JPS5855452Y2 - Load grounding type constant current device - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a load grounding type constant current device that is often used as an operation output source or a signal source for setting signals in, for example, a process control device.

Conventionally, as this type of constant current device, a configuration using an operational amplifier Q1 as shown in FIG. 1 has been widely used.

In FIG. 1, resistor R1 is a current sensing resistor.

One end of this current detection resistor R1 is connected to the positive electrode side of the power source 1, and the other end is connected to one end of the current control active element 2.

The control active element 2 is constituted by a Darlington connection between a junction field effect transistor Q2 and a transistor Q3.

One end of the current sensing resistor R1 is connected to the connection point between the Darlington-connected drain of the field effect transistor Q2 and the collector of the transistor Q3, and the other end of the control active element 2, that is, the emitter of the transistor Q3, is connected to the load RL.
It is connected to the common potential point 3 through.

The output of the operational amplifier Q is supplied to the control terminal of the control active element 2, that is, the gate of the field effect transistor Q2 here, and the output of the operational amplifier Q1 is supplied to one input terminal of the operational amplifier Q1, that is, the non-inverting input terminal (+) here. The voltage at the connection point between the current detection resistor R1 and the control active element 2 is supplied, and the inverting input terminal (
-) is given the set voltage ■s from the set voltage source 4.

Note that a resistor R2 is connected between the base and emitter of the transistor Q3 of the control active element 2.

According to this configuration, the current flowing through the control active element 2 is controlled by the output of the operational amplifier Q1, and the voltage drop in the current detection resistor R1 and the voltage v5 set in the set voltage source 4 are always equal to each other. The load RL is controlled to have a constant current i.

supply.

However, the constant current device having this configuration has the drawback that, for example, large noise is applied from the load side, or latch-up phenomenon tends to occur when the power is turned on.

The latch-up phenomenon occurs, for example, when a large level of noise is applied from the load RL side.

In other words, the load RL is often installed far away from the constant current device.

Therefore, the wiring between the constant current device and the load RL becomes long, and it becomes easy to receive external induction.

Therefore, sometimes large levels of noise can be induced.

When a large level of positive noise is applied to the emitter of transistor Q3, transistor Q3 is instantaneously turned off.

When transistor Q3 turns off, current sensing resistor R1
Since the voltage drop at is zero, only the positive setting voltage v5 is applied to the non-inverting input terminal (-) of the operational amplifier Q1, so the output of the operational amplifier Q1 is biased in the positive direction.

Therefore, at this time, a current 11 flows into the load from the operational amplifier Q1 through the gate of the junction field effect transistor Q2 and the base of the transistor Q3.

This current 11 is a constant current i when the transistor Q3 is turned on.
.

Even if it starts flowing, it does not disappear instantly. Therefore, the current i.

When the current i tries to restore to its original current value.

and the operational amplifier 11 flow in a superimposed manner. Therefore, load RL
ni i.

+11 flows, the voltage at the load RL becomes higher than normal.

If the field effect transistor Q2 is a junction type, its gate current is relatively large, so the current 11 is large.

As a result, the emitter voltage of the transistor Q3 increases, preventing the restoration of the current io, and stabilizing the state in which only the current 11 flows into the load RL.

In this state, transistor Q2. Since Q3 is not controlled, the current flowing through the load RL is not a constant current.

This state is called latch-up and may occur even when the power is turned on.

One possible way to avoid the latch-up phenomenon is to use a MOS field effect transistor with a small gate current as the field effect transistor Q2, but MOS field effect transistors are easily damaged by noise. It has the drawback of lack of reliability.

The purpose of this invention is to provide a constant current device of this type in which latch-up does not occur without deteriorating reliability.

FIG. 2 shows an embodiment of this invention.

In this invention, a current limiting resistor R4 is inserted in series between the output of the operational amplifier Q1 and the control electrode of the control active element 2 in the constant current device as explained in FIG. A feedback capacitor C1 is provided.

That is, the negative feedback capacitor C1 has one end connected to the operational amplifier Q1.
The other end is connected to the inverting input terminal (=) of the operational amplifier Q1.

With this configuration, if a large level of noise is applied from the load RL side, the transistor Q3 is turned off momentarily, and the noise is transmitted from the operational amplifier Q1 to the load RL through the gate of the field effect transistor Q2 and the base of the transistor Q3. Even if current 11 flows, this current 11 is limited by resistor R3 and can be suppressed to a sufficiently small current value.

Therefore, when the transistor Q3 is turned back on, a current 11 and i flows through the load RL.

Even if the current i1 flows in a superimposed manner, the voltage drop in the load RL will not become abnormally large because the current i1 is limited to a sufficiently small value, and therefore the emitter potential of the 1-transistor Q3 will not rise abnormally. The collector current io of the transistor Q3 can be quickly restored to its original current value.

In order to limit the current 11 to a sufficiently small value, the voltage of the power supply 1 is 24V and the load current i.

When is 20mA, resistor R3 needs to be 50Ω or more.

However, in this invention, since the negative feedback capacitor C1 is connected to the operational amplifier Q1, the circuit does not oscillate.

That is, by inserting the resistor R3, the delay time constant of the signal system increases, and the system becomes more susceptible to oscillation.

Therefore, by connecting the negative feedback capacitor C1 to the operational amplifier Q1, sudden signal components such as noise can be removed by connecting the negative feedback capacitor C1 to the operational amplifier Q1.
1 causes negative feedback and cancels it out, suppressing oscillation.

As explained above, according to this invention, the operation of a constant current device using an operational amplifier can be stabilized with a simple configuration, and the effect is extremely large in practical use.

Although an example in which the transistor Q3 is an NPN transistor has been described above, it is easily understood that the transistor Q3 can also be constructed using a PNP transistor.

[Brief explanation of drawings]

FIG. 1 is a connection diagram for explaining a conventional load grounding type constant current device, and FIG. 2 is a connection diagram showing an embodiment of this invention. Ql: operational amplifier, 1: power supply, 2: active element for control, 3
: common potential point, R1: current detection resistor, RL: load,
R3: Current limiting resistor, C1: Negative feedback capacitor.

Claims (1)

[Scope of utility model registration request] One end of the control active element is connected to a power supply through a current detection resistor, the other end of the control active element is connected to a common potential point through a load, and the connection point between the current detection resistor and the control active element is connected to a common potential point through a load. is connected to one input terminal of the operational amplifier, applies a set voltage to the other input terminal, and is provided with a negative feedback capacitor to connect the output of the operational amplifier to the control terminal of the active control element. A load-grounded constant current device consisting of a current limiting resistor inserted in series.