JPS6117129B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a drive circuit for an electromagnetic switch or the like in which a switching element is driven by a solenoid such as a relay or a plunger.

For example, as shown in FIG. 1, a conventional drive circuit drives a solenoid 1 (internal resistance r) that opens and closes a contact 1a, and a switch 2 connected in series to the solenoid 1 (internal resistance r).
It depends on opening and closing. However, in this solenoid 1, a reverse voltage V _L is generated due to a back electromotive force when the current suddenly changes, and this reverse voltage V _L is absorbed by the power supply 3 and capacitor 4 when the switch 2 is closed. However, when the switch 2 is open, it is added to the switch 2, and particularly when the switch 2 is an electronic switch such as a transistor, it has a large effect on its withstand voltage. The characteristics of the reverse voltage V _L during this open circuit are shown in Figure 2A.

To prevent this, conventionally, a voltage absorbing element such as a diode 5 is used to convert the current into a _current as shown in FIG.

However, in this case, the current due to the back electromotive force continues to flow through the solenoid 1, and the current becomes zero only when the voltage drops below the threshold voltage Vs of the diode 5, so that the contact 1a, which should be opened, is , due to the above current, solenoid 1
Due to the transient magnetic flux generated during this period, the contact 1a remains closed for a predetermined period of time, and the response speed of the contact 1a is delayed.

As described above, in the past, element breakdown voltage and responsiveness have a contradictory relationship, and it has not been possible to satisfy both at the same time.

The present invention has been developed in view of the above-mentioned points.When the solenoid is activated, the back electromotive force is canceled by the voltage of the capacitor that is in a charged state, and after the solenoid is activated, its driving current is reduced, so that when the solenoid returns, the back electromotive force is canceled out. It is an object of the present invention to provide a drive circuit that reduces the back electromotive force of the switching element, eliminates the need to pay undue attention to the withstand voltage of the switching element, and increases the response speed.

Generally, in solenoid elements such as plungers and relays, the current flowing through the solenoid and
There is a certain hysteresis relationship between the contact piece and the contact piece that responds to the current change. This hysteresis relationship is
Generally, when a drive current is applied from a state of zero current, there is _a difference between the drive current I _ON that is sufficient to operate the movable piece and the current I _OFF that returns the movable piece.
> I _OFF , and the current I _F with the relationship I _ON > I _F > I _OFF is usually called a holding current. Therefore, if this holding current I _F is applied to the solenoid after driving, the current change when the solenoid is reset next time will be I _F , and the generation of back electromotive force will be reduced.

An embodiment focusing on these points will be described below. First, in FIG. 3, a resistor 6 is connected in series to a solenoid 1 having an internal resistance r, and a capacitor 7 is connected in parallel to this resistor 6. In this case, the resistor 6 is connected to the solenoid 1 by the internal resistance r.
Set to a value that provides a holding current I _F close to _OFF . Then, a diode 8 is connected between the ground, the switch 2, and the resistor 6 so as to be in the opposite direction to the power source 3.

In FIG. 3, when the switch 2 is closed, the charge on the capacitor 7 is zero before that, so the resistor 6 is short-circuited by the capacitor 7, and a large drive current is generated. I
_ON flows, and thereafter charging of the capacitor 7 progresses, and the current decreases to I _F determined by the voltage of the power supply 3 and the resistors r and 6, and this state continues thereafter. Contact 1a
is closed by the first drive current I _ON .

In this way, if the switch 2 is opened after the current reaches the holding current _IF , a counter electromotive force of -L・di _F /dt will be generated, assuming that the inductance of the solenoid 1 is L; As mentioned above, the reverse voltage V _L due to the electromotive force does not become very large.

At this time, the capacitor 7 is charged with the voltage Vc, and if the time constant of the capacitor 7 and the resistor 6 is set to be sufficiently larger than the extinction time of the reverse voltage V _L , the capacitor 7 can now be considered as a battery with voltage Vc. On the other hand, the loop of current that should flow due to the reverse voltage I _L is a loop with the diode 8 in the forward direction.

Therefore, the voltage Vc of the capacitor 7 and the reverse voltage V _L cancel each other out, and since the diode 8 is in the forward direction with respect to this reverse voltage V _L , in order to cause current to flow through the solenoid 1. The voltage difference between the two is V _L -(Vc+Vs), where the threshold voltage of the diode 8 is Vs. That is, the reverse voltage V _L flows through the solenoid 1 only for a time period during which the reverse voltage V L decreases to Vc+Vs. Moreover, when this current becomes less than the contact current I _F , contact 1a opens, so
The time from when the filter switch 2 is opened to when the contact 1a is opened is extremely short. The states of voltage changes at points related to the operation of the solenoid 1 described above are shown in FIG. 4B.

On the other hand, as shown in Figure 4A, the voltage applied to the switch 2 is the voltage Vo of the power supply 3 and the threshold voltage of the diode 8 when it is open.
Since the sum with Vs becomes Vo+Vs, there is no need to particularly consider its withstand voltage.

As can be seen from the above, according to the present invention, the current consumption of the circuit can be reduced, the responsiveness can be extremely high, and the reverse voltage to the switch section can also be about the same as the power supply voltage.

For this reason, it is suitable for use as a protection circuit for power amplifiers, etc., for drive circuits that cut off the load in the event of abnormal operation, and for other drive circuits that require high-speed operation, especially when there are restrictions on element withstand voltage. .

[Brief explanation of the drawing]

FIG. 1 is a conventional circuit diagram, FIG. 2 is a diagram of its operating characteristics, FIG. 3 is a diagram of an embodiment of the present invention, and FIG. 4 is a diagram of its operating characteristics. 1... Solenoid.

Claims (1)

[Claims] 1. Connect a parallel-connected resistor and capacitor between one end of the solenoid that drives the contact and the switch element, and connect the other end of the solenoid and the non-power side of the switch element with respect to the polarity of the power supply. A diode in the opposite direction is connected, and the time constant of the resistor and capacitor connected in parallel is set to be sufficiently longer than the extinction time of the back electromotive voltage generated in the solenoid when the switch element is turned off in a steady state where power is supplied. A drive circuit characterized by: