CS230771B1 - Connexion for brige circuit measure resistance - Google Patents

Abstract

The purpose of the invention is to create a simple one circuit for measuring or indicating resistance values, suitable especially for applications with lower demands on accuracy. Its essence is that bridge it is powered to its first node, yet what is attached to the opposite third node the collector of the first transistor whose the control electrode is connected to the other node and is connected to the fourth node control electrode of the second transistor. The first transistor keeps the voltage on the second node equal to the control threshold voltage electrodes and thus actually compensates its influence. The Y circuit can be used advantageously field-effect transistors.

Description

The object of the invention is to connect a bridging circuit for measuring resistors. In many fields of human activity, there is a need to measure electrical resistance. In this case, it is often sufficient to know only the resistance value or to indicate whether the measured resistance is less than or greater than the set value. Many different simple connections are known for this purpose. Their common disadvantage is that their operation is highly dependent on the voltage between the control electrode and the emitter of the transistors used. Therefore, otherwise very advantageous field-controlled transistors which have a considerable variation in the threshold voltage of the control electrode cannot be used therein.

This problem solves the connection of a bridge circuit for measuring resistors whose essence is that the bridge is

- a first node, while a collector of an auxiliary transistor whose control electrode is connected to the opposite third node

- is connected to the second node and the emitter is connected to the common conductor and to the fourth node is connected the control electrode of the second transistor whose emitter is connected to the common conductor. According to further features, the bridge is fed to its first node either through a power resistor or from a power source.

The connection of the bridge circuit according to the invention compensates in a simple way for the influence of the threshold voltage of the control electrode.

The operation of the circuit is influenced only by the difference in the threshold voltage of the control electrodes of the two transistors, which can be kept within relatively narrow limits either by pairing or by selecting transistors contained in only the element.

230 771

Fig. 1 shows the principle of connection of a bridge circuit according to the invention; Fig. 2 shows an example of a particular embodiment.

The bridge circuit in Fig. 1 is formed by four resistors JL, 2, 3 and 4. The first bridge node O1 is connected to the power terminal 8 via a power resistor 81. The collector of the first transistor 5, whose control electrode is connected with the second node 02 and the emitter I connected to the common conductor 7. The fourth node is connected to the control electrode of the second transistor 6, the emitter of which is connected to the indicator circuit (not shown).

In Fig. 2, the first bridge node 01 is connected via a power resistor 81 to the power terminal 8 and is connected via a potentiometer 12 to a third node 03. The third node 03 is connected to a collector of the first transistor 5 the electrode is connected to the slider 02 of the potentiometer

12. The third node 03 is connected by a third resistor 3 to the fourth node 04, which is further connected by a protective resistor 61 to the control electrode of the second transistor 6, whose emitter is connected to a common conductor 7 and the collector is connected to an indicator circuit (not shown). The control electrode of the second transistor 6 is connected to the common conductor 7 by a blocking capacitor 62.

The values of the bridge resistors 1, 2, 3 and 4 and the supply resistor value 81 should be selected such that the voltage drop across the second resistor 2 is not greater than the control electrode threshold voltage of the first transistor 5. Then the first transistor 5 automatically maintains voltage between the second node. 02 and a common conductor 7 equal to the threshold voltage of its control electrode. For example, if this voltage was higher, the first transistor 5 would open more, resulting in a decrease in the voltage at the third node 03 and thus at the second node 02, thereby offsetting the difference. With a balanced bridge, there is the same potential at the second node 02 and the fourth node 04 and therefore there is a voltage equal to the control electrode threshold voltage between the control electrode and the emitter of the second transistor 6. The second transistor 6 is then at the opening limit as indicated by the indicator circuit.

230 771

In the circuit shown in Fig. 2, the bridge is balanced by potentiometer 12. As mentioned above, the first transistor 5 maintains at the slider 02 of the potentiometer 12 a voltage equal to the threshold voltage of its control electrode. The measured resistance (not shown) is connected between the first node 01 and the fourth node 04. The protective resistor 61 serves to protect the control electrode of the second transistor 6 from damage, the blocking capacitor 62 serves to limit the influence of any induced interference voltages. If the bridge is balanced, the second transistor 6 is at the switch-on limit. If the measured resistance is less than the setting of the potentiometer 12, the second transistor 6 is closed; if the measured resistance is greater, the second transistor 6 is open.

The bridging circuit for measuring the resistances of the present invention has been designed for use in an electroacupuncture apparatus to locate those skin areas that have a much lower electrical resistance than the surrounding area. In addition to the advantages mentioned above, the bridge circuit according to the invention also satisfies the requirement that the open-circuit voltage at the measuring terminals should not exceed about 3.5 V, which would otherwise be difficult to achieve with field-controlled transistors. The bridging circuit according to the invention is advantageous wherever the resistance value is to be measured or indicated without great precision. Location of measured and variable resistance, resp. The circuit can also be implemented with bipolar transistors.

Claims (3)

1. Connection of a bridge circuit for measuring resistors with a bridge composed of four resistors, one of which is an unknown measured resistance and the other at least one resistance is variable, characterized in that: the power terminal (8) is connected to the first node (Ol) ), wherein the collector of the first transistor (5) is connected to the opposite third node (03), the control electrode of which is connected to the second node (02) and the emitter is connected to the common conductor (7), while the fourth node (d4) is connected a control electrode of the second transistor (6), the emitter of which is connected to a common conductor (7). Bridge circuit connection according to claim 1, characterized in that the power terminal (8) is connected to the first node (Ol) via a power resistor (81). Bridge circuit connection according to claim 1, characterized in that the power terminal (8) is connected to a power source.