GB2478597A

GB2478597A - Inrush current protection device

Info

Publication number: GB2478597A
Application number: GB1004142A
Authority: GB
Inventors: Christopher Charles Brighty
Original assignee: Burland Technology Solutions Ltd
Current assignee: Burland Technology Solutions Ltd
Priority date: 2010-03-12
Filing date: 2010-03-12
Publication date: 2011-09-14
Anticipated expiration: 2030-03-12
Also published as: GB2478597B; GB201004142D0

Abstract

Input terminals 12 of an inrush current protector are connected to an AC mains power source and output terminals 13 are connected to a capacitive load 22 such as a computer monitor. The inrush current protector comprises series current limiting resistor R6 connected between the input and output terminals; a switchable shunt 18 to shunt power around the current limitation resistor; and a controlling circuit 20 to close the switch when an RMS voltage on the output terminals rises from a lower to a comparatively higher level as the circuit is initially energised. The time taken to reach the threshold, before the shunt resistance is switched out, depends on and automatically adjusts to the capacitance of the load. The inrush protector can comprise a power supply stage (fig 2, 24); an opto-triac (fig 2, U1); a trigger circuit (fig 2, 26) having two transistor networks; and a double pole relay (fig 2, 18). The switch can be latched closed and the trigger circuit can be de-energised when power is shunted around the current limiting resistor.

Description

Inrush Current Protector Device and Method

BACKGROUND

a. Field of the Invention

The present invention relates to an electronic device for guarding a circuit against inrush current owing to a when a capacitative load when the circuit is initially energised to provide alternating current mains power to the load, and to a method for performing the same.

b. Related Art In modern offices and control rooms it is common for a large number of flat screen monitors to be connected to a mains power supply. In most parts of the world, mains power is usually 230 V-240 V or about 120 V. Modern computer monitors use switch mode power supplies to derive power from a mains power supply. When such a monitor is switched on a capacitor on the input of the power supply has to be charged which results in a short high current pulse that lasts for the order of milliseconds. This significant increase in the initial current draw is typically referred to as inrush current.

In situations where a single flat screen monitor is switched on this does not present a problem as the peak current is not significant, however, when multiple monitors are switched on simultaneously the magnitude and duration of the current pulse is sufficient to trip circuit breakers or weld the contacts of switching components. Both occurrences are undesirable.

One known way of limiting inrush current is to provide a negative temperature coefficient resistor in series with a load. The resistance of such resistors decreases as the temperature of the resistor increases. The initial current inrush causes the temperature of the component to rise so its resistance rapidly decreases, such that the effect of the component on the remainder of the circuit is reduced. After the initial switch on and after the inrush current has finished the inrush limiting resistance is switched out of the circuit so that this no longer limits the current.

A disadvantage of this approach is that after the resistor has been used to limit the inrush current, time must be allowed to allow this to cool to room temperature before the resistor is fully effective as a protection device. The time taken to cool is typically of the order of several minutes. When limiting the inrush of a bank of simultaneously switched monitors, this can present a problem because situations arise when the power supply is removed and is then restored again in a time less than the time required for the device to cool. This situation may occur for example when the monitors are switched on and off by an energy saving unit that detects the presence or absence of an operator. In these situations the inrush current protection should ideally be available almost as soon as the power is switched off.

It is an object of the present invention to provide a more convenient inrush current protector device and method of limiting inrush current from a mains power source when a capacitative load is initially energised.

