How to Simplify Current Monitoring by Using Diode?

The diode and the negative end of the power supply are connected in series to monitor the current, and the fixed range digital multimeter (DMM) is used to detect the current. This simple design example can realize the current monitoring from a number of μA to 100mA in a single range. This design example has proved to be very useful and simple. Only 3 to 4 modules are needed to monitor the current from the μA over to 100mA in a single range.

Home Energy Monitor Project: Current

As defined by the diode formula IF≅I0 × exp (eVF/kT), the voltage on the diode increases with the logarithmic current flowing through it. Where IF is a forward current, IO is a reverse saturation current, the charge is (1.602 × 10 ~ (-19) C) V _ F is a forward voltage T is the temperature (K), k is the Boltzmann constant (1.380 × 10 ~ (-23) J/K).

Depending on the purpose, the following formulas can be extracted: VF∝logIF(temperature fixed)

Catalog

I Shunt Diode

II Adding Extra Diodes

III. LTspice

IV Conclusion

FAQ

I Shunt Diode

Now let ' s look at a diode with a measuring instrument . When the current is low , it indicates the milliampere ( mA ) level current that flows through the meter rather than the diode; while in a large current it displays the voltage on the diode, and the logarithm of the current thus derived ( imagining the diode as a self-adjusting shunt ) . Therefore the bottom of the meter scale is therefore quite linear and the top has enough logarithmic properties and the middle is a transition phase , so the entire range is very useful.

As shown in Fig.1, using a Schottky rectifier, a 100μA/1.7kΩ meter and an appropriate series resistor can monitor the current from 10 μA to over 100mA within a single range, and the indicated speed is limited to the pendulum speed of the meter.

Schottky rectifier, 100 μ A / 1.7 kΩ meter and suitable series resistance

Fig. 1 Schottky rectifier, 100 μ A / 1.7 kΩ meter and suitable series resistance

This simple circuit often has more problems than the number of components, in addition to the high-precision calibration process, the circuit also has two main drawbacks: series voltage drop and temperature stability. The diode voltage drop is as high as 400mV, so it is best to use a new or charged battery when monitoring, otherwise your measured components may show that the battery is low. Or treat the circuit as a convenient low-voltage test circuit that might add a short-circuit switch.


II Adding Extra Diodes

At the bottom of the scale, almost all current flows through the instrument, which is limited by the machine and magnetic temperature coefficients, and the measured temperature coefficient is very low. But at large currents, a voltage drop can be seen on the diode, which will drop at a rate of about 2mV/K, as predicted by the diode formula. This not only affects the slope of low of logarithm, but also affects the transition point from linear to logarithmic. In addition, the meter windings account for a large part of the total series resistance, and the TCR of copper at room temperature is 3930 ppm/kg. Fig.2 shows the relation curves of deviation and current of 1N5817 at 0℃, 25℃ and 50℃. These curves take into account the TCR of the measuring circuit and the temperature coefficient of the diode, but ignore the self-heating effect of the latter, but there is no problem at relatively stable temperature.

Deviation and current curve

Fig. 2 Deviation and current curve

Self-heating mainly exists in D1 will have no impact on current. Suppose the current flowing through is 100mA, the voltage drop D1 is 400mV—that's 40mW. According to the manual, the basic thermal resistance of a D0-41 1N5815 with a slightly longer pin and a large amount of radiating copper is 50 K/W. When these data are taken into account, the temperature rise of the node is only 2℃ at 100mA, which is equivalent to the reduction of VF by about 4mV, or the error of about 1% at full scale. Try to keep the diode to a short pin and high thermal quality, noting that there may be high transient currents during conduction, as these can lead to errors until the temperature of the node cools again.

An improved version of the offset temperature coefficient

Fig. 3 An improved version of the offset temperature coefficient

The bias and current curve after adding a diode

Fig. 4 The bias and current curve after adding a diode

Fig. 4 shows the curve of the circuit. Note that most of the curve is now in logarithmic form, and that extra diode effectively suppresses the initial linear region. However, the selection of this diode is critical because the forward voltage of D2 should be slightly lower than that of D1, but other features should match.


III LTspice

D1 using 10MQ060N and D2 using BAT54—this is the first pair of components emulated. Both are cheap, modeled by LTspice and are therefore recommended components. A pair of 10MQ060N works almost consistently (but a pair of BAT54 is inconsistent). In most of the time, this group combines with other components showing worse temperature variations and strange indications, so it is necessary to  model the circuit before building it. If the sensitivity and resistance of the instrument are appropriate, R1 can be omitted. On the same thermal properties, the D1 and D2 can track mutual temperature changes.

