The source of constant current to grounded load

 

(57) Abstract:

Usage: multifunctional measuring instruments (multimeters) when improving ammelrooy their part. The inventive source of constant current in the operational amplifier (op-amp) 6 and four precision resistors 1-4, two of which are connected to the inverting input of the OS, and the other two-to the not inverting input, the trimmer resistor 8 in series with a resistor in the circuit non-inverting input of the OS - the output of the shelter and two kinematically connected to the switch 9, the movable contacts of which are connected with the resistors connected to the not inverting input of the operational amplifier, the stationary contacts of the first switch are connected with the poles of the source of the input voltage, and the fixed contacts of the second switch are connected one with the earth, and the second with the output of the operational amplifier. When checking (setting) device circuit is converted by using the above-mentioned switches in the differential amplifier, trimmer, the output voltage is reduced to zero. 1 Il.

The invention relates to the circuitry of electronic devices, namely, integrated electronics, use the power load. Primary area of use in measurement technology - ohmmeters measure resistance DC grounded conclusion of the test resistor.

A well-known source of constant current on the basis of an operational amplifier with negative and positive feedbacks [1] . The principal disadvantages of this device lies in the fact that all five of the resistors included in the combined negative and positive feedback must be precisely aligned, as in the absence of approval of the current through the load resistance will be determined with great accuracy and, in addition, the transition from one value of the reference current to another requires the simultaneous change of the resistance of all five resistors.

Known also another source of constant current, containing two operational amplifier and five precision resistors [1] .

The disadvantages of this device are its complexity and the need for precise coordination of all five resistors.

The closest in technical essence to the invention is a constant current source, containing operational amplifier, four precision resistor, historiquement source of input voltage through another precision resistor with the output of the operational amplifier. Non-inverting input of the shelter through the third precision resistor connected to the common point of the device ("earth") and through the fourth precision resistor with the output of the shelter. The load is connected between reinvestiruet entrance of the shelter and "earth". The device is operable only when precise coordination of the above precision resistors, forms in load current equal to the ratio of the magnitude of the input voltage to the resistance value of the resistor connected between reinvestiruet input and ground.

The disadvantage of the prototype is the need for precise matching of the resistors, because otherwise stable load current is formed with a large error. For example, when using resistors with the deviation from the nominal values of the resistors 1% , stable current will be determined with an error of 4% or more.

The disadvantage of the prototype is that even if the condition of the exact matching of the resistors, at higher output currents, the resistor must be small, thus limited output range, i.e., this scheme is used in practice rarely.

The technical result, which provides the invention is to increase technasia four precision resistor, the source of the input voltage, operational amplifier, a resistor load, the first output of the first resistor is included with the pole of the input voltage, the other pole is connected to a common point, the other of the resistor connected to the inverting input of the operational amplifier and to the output of the second resistor, the second terminal of which is connected to the output of the operational amplifier, one output of the fourth resistor is connected to the output of the load resistor and not inverting input of the operational amplifier.

New in the invention is the fact that it introduced: a trimmer and two kinematically connected to the switch, and the movable contact of the second switch is connected to one output of the third resistor, the other output of which is connected to the output of the load resistor and to the not inverting input of the operational amplifier, the free conclusion of the fourth resistor is connected to the moving contact of the first switch, its one stationary contact connected to a common point, the other pole of the source of the input voltage, to the common point connected to one fixed contact of the second switch, the other stationary contact is connected to the output of the operational amplifier.

On chesector 1, the second resistor 2, the third resistor 3, the fourth resistor 4, input 5 conclusions for connection to a voltage source, an operational amplifier 6, the load resistor 7, the trimmer resistor 8, the first switch 9, kinematically associated with the second switch 10.

The first resistor 1 single output is connected to the pole of the input voltage, the other pole of which is connected to a common point, the other output of the first resistor connected to the inverting output of the amplifier and one of the conclusions of the second resistor 2, the second terminal of which is connected to the output of the operational amplifier 6, one of the conclusions of the third resistor 3 is connected with a movable contact 11 of the second switch 10, and the other with the output of the trimmer resistor 8; trimmer resistor 8 is connected by its output to the not inverting input of the operational amplifier 6 and the output of the load resistor, this connection point is connected to one of terminals of the resistor 4, the other of the resistor is connected with a movable contact 13 of the first switch 9, the stationary contacts of which includes one 14 to the output of the first resistor 1, the other 12 are connected to a common point and the fixed contact 15 of the second switch 10 is connected to a common point, the other 16 - to Imagna principle of operation for the circuit elements instead of digital used the appropriate letter designation:

5-E, 1-R1, 2-R2, 3-r3,

8 - r3, 4R4, 7-RL, 9-S1THE 10-S2;

R3= r3+ r3< / BR>
S1switch (pins 12, 13, 14),

S2switch (pins 11, 15, 16).

The device operates as follows. First, make adjustment. Switches S1and S2set in the position "setting" (the movable contacts 11 and 13 are connected with the stationary contacts 14 and 15), and the load is disconnected (R4= = ). The scheme at this moment transformed (converted) into a differential amplifier (2), to the inverting and non-inverting input of which is filed the same voltage that is the Voltage at the output of the operational amplifier (op-amp) is absent, unless the condition:

R1/R4= R2/R3(1)

If this condition is not met, then the output of the operational amplifier there is a voltage offset of the U. for Example, if E = I, R2= R3and when the deviation of R1from R41% , U = 5 mV.

Dragging the slider to the resistor r3in one way or another, we have U = 0, i.e., achieve compliance with the conditions above.

When the transfer switches S1and S2in position "form stable load current.

The principle of the scheme is that as you increase the load resistance is proportional to the increase in positive feedback, which ensures the stable load current. When the load resistance is equal to zero (Rn= 0) positive feedback is absent (resistor R4between reinvestiruet entrance of the shelter and a common point when shorted). The scheme turns into an ordinary inverting amplifier, the output voltage is determined by the equation

Uo= -E R2/R1< / BR>
When Rn= 0, the current through the load is equal to the output voltage divided by the resistance R3, i.e.

In= Uo/R3= -E R2/R1R3.

From condition (1) implies that

R2/R1R3= 1/R4and thus, finally

In= -E/R4.

When increasing the load resistance increases positive feedback, i.e., increases in proportion to the output voltage, and the current through the load is not changed. This is up until the output voltage is less than the maximum allowable under the terms of this operational amplifier.

and the output voltage of the operational amplifier Uo= where m = R1/R2= R4/R3< / BR>
Using formula (2), easy to install maximum load resistance for which the operational amplifier can provide a specified current.

For example, when In= 2 mA, m = 1, R4= = 1 kOhm and the maximum allowable output voltage of the operational amplifier Uo= 10, we get:

Rn= = = 2000 Ohms (56) 1. Gutnikov C. C. Integrated electronics in the measuring devices. L. ; Energoizdat, 1988 , page 70.

2. Horowitz P. , hill U. , Art circuitry in 2 volumes, volume I, translated from English. M. : Mir, 1984, pp. 162, 163.

The SOURCE of constant CURRENT TO GROUNDED LOAD, containing the first, second, third and fourth precision resistors, the input leads for connection of the input voltage, operational amplifier, a resistor load, the first output of the first resistor is included with one input / output, another input the output is connected to a common point, the other output of the first resistor connected to the inverting input of the operational amplifier and to the output of the second resistor, the second terminal of which is connected to the output of the operational amplifier, one output of the fourth resistor is connected to the output resistor load is a torus and two kinematically associated switch, moreover, the movable contact of the second switch is connected to one output of the third resistor, the other output of which is connected to the output of the trimmer resistor, the other output of which is connected to the output of the load resistor and to the not inverting input of the operational amplifier, and a free output of the fourth resistor is connected to the moving contact of the first switch, its one stationary contact connected to a common point, the other to another input output to a common point connected also fixed contact of the second switch, the other stationary contact is connected to the output of the operational amplifier.

 

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