SUMMARY OF THE INVENTION

According to the invention, there is provided an inrush current protector device for limiting inrush current from a mains power source when a capacitative load is initially energised from said source, the device comprising a pair of mains input terminals for connection to an alternating current (ac) mains power supply, a pair of mains input lines leading away from the mains input terminals, a pair of output terminals for connection to a capacitative load, a pair of output lines leading to the output terminals, a current limitation resistor, an electrical shunt across the current limitation resistor, a first switch for opening and closing the shunt, and a controUing circuit for controlling the operation of said switch, wherein: -the current limitation resistor is in a series electrical connection with electrical current passing between said input and output terminals such that, in use, ac mains electrical power is conveyed from the input terminals to the output terminals either through the current limitation resistor or through the shunt, depending on whether said switch is, respectively, open or closed; -the controlling circuit having an output, said output being inked to said switch; -the controlling circuit is connected to said output terminals such that the electrical power supplied to energise the controlling circuit depends on the RMS voltage across the output terminals; and -the arrangement is such that, in use, (i) when the controlling circuit is unenergised, said switch is open so that ac mains electrical power passes through the current limitation resistor, and ii) when said switch is open and ac mains electrical power is initially conveyed through the current limitation resistor to a capacitative load and said RMS voltage rises from a relatwely low level to a relatively higher level, the controlling circuit then being energised causing said controlling circuit output to close said switch such that mains electrical power is shunted past the current limitation resistor.

Also according to the invention, there is provided a method of limiting inrush current from a mains power source when a capacitative load is initially energised from said source, using an inrush current protector device comprising input terminals, output terminals, a current limitation resistor in a series electrical connection between said input and output terminals, a switchable electrical shunt for shunting electrical power around the current limitation resistor, a pair of output lines leading to the output terminals, and a controlling circuit operatively connected to the switchable electrical shunt, the method comprising the steps of: i) connecting the output terminals to a capacitative load with no alternating current (ac) mains power being supplied to the input terminals such that the controlling circuit is inactive to control the switchable electrical shunt and the switchable electrical shunt is inoperative for shunting electrical power around the current limitation resistor; ii) connecting the input terminals to an alternating current (ac) mains power source to cause electrical current to pass through the current limitation resistor while at the same time energising the controlling circuit using a rising RMS voltage derived from the output lines; and iii) using said rising RMS voltage to activate the controUing circuit in order to cause the switchable electrical shunt to become operative to shunt e'ectrical power around the current limitation resistor when said rising RMS voltage reaches a threshold value.

In the context of the present invention the term "capacitative load" means any reactive load (i.e. a load which is not purely resistive) where the capacitance predominates over any inductance, such that the voltage at the input to the load lags the current into the load.

The time taken to reach the threshold value will depend on the capacitance of the load, and hence the time taken before the shunt resistance is switched out of the series connection with the load will automatically adjust to the capacitance of the load.

In a preferred embodiment of the invention, the controlling circuit comprises a power supply stage and a switching stage, the power supply stage, in use, providing electrical power to the switching stage and the switching stage providing the output of the controlling circuit. The power supp'y stage is connected to the output terminals such that the &ectrical power supplied to energise the switching stage depends on the RMS voltage across the output terminals.

Then, when the power supply stage is unenergised, the first switch is open so that ac mains e'ectrical power passes through the current limitation resistor. Following this, when the first switch is open and ac mains electrica' power is initially conveyed through the current limitation resistor to a capacitative load and the RMS voltage across the output Unes rises from a relatively low level to a relatively higher level, the power supply stage is then energised causing the power supply stage output to close the first switch so that mains electrical power is shunted past the current limitation resistor.

Preferably the current limitation resistor is in series between the input a first one of the input lines and a first one of the output lines leading to a first one of the output terminals, a second one the input lines being connected to a second one of the output lines, such that the RMS voltage is between the first and second output lines.

It is desirable for the output of the switching stage to comprise an opto-triac device, such that there is no electrical contact between the components and no moving parts.

Preferably the switching stage comprises a first transistor switch network and a second transistor switch network, the first network when powered by a rising supply voltage from the power supply stage having an output that deactivates the output of the switching stage and the second network when powered by a the same rising supply voltage from the power supply stage having an output that switches off the second network in order to activate the output of the switching stage, the activation of the output of the switching stage In a preferred embodiment, the controlling circuit is a trigger circuit which, in use, is automatically de-energised after the switched is closed when electrical power is shunted around the current limitation resistor.

In some embodiments, the trigger circuit is automatically de-energised by a second switch, the operation of the second switch being linked to the operation of the first switch so that both of the switches are activated in concert.

Preferably the first and second switches are part of a double pole double throw switch unit, and preferably the first switch is a relay switch.

In a preferred embodiment of the nrush current protector device, the device comprises means to latch the first switch closed after the capacitative load is energised and when electrical power is shunted around the current Iimtation resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a simplified schematic diagram of an inrush current protector device for Iimitng inrush current from a mains power source when a capacitative load is initially energised from said source, according to a preferred embodiment of the nvention; Figure 2 is a detailed schematic circuit diagram of the inrush current protector device of Figure 1; and Figure 3 is a block flow diagram illustrating a method of limiting inrush current from a mains power source when a capacitatve load is initially energised from said source, according to a preferred embodiment of the invention;

DETAILED DESCRIPTION

Figure 1 shows a schematic diagram of an inrush current protector device 10 according to the present invention. A mains power supply (not shown), typically 230-240 V, is connected to a pair of mains input terminals 12, 12'. Mains input Unes 14 lead from the mains input terminals 12, 12' and a pair of output Unes 15, 15' lead to a pair of output terminals 13, 13'. A wire-wound 10 current limitation resistor R6 is connected in series between the mains voltage input line 14 and the live mains voltage output line 15. An electrical shunt 16 is provided across the current limitation resistor R6, and a switch unit 18 is provided in the shunt 16 for opening and closing the shunt 16.

The switch unit 18 is controlled by a controlling circuit 20 which will be described in more detail hereinafter. The controlling circuit 20 is connected in parallel between the output lines 1 5, 15' and an output of the controlling circuit 20 is operably connected to the switch unit 18 such that the controlling circuit 20 opens and closes the switch 18 depending on the voltage across the output lines 15, 15. A bank of flat panel monitors 22 is connected to the output terminals 13, 13 of the inrush current protector device 10. The bank of monitors 22 form a capacitive load which draws current through the inrush current protector device 10 when the mains power supply is initially connected to power the monitors 22.

Figure 2 shows a schematic circuit diagram of the inrush current protector device 10. The output terminals 13, 13' which are electrically connected to the capacitive load, which in this embodiment is a bank of monitors, are shown on the right hand side of the circuit diagram. The input terminals 12, 12', which are electrically connected to a mains power supply are shown towards the centre of the diagram.

In addition to the current limitation resistor R6, three further resistors are shown connected in series in the mains voltage line of the circuit. These resistors, Ri, R2 and R3 act to decrease the voltage supplied to the power supply stage of the controlling circuit. It is preferable to provide three resistors as a failsafe so that if one resistor shorts out, the remaining resistors continue to provide protection to the controlling circuit 20. The resistors Ri, R2 and R3 also protect the controlling circuit from spikes in the mains power supply.

The controlling circuit 20 comprises a power supply stage 24 and a switching stage 26. The power supply stage includes a rectifier circuit having four diodes Dl, D2, D3 and D4 and a capacitor 02. The power supply stage 24 provides direct current power to the switching stage 26 of the controlling circuit 20.

In this embodiment, the switch unit 18 n the electrical shunt is in the form of relay RL1, which is a double pole double throw relay having an activation coil 28. The unit 18 therefore has a first switch 30 and a second switch 32 which switch together in concert.

Referring to the circuit diagram shown in Figure 2, initially all the controlling circuit capacitors Ci, 02 and 03 are discharged. The contact arms 31 of the relay RL1 are biased by a spring (not shown) so that these are positioned so as to connect lines 33 and 34, and 36 and 37, as shown in Figure 2. Mains voltage is connected to Ji; live to line 43 and neutral to line 2. With the relay 18 in this nitial position, current from the mans supply flows through the resistor R6, and then through resistors Ri, R2 and R3 to the power supply stage 24 of the controlling circuit 20.

