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What resistance should be selected when the power-on current is too large

NTC thermal Resistors One benefit of having an active circuit for suppressing inrush current is lower component cost. Inrush current limiters are generally cheaper than active circuit components. However, the proper cost of each component depends on the power level of the power supply it is targeted for. Generally, the higher the power level, the larger the components and the higher the cost. The following example based on a 300W power supply can easily illustrate the cost advantages of an inrush current limiter with respect to active circuits. To provide surge protection with an inrush current limiter, the only components required are two inrush current limiters, and the total cost of the two thermistors is $ 0.80 or less.

The benefit of the second inrush current limiter is that the planning is simpler. Because surge suppression circuit plans based on inrush current limiters typically touch fewer components, they are less messy than those that touch active circuits. In turn, the simpler planning reduces the time required to plan the inrush current of the power supply to restrain the talent. As a general rule of thumb, using an inrush current limiter for inrush current restraint requires only about one-fifth of the time required to plan a comparable active circuit. In addition, the inrush current limiter plan provides greater manufacturing simplicity and lower scrap rates. Fewer components means fewer assembly processes. A lower component count also reduces the possibility of manufacturing scrap due to component defects. The surge restraint plan based on the inrush current limiter usually requires much less space than the active circuit on the power circuit board. Regarding the very expensive use of available space, this may be a key consideration. The disadvantages of using active circuits with resistors often require replacement parts. However, the surge suppression according to the inrush current limiter is self-protected in the disadvantaged way because their resistance decreases with increasing temperature.

Active circuit for inrush current

As mentioned above, some applications Sometimes, various types of "active" circuits are used as replacements for NTC thermistors (inrush current limiters). These component replacements include triacs, resistors and thyristors. Active circuit alternatives are triacs (300W power supplies usually cost about $ 1.00), others have a resistor (approximately $ 0.60), and are required to drive triacs Circuit ($ .20 or more), it is calculated that although the economic and planning benefit is that the inrush current limiter supplies inrush current restraint, in some cases, the active circuit may provide a more suitable processing solution.

For example, when the “thermal start” function is critical, the active circuit may sometimes be a better choice than the inrush current limiter. This benefit touches the resistance of the inrush current limiter enough to provide the required cooling / recovery time for the required level of inrush current protection. Assuming that the power reduction time is shorter than the specified recovery time, such as a few seconds, the thermistor will have no opportunity to cool down and return to its initial resistance level. When the power is turned on from the beginning, the resistance is too low to provide satisfactory surge protection, so the circuit may be damaged, the fuse blown, etc. The power consumption of the active circuit is lower than the inrush current limiter when the rated power is higher (usually higher than 300W).

NTC thermistor and active circuit combination

engineers can use a plan Technically eliminate the problem caused by the cooling / recovery time required for the inrush current limiter to return to the initial resistance. Essentially, this involves planning for inrush current protection to drop the inrush current limiter out of the circuit after performing its function. When the initial surges are now available, by removing them from the circuit, the thermistors have an opportunity to cool, so they are ready to take care of the subsequent surges after a power outage. The technical need to add a relay or triac control switch in parallel with the inrush current limiter, and the circuit required to control it. All components of the protection circuit will be connected in series with the input of the line.

When the thermistor absorbs the inrush current, the triac controls the switch or the relay closes. The simplest way to power these components is from the power supply itself. When the power is activated, it will turn off the relay or cause the triac to control the switch, which will Thermistor fell out of the circuit and allowed it to cool and restore its initial resistance, so it was ready to provide surge current protection.

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