V. I. Ivolgin, Tambov
Any electronic device has a power source, due to the energy of which it performs its functions. And it is not surprising that significant space in the press is devoted to their descriptions, design recommendations, consideration of the operation of individual components, and proposals for their improvement.
It should be noted that modern power supplies, as a rule, have a fairly low output impedance. And for this reason, in emergency situations, even at low voltages at their output, significant current overloads cannot be ruled out, leading to damage to the source or the device itself. In this regard, power supplies are usually equipped with protection systems. They are quite diverse and have more or less autonomy relative to the design of the source itself.
One of the options for such a device, which can be used as a stand-alone unit, is offered in. Its operating principle is based on limiting the current consumption, the sensor of which is a low-resistance resistor connected in series to one of the wires between the power source and the load. The voltage from the sensor, proportional to the current consumed, after amplification, is used to control the pass transistor. By changing its operating mode at the right time, direct overload protection is provided.
In this article, a well-known structure based on two bipolar transistors is given as a prototype (Figure 1). The main disadvantage of the device is the significant voltage drop across it, which reaches its maximum value at the maximum operating current. According to the author, it is approximately 1.6 V, and about 1 V drops on the pass transistor VT1, and the remaining 0.6 V on the current sensor Rs. In connection with this, the author proposes another circuit that allows reducing the voltage drop across it to 0.235 V at limit current is 1.3 A. This value is quite small, although it is achieved using a more complex circuit containing about 20 elements.
On the other hand, this design, in comparison with the one proposed by the author, is attractive in its simplicity. And in this regard, the question arises: is it possible, while remaining within such a simple structure, to achieve a reduction in the voltage drop across such a fuse without noticeably complicating it? And how?
As follows from the given numerical data for the prototype, the largest voltage drop occurs across the pass-through bipolar transistor VT1. Analysis shows that with such a switch on, it is impossible to achieve its saturation, and thereby achieve low voltage drop values, without an additional power source. But introducing it only for this purpose would be expensive. And although it would probably be possible to propose some other ways to reduce these losses on VT1, it would be more rational to immediately replace the bipolar transistor with a field-effect transistor with a low channel resistance value. This will reduce both the voltage drop across the control transistor and the limiter’s own consumption by reducing control currents. In addition, it is advisable to change the connections between the transistors so as to convert the limiter into a system of two amplifier stages, instead of just one in the original structure. Ultimately, the circuit diagram of the limiter under study will look like this (Figure 2), which can also be considered as a simplified version of the device shown in.
Testing the functionality of the proposed limiter, as well as performing measurements, were carried out on a breadboard, in which a field-effect transistor mounted on a radiator was used as VT1, VT2 - a transistor with β ≈ 300, RS - a 1.2 Ohm resistor, R1 - 4.2 kOhm, and the load was a set variable wire resistors of the required power. The voltage at the limiter input was 12 V. The measurement results are shown in Figure 3.
Testing the limiter with a short circuit showed that when this manipulation is performed, the current through the pass transistor is set at 0.5 A at a voltage on the current sensor of 0.60 V. And, thus, such a current limiter is quite functional. One can also note its rather high output resistance in current limiting mode - when the voltage at its output changes in the range of 0...11.3 V, the current through the load practically remains equal to 0.5 A. In addition, due to the known dependence of transistor parameters on temperature, the dependence was checked heating current limit values VT2. As it turned out, its value was only about -0.2% relative error per degree.
From the analysis of the graphs it follows that the voltage drop across a pass transistor of this design is already quite small and even at the edge of the current range does not exceed 0.1 V. It can also be noted that on the graph of the voltage drop across VT1, two intervals can be visually distinguished. In the first of them, at currents from 0 to 0.45 A, the increase in voltage drop is its linear function, which indicates saturation of the transistor in this part of the range. Indeed, the transistor channel resistance calculated from these data is approximately 0.125 Ohms, which practically coincides with the passport data of the used transistor VT1. At higher currents, in the range of 0.45 - 0.5 A, there is first a slow and then a sharp nonlinear increase in this value, associated with the activation of the current limiting mechanism.
Thus, from the above data it follows that the total voltage drop across the limiter has noticeably decreased, and is already determined mainly not by the voltage drop across VT1, but by the voltage of the sensor R S. How can you reduce the last value?