Capacitor 02 in the power supply stage 24 starts to charge due to the current flowing through Ri, R2 and R3, and the rectifier formed by diodes Di, D2, D3 and D4. When the voltage across 02 reaches around 3 V there is sufficient base current flowing through resistors R8 and RiO to turn on transistor Ti and an LED of the opto-triac device Ui is bypassed. As the voltage across C2 increases past V a zener diode D5 starts to conduct and holds the base of T2 at around 10 V. When the voltage across 02 reaches 10.7 V, T2 turns on and Ti is turned off so removing the bypass to the LED of Ui. This activates the triac of Ui and the coil 28 of the relay RL1 is energised. This causes the switch unit contact arms 31 to switch so that lines 34 and 35, and lines 37 and 38 are connected.

The connection between lines 34 and 35 closes the first switch 30, which is in series in the shunt, so that resistor R6 is shorted so that current flows from the mains supply, through the relay first switch 30 and to the output terminals 13, 13' connected to the bank of monitors (as shown n Figure 1).

Current no longer flows to the controUing circuit 20 because the switch contacts connected to ines 34 and 33 have been broken by the relay RL1 energising.

Therefore, 02 discharges ready for the next cycle. In addition, the opto-triac device Ui de-activates. This would cause current to stop flowing to the relay coil 28, however, when the coil 28 of the re'ay RL1 is first energised, the relay 18 is latched on by the contact made between lines 37 and 38. Current flows from the mains though the relay switch 18 and then through the coil 28 of the relay, such that the coil 28 remains energised even when current no longer flows to the controlling circuit 20.

Capacitors Ci and 03 are included in the switching stage 26 of the controlling circuit 20 to reduce susceptibility to noise.

Figure 3 shows a flow chart 50 which summarises the method according to a preferred implementation of the invention. First, the output terminals 13, 1 3' are connected 51 to a capacitative load 22 with no alternating current (ac) mains power being supplied to the input terminaisl2, 12' such that the controlling circuit is inactive to control the switchable electrical shunt 16 and the switchable electrical shunt is inoperative for shunting electrical power around the current limitation resistor R6. Then, the input termina's 12, 12' are connected 52 to an alternating current (ac) mains power source to cause electrical current to pass through the current limitation resistor R6 while at the same time energising the controlling circuit 20 using a rising RMS (root means square) voltage derived from the output lines 15, 15'. FinaUy, the rising RMS voltage is used to activate 53 the controlling circuit 20 in order to cause the switchable electrical shunt 16 to become operative to shunt electrica' power around the current limitation resistor R6 when the rising RMS voftage reaches a threshold value.

In use the invention can be conveniently incorporated within a power strip (not shown) comprising a bank of mains power sockets and an electrical plug that can be plugged into a wall socket. However, the invention could be made part of any other eectrica component used to provide power to eectrca equipment having a predominantly capacitaUve reactive oad, for example a non-interruptible power supply or mains filtering equipment.

It should be understood that the invention has been described above by way of example only and that modifications in detail may be made without departing from the scope of the invention as set out in the claims. -11 -