Silicon P-N junction diodes generally have a very straight (log IF) / VF relation, while Schottky's straight line is not. This is because their structures have higher series resistance, are more closed to linear than logarithmic at very low currents, and have protection loops to control the potential gradient of P-N diodes that are parallel to Schottky nodes. Therefore, in practice, the exact logarithmic law will change with the current and the type of component.

Although a used diode may be fine for the first pair, due to the inevitable inaccuracy of the circuit, the double diode design still needs to be carefully selected. Schottky diodes can provide more reference resources.

100 μ A /1700 Ω indicators, which are very common, very tightly connected, very useful, and their linear and structure are well consistent with units, just match the 35mm × 14mm aperture, so select them.

The calibration points used in Fig.5 are generated by arranging a series of combinations of monitors, batteries, fixed and variable resistors, and the DMM series. Existing test scales are marked at the appropriate points and then removed and scanned, which are used as templates for the final layout. 

The simulation results are used to generate the reference point in Fig.5 (left), and the results well reflect the actual operation, although the multimeter is poor. These scales can save time, but are not as accurate as they are newly made (obviously these measuring structures need different scales), and R1 can be calibrated slightly (the instrument is set at ±20%). Both scales consider the non-linearity of the instrument structure.

derives from the calibration point (right) of the monitor, battery, fixed and variable resistor, and DMM combination.

Fig.5 The calibration point (right) of the monitor, battery, fixed and variable resistor, and DMM combination

IV Conclusion

Whatever, now that these circuits are embedded in most of my development projects and even in production testing devices, they are effective in finding a variety of faults and problems, from power lines short-circuiting to the pull-up pins of miscoding.

In order to facilitate the monitoring of the current, it is necessary to connect the appropriate diode with the negative end of the power supply and monitor its forward voltage drop. After some simple calibration, you can monitor the supply current in full sync with the other parameters you want to detect.


FAQ

1. What is a shunt diode?

In electronics, a shunt is a device that creates a low-resistance path for electric current, to allow it to pass around another point in the circuit. ... The origin of the term is in the verb 'to shunt' meaning to turn away or follow a different path.

2. What is shunt and its uses?

shunt is a device which allows electric current to pass around another point in the circuit by creating a low resistance path. A shunt (aka a current shunt resistor or an ammeter shunt) is a high precision resistor which can be used to measure the current flowing through a circuit.

3. How does a shunt diode work?

The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load. One of the most common examples of the shunt regulator is the simple Zener diode circuit where the Zener diode acts as the shunt element.

4. What are the disadvantages of shunts?

a. It has poor efficiency for large load currents.

b. It has high output impedance.

c. The output DC voltage is not absolutely constant because both VBB and VZ voltages decrease with increase in room temperature.

5. Where is shunt used?

The shunt is used in the galvanometer for measuring the large current. It is connected in parallel to the circuit of the galvanometer. The galvanometer is the current sensing devices. The direction of flow of current inside the circuit is determined by the pointer of the galvanometer.

6. Why shunt is always connected in parallel?

A shunt resistance should be connected in parallel to the galvanometer so as to keep its resistance low. Such low resistance galvanometer ( ammeter) is used in series with the circuit to measure the strength of current through the circuit.

7.How is shunt current calculated?

How to Calculate a Shunt:

a. Write down the Ohm's law expression of "V = I * R" where "V" is the voltage drop across shunt resistor, "I" is the current flowing through shunt and "R" is the shunt resistance.

b. Substitute value of voltage "V" and current "I" in the Ohm's law expression.

8. What size shunt do I need for battery monitor?

A 100 amp shunt would be plenty if you are only using 12v devices like water pump, furnace blower and lights. We have an inverter and pass up to 200 amps sometimes. The shunt that came with our monitor is good for 500 amps. It doesn't hurt to have a shunt larger than you need.

9. Why shunt is used in galvanometer?

Since galvanometer is a very sensitive instrument that it can not measure the heavy currents . to do so A shunt is connected with parallel with galvanometer to convert it into ammeter. ... so after that it can measure heavy currents in the circuit.

10. Is a shunt a resistor?

A shunt is a low-ohm resistor that can be used to measure current. Shunts are always employed when the measured current exceeds the range of the measuring device.

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