voltage at node  313  is lower than the breakdown voltage of D Z . The voltage at node  314  is therefore at ground potential and transistors Q 1  and Q 2  are off. As C 4  continues to charge up, the voltage at node  313  rises. Once the voltage at node  313  has risen sufficiently, is the zener diode D Z  will start to conduct and the voltage at node  314  will start to rise. Once the voltage at node  314  has risen sufficiently, Q 1  and Q 2  will switch on. As Q 2  switches on, the voltage at node  315  rises rapidly. The increase in voltage at node  315  is translated back to node  314  through capacitor C 3 . This results in positive feedback. A discharge path for C 4  is created due to the switching on of Q 2 . Because of positive feedback, C 4  is rapidly discharged, causing the voltage at node  313  to drop very quickly. This causes the voltage at node  314  to drop, switching off Q 1  and Q 2 . As Q 2  is switched off, the voltage at node  315  drops back to ground potential. Due to this short-lived switching on and off of the transistors, a voltage pulse is seen at node  315 . This pulse is coupled to node  309  via capacitor C 2 . If a load is present across nodes  111  and  113 , this pulse will go through the load and return to ground at node  311  via capacitor C 1 . As transistors Q 1  and Q 2  are turned off, C 4  will start to charge up again so that the cycle repeats.  
         [0064]     The voltage at node  313  has the form shown in  FIG. 4  and the voltage at node  315  has the form shown in  FIG. 5 .  
         [0065]     Referring once again to  FIG. 3 , the circuitry of the sensor is shown in box  305 . The sensor is simply a transformer X 2 . The primary winding of the transformer X 2  forms part of the line from the secondary winding X 1   b  of transformer X 1  through node  311  to load output node  113 . The secondary winding of the transformer X 2  is connected to the control  307 . When no load is connected at output nodes  111 ,  113 , no return path for the pulsed signal is provided, so no pulse is picked up at the primary winding. On the other hand, when a load is connected at output nodes  111 ,  113 , the pulse is picked up at primary winding of transformer X 2  and hence at the secondary winding of transformer X 2 .  
         [0066]     Referring once again to  FIG. 3 , the circuitry of the control is shown in box  307  and the circuitry of the switch is shown in box  109 . The control comprises transistors Q 3  and Q 4 , diode D 1  and capacitor C 5 . The switch comprises a relay having a switch S 1  and a coil CO 1 . With each current peak through the secondary winding of X 2 , the capacitor C 5  charges up a little. Once capacitor C 5  has charged up sufficiently to switch on transistor Q 3 , current starts to flow from rectifier  117 , through the coil CO 1  and through transistors Q 3  and Q 4 . The current through the coil CO 1  causes switch S 1  to close. When the load is disconnected so that there are no current peaks through the secondary winding of X 2 , the voltage across capacitor C 5  begins to fall, until the transistor Q 3  is switched off. Then, there is no current through the coil CO 1  and the switch S 1  opens.