# Analog combination meter 2

### Analog combination meter part 2

Among the gauges in the combination meter, the oil pressure gauge, the fuel gauge, the water temperature gauge and the charge display device (charge lamp) are combined with a sender unit that detects various measured values ​​and a receiver unit that displays to the driver.

These units are classified into several types according to the operating mechanism, but are used in combination as appropriate depending on the application.

#### Connection method of various gauges

Gauges that use an electric receiver unit and sender unit have the receiver unit and sender unit connected in series with a single wire, and are used with a 12V or 24V power supply. Therefore, the voltage of 12V or 24V is not applied to the receiver unit or the sender unit respectively, but this voltage also differs depending on the gauge method, so if you do not handle it with familiarity with the gauge method, the parts will be damaged. You need to be careful because you will make a wrong decision when troubleshooting.

##### Circuit that combines a resistance type sender unit and a bimetal type receiver unit

In the gauge of the type in which the receiver unit and the sender unit are connected in series, there is no mechanism to control the time when the current flows on the sender unit side, so the current change of the heating wire of the receiver unit will occur as an error. , Voltage compensation is required. Figure 10: Operation of bimetal and resistance meter

Therefore, as shown in FIG. 10, a voltage regulator for the meter is provided on the power supply side of the receiver unit to prevent an error from occurring.

The voltage regulator for the meter is connected to each gauge as shown in FIG. 11, and is called a contact type voltage regulator for the meter. Figure 11: Connection of voltage regulator for meter (contact type)

In this operation, when the key switch is turned on, a current flows through each gauge circuit through the contacts of the voltage regulator. At the same time, a current flows through the heating wire of the voltage regulator to heat the bimetal, and after a certain period of time, the bimetal bends, the contacts are separated, and the current to the gauge circuit is cut off.

When no current flows through the heating wire of the voltage regulator, the bimetal cools and returns to its original state, the contacts close, and current flows back into the circuit. Since this operation is repeated while the key switch is ON, a periodically intermittent current flows through the gauge circuit.

Therefore, the opening / closing cycle of the voltage regulator changes according to the fluctuation of the power supply voltage, a constant amount of electricity is supplied to the circuit, and the receiver unit is not affected by the voltage fluctuation and prevents the indication error. be able to. Figure 12: Meter contact voltage regulator

The contact type voltage regulator for meters is usually installed integrally with the receiver unit as shown in Fig. 12 (1), but it is assembled in a case made of insulation as shown in Fig. 12 (2). It may be installed and used separately.

##### A circuit that combines a resistance type sender unit and a cross coil type receiver unit

About the fuel gauge as shown in Fig. 13 below Figure 13: Cross-coil fuel gauge Figure 13: Cross-coil fuel gauge

As shown in FIG. 13 (2), one coil L1 and L2 of the sender unit and the receiver unit are connected in series, but the other coil L3 and L4 are connected in parallel with the sender unit.

In this sender unit method, the pointer is moved by the action of a combined magnetic field generated by a rotor made of a magnet and a coil wound at right angles to the rotor. Therefore, even if the power supply voltage fluctuates, the magnetic fields created by both must increase or decrease at the same rate. Therefore, by making the ratio of the magnetic fields the same, the circuit structure does not cause an error in the pointer instruction.

The receiver unit has a coil crossed around a rotor (magnet) and is operated by the magnetic field. Figure 14: Winding direction of cross coil

In the cross coil, as shown in Fig. 14, L1 and L3 are coaxially wound in the A and C directions in opposite directions, and L2 and L4 are wound in the B and D directions in the opposite directions. These coils generate a magnetic field that operates the rotor. Figure 15: Cross section of cross-coil gauge

In this meter, in order to control the movement of the rotor, silicone oil is injected around the rotor as shown in FIG. (By balancing with the force of the magnetic field, the behavior and balance of the rotor movement are maintained.)

• Operation of cross-coil fuel gauge
• When the resistance value of Rs is 0 When the resistance value of Rs in FIG. 16 is 0, the voltage Vs is also 0 potential, the constituent circuits are L1 → L2 → ground, and the current is excited only in L1 and L2.

Therefore, the rotor pointer stops at the position of the combined magnetic field of L1 and L2.

L1 and L2 intersect 90 degrees, but since L1 has more turns than L2, the magnetic field of L1 is larger than that of L2, and the pointer of the rotor approaches the magnetic field of L1. It stops at the position of the combined magnetic field (angle θ1) in FIG. Figure 17: When the resistance value of Rs is 0

• When the resistance value of Rs rises When the resistance value of Rs rises, the voltage Vs also rises proportionally, and the circuit configuration becomes L1 → L2 → Rs → ground at the same time as L1 → L2 → L3 → L4 → ground.

Therefore, since current flows through all the coils, L1, L2, L3, and L4 are excited.

Since L1 and L3 have opposite coil winding directions, the magnetic field of L3 acts in the direction of reducing the magnetic field of L1.

Similarly, since the coil winding directions of L2 and L4 are opposite to each other, the magnetic field of L2 acts to reduce the magnetic field of L4.

When the movement of the pointer of the receiver unit is considered in units of 0 ° to 180 ° (half scale), the combined magnetic field of all coils is as shown in Fig. 18 below, and the pointer of the rotor is on a half scale (angle θ2). ) Stops at the rotated position. Figure 18: When the resistance value of Rs increases

#### Needle type receiver unit Figure 19: Needle type receiver unit (1)

Generally, gauges are in a state where each pointer has returned to its original position unless the key switch is turned on and current does not flow. With a fuel gauge, it is convenient if the remaining amount at that time can be confirmed as shown in Fig. 19, regardless of the key switch. Figure 20: Needle type receiver unit (2)

Therefore, if the part where the rotor of the coil type receiver unit is placed is sealed as shown in Fig. 20 and highly viscous silicone oil is sealed in it, the viscosity of the silicone oil will cause it even after the power is turned off. The rotor stops as it is, and the pointer shows the remaining amount without returning. A receiver unit having such a structure is called a needle type receiver unit.

#### Fuel level indicator

As shown in FIG. 21, the fuel level indicator is composed of an indicator lamp and a thermistor, and lights the lamp when the remaining fuel in the fuel tank is low to give a warning to the driver. Figure 21: Fuel Level Indicator

If there is a lot of fuel in the tank, the temperature is low because the thermistor is submerged in gasoline.

Therefore, the resistance of the thermistor is large and the current flowing through the circuit is small, so that the lamp does not light.

When the amount of gasoline in the tank is low and the thermistor is exposed to the outside of the gasoline, the temperature of the thermistor rises and the resistance decreases, so the lamp lights up.

Generally, the thermistor is used by assembling it to a fuel sender unit.