The answer suggests itself - you need to reduce the value of RS, as was done in, and use an additional amplifier to compensate for the decrease in the sensor signal level. But on the other hand, in the circuit discussed above (Figure 2) such an amplifier, made on transistor VT2, already exists. However, its parameters do not allow the voltage drop RS to be reduced to lower values, although it has a fairly high gain. In connection with this problem, let us consider in more detail the features of the operation of VT2 as a pre-amplifier of the signal from the current sensor.
As follows from the circuit diagram (Figure 2), the current limitation through VT1 occurs due to a change in the voltage at its gate, which occurs when the collector current of the transistor VT2 changes. Its mode is controlled by voltage from the resistor of the current sensor R S. And, as follows from the latest measurement data (Figure 3), the device reaches full current limitation only at voltages of about 0.6 V at its base relative to the emitter. This circumstance determines the resistance value of the resistor R S .
But it is characteristic that part of the voltage on the sensor in the range from 0 to 0.55 V can be considered “extra”, since in this interval VT2 practically does not “feel” it, and only the interval 0.55 - 0.6 V will be truly “working” for it. If the lower limit of the amplifier sensitivity, visually 0.55 V, is set to zero, it will be possible to solve the problem of reducing the value of R S .
Technically, this result can be achieved, for example, by introducing a separate 0.55 V auxiliary source into the circuit between the base of VT2 and the right terminal of R S. But it is more convenient to form it by using a divider of two resistors connected between the common wire and the emitter of transistor VT1 (resistors R2, R3, Figure 4). And its parameters should ensure a voltage drop across R2 equal to 0.55 V. To make this value less dependent on the input current of the transistor, it is advisable to keep the current of this divider within 0.5 - 1 mA. Under these conditions, an insignificant voltage on RS will put transistor VT2 into the active start-limiting mode, and complete current limitation will occur when the voltage drop on RS is only a little more than 0.05 V. It is clear that by changing these resistors it will be possible to change the current limiting threshold. And this will be more convenient than selecting the value of RS.
A new version of the limiter circuit diagram, already taking into account the above considerations, is presented in Figure 4. Its test layout was made while maintaining the details of the device of the previous version with a change in resistance R S by 0.2 Ohm, and the installed additional resistors R2 and R3 have values of 680, respectively. Ohm and 15 kOhm. The test and measurement conditions remain the same as before.
The main test results, as follows from the presented graphs (Figure 5), are as follows. As before, the short circuit current of the device is 0.5 A. More precisely, in reality, with the indicated values of resistors R2, R3, it was 0.48 A, but this value was corrected by connecting an additional variable resistor in series with R3. As for the maximum value of the voltage drop on the sensor RS, it fell in proportion to the decrease in the value of the set RS and amounted to only about 0.1 V. The graph of the voltage drop on the control transistor, compared with the same parameter of the previous circuit, in general, retained its features, although somewhat changed. So, for example, you should pay attention to the fact that this time the region of sharply nonlinear increase in the voltage drop across the pass transistor has shifted to the range of 0.4 - 0.5 A, and in the rest it grows almost linearly. It follows from this that there is still a certain reserve for reducing the voltage drop at the current sensor R S.
As already noted, a minor correction of the limiting current in this design was carried out by changing the resistance R3, but when a significant change is required, it is more convenient to use R2. When calculating its value, it is advisable to first set the value of the maximum voltage drop V SM on the current sensor RS in the limiting mode. In principle, this value can be anywhere from 0 to 0.6 V. But you need to keep in mind that as it decreases, the temperature stability of the proposed solution deteriorates. So, at V SM = 0.6 V, the temperature coefficient of the dependence of the change in the current limit limit in the room temperature region does not exceed 0.2% per degree, and at V SM = 0.1 V, this indicator increases to 1.5%. This value in some cases may still be acceptable, and it can conditionally be taken as the lower limit of the range of permissible values V SM, while the upper limit will be determined by the maximum voltage drop at the base of transistor VT2 in current limiting mode. If for the calculation we select V SM equal to 0.15 V, then from this condition at a given limiting current I M , for example, 1.5 A, the value will be determined
With V VX = 12 V and R3 = 15 kOhm, we obtain that R2 = 0.58 kOhm.
If necessary, this resistor, if replaced with a variable one, can quickly change the limiting current within significant limits, which, however, will be accompanied by a change in the maximum voltage drop V SM and a corresponding change in the temperature coefficient of instability.