Claims

CLAIMS1. An inrush current protector device for limiting inrush current from a mains power source when a capacitative load is initially energised from said source, the device comprising a pair of mains input terminals for connection to an afternating current (ac) mains power supply, a pair of mains input lines leading away from the mains input terminals, a pair of output terminals for connection to a capacitative load, a pair of output lines leading to the output terminals, a current limitation resistor, an electrical shunt across the current limitation resistor, a first switch for opening and closing the shunt, and a controlling circuit for controlling the operation of said switch, wherein: -the current limitation resistor is in a series electrical connection with electrical current passing between said input and output terminals such that, in use, ac mains electrical power is conveyed from the input terminals to the output terminals either through the current limitation resistor or through the shunt, depending on whether said switch is, respectively, open or closed; -the controlling circuit having an output, said output being linked to said switch; -the controlling circuit is connected to said output terminals such that the electrical power supplied to energise the controlling circuit depends on the RMS voltage across the output terminals; and -the arrangement is such that, in use, (i) when the controlling circuit is unenergised, said switch is open so that ac mains electrical power passes through the current limitation resistor, and ii) when said switch is open and ac mains electrical power is initially conveyed through the current limitation resistor to a capacitative load and said RMS voltage rises from a relatively low level to a relatively higher level, the controlling circuit then being energised causing said controlling circuit output to close said switch such that mains electrical power is shunted past the current limitation resistor.
2. An inrush current protector device as claimed in Claim 1, in which -the controlling circuit comprises a power supply stage and a switching stage, the power supply stage, in use, providing electrical power to the switching stage and the switching stage providing said output of the controlling circuit; -the power supply stage is connected to said output terminals such that the electrical power supplied to energise the switching stage depends on the RMS voltage across the output terminals.
3. An inrush current protector device as claimed in Claim 1 or Claim 2, in which the current limitation resistor is in series between the input a first one of the input lines and a first one of the output lines leading to a first one of said output terminals, a second one the input lines being connected to a second one of the output lines, such that said RMS voltage is between the first and second output lines.
4. An inrush current protector device as claimed in any preceding claim, in which the output of the switching stage comprises an opto-triac device.
5. An inrush current protector device as claimed in any preceding claim, in which the switching stage comprises a first transistor switch network and a second transistor switch network, said first network when powered by a rising supply voltage from the power supply stage having an output that deactivates the output of the switching stage and said second network when powered by a the same rising supply voltage from the power supply stage having an output that switches off said second network in order to activate the output of the switching stage, said activation of the output of the switching stage
6. An inrush current protector device as claimed in any preceding claim, in which the controlling circuit is a trigger circuit which, in use, is automatically de-energised after the switched is closed when electrical power is shunted around the current limitation resistor.
7. An inrush current protector device as daimed in Claim 6, in which the trigger circuit is automatically de-energised by a second switch, the operation of the second switch being linked to the operation of the first switch so that both of said switches are activated in concert.
8. An inrush current protector device as claimed in Claim 7, in which said first and second switches are part of a double pole double throw switch unit.
9. An inrush current protector device as claimed in any preceding claim, in which the first switch is a relay switch.
10. An inrush current protector device as claimed in any preceding claim, comprising means to latch the first switch closed after the capacitative load is energised and when electrical power is shunted around the current limitation resistor.
11. A method of limiting inrush current from a mains power source when a capacitative load is initially energised from said source, using an inrush current protector device comprising input terminals, output terminals, a current limitation resistor in a series electrical connection between said input and output terminals, a switchable electrical shunt for shunting electrical power around the current limitation resistor, a pair of output lines leading to the output terminals, and a controlling circuit operatively connected to the switchable electrical shunt, the method comprising the steps of: i) connecting the output terminals to a capacitative load with no alternating current (ac) mains power being supplied to the input terminals such that the controlling circuit is inactive to control the switchable electrical shunt and the switchable electrical shunt is inoperative for shunting electrical power around the current limitation resistor; ii) connecting the input terminals to an alternating current (ac) mains power source to cause electrical current to pass through the current limitation resistor while at the same time energising the controlling circuit using a rising RMS voltage derived from the output lines; and iii) using said rising RMS voltage to activate the controlling circuit in order to -14-cause the switchable electrical shunt to become operative to shunt e'ectrical power around the current limitation resistor when said rising RMS voltage reaches a threshold value.
1 2. An inrush current protector device, substantiaUy as herein described, with reference to or as shown in the accompanying drawings.
13. A method of limiting inrush current from a mains power source, substantially as herein described, with reference to the accompanying drawings.