Summarizing the discussion on the design of a simple current limiter (Figure 4), we can conclude that the changes made to the structure of the prototype (Figure 1) ultimately made it possible to reduce the voltage loss across it to tenths of a volt. It should also be added that its work was selectively tested in other modes not reflected in the article. In particular, with limiting currents in the range from 10 mA to 5 A and input voltages of 7, 12 and 20 V. To adapt to these conditions, only the values of R S were changed (0.05, 0.2 and 1.2 Ohm), and to set the limiting current as R2 a 1 kOhm variable resistor was used, the resistance of which was set in accordance with the calculation according to (2). All other elements, including transistors, remained the same.
Current limiter is a device designed to exclude a possible increase in current in the circuit above a specified value. The simplest limiter is an ordinary fuse. Structurally, the fuse is a fuse-link enclosed in an insulator - a housing. If, for one reason or another, the current consumed by the load increases in the circuit, the fuse link burns out and power supply to the load is cut off.
Types of limiters
With all the advantages of using a fuse, it has one serious drawback - low performance, which makes it impossible to use in some cases. The disadvantages include the disposability of the fuse - if it blows, you will have to look for and install a fuse exactly the same as the blown one.
Much more advanced than the fuses mentioned above are electronic limiters. Conventionally, such devices can be divided into two types:
Separately, it is worth mentioning the so-called passive protection devices. Such devices are designed for light and/or sound signaling in situations where the permissible current in the load is exceeded. For the most part, such schemes alarms are used in conjunction with electronic limiters.
The simplest solution when it is necessary to limit the direct current in the load is to use a field-effect transistor circuit. The schematic diagram of this device is shown in Fig. 1:
Rice. 1 - Field-effect transistor circuit
The load current when using the circuit shown in Fig. 1 cannot be greater than the initial drain current of the applied transistor. Therefore, the limiting range directly depends on the type of transistor. For example, when using the domestic transistor KP302, the limitation will be 30-50 mA.
The main disadvantage of the scheme described above is the difficulty of changing the limit limits. In more advanced devices, to eliminate this drawback, an additional element is used that performs the functions of a sensor. As a rule, such a sensor is a powerful resistor that is connected in series with the load. At the moment when the voltage drop across the resistor reaches a certain value, the current will automatically be limited. The diagram of such a device is shown in Figure 2.
Rice. 2 - Bipolar transistor circuit
As you can see, the basis of the circuit are two bipolar transistors of the n - p - n structure. A resistor R 3 with a resistance of 3.6 Ohms is used as a sensor.
The operating principle of the device is as follows: voltage from the source is supplied to resistor R 1, and through it to the base of transistor VT 1. The transistor opens, and most of the voltage from the source is supplied to the output of the device. In this case, transistor VT 2 is in the closed state. At the moment when the voltage drop across the sensor (resistor R 3) reaches the opening threshold of transistor VT 2, it will open, and transistor VT 1, on the contrary, will begin to close, thereby limiting the current at the output of the device. The HL 1 LED is an indicator that the limiter has been activated.
The response threshold depends on the resistance of resistor R 3 and the opening voltage of transistor VT 2. For the described circuit, the limit threshold is: 0.7 V / 3.6 Ohm = 0.19 A.
In some cases, a device is required with the ability to manually change the current limit value in the load, for example, when it comes to the need to charge car batteries. The diagram of the adjustable device is shown in Figure 3.
Rice. 3 - Circuit with current limit adjustment
Device specifications:
The main feature of the circuit is the ability to both change the value of the current limit in the load and the ability to adjust the output voltage. The current limit is set by variable resistor R 5, and the output voltage is set by variable resistor R 6. The current limit range is determined by the resistance of the current sensor - resistor R2.
When designing such a device, it is worth remembering that quite a lot of power is allocated to VT 4, so to eliminate the likelihood of the element overheating and failure, it must be installed on a radiator. Also note that variable resistors R 5 and R 6 must have a linear adjustment relationship for more convenient use of the device. Possible analogues of the parts used:
It cannot be said that one or another current limiting method is better or worse. Each has its own advantages and disadvantages. Moreover, the use of each is advisable or completely unacceptable in a certain specific case. For example, using a fuse in the output circuit of a switching power supply is in most cases impractical, since the fuse as a protection element is not fast enough. In simpler terms, the fuse may burn out after the power elements of the power supply become unusable due to overload.
In general, the choice in favor of one or another limiter should be made taking into account the circuitry, and sometimes design features of the input voltage source and the characteristics of the load.
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