The pptc self-recovery fuse has become more and more popular, so what problems should be paid attention to when selecting the pptc self-recovery fuse?

1. Normal working voltage of pptc self-recovery fuse

  First of all, we need to understand where the PPTC self-recovery fuse is used in the product (for example: input, output, signal terminal, control terminal, etc.), and determine the voltage range that PPTC will withstand after protection at this location (PPTC series connected When the current of the circuit is abnormal, PPTC will change from low resistance to high resistance within a certain time range, thus preventing the flow of abnormally large current and protecting subsequent circuits from damage by large current. Almost all the voltage is applied to PPTC.) This is the basic condition for selection.

2. The normal working current of the product

  It is necessary to fully understand the normal operating current of pptc self-recovery fuses. If this item is not fully understood, it will cause an error in type selection.

  1) Pay special attention to the fact that in the early stages of understanding the normal operating current of the product, it is often not particularly accurate. When an electronic engineer chooses the model of the device, he often performs some natural and habitual amplification of the parameters, for example: the actual measurement value is 0.42A, but the habit will be set to 0.5A, and may even be considered For the sake of insurance, some amplifications are made and it is set to 0.8A. This is the usual habit of electronic engineers when selecting self-recovery fuses.

  2) For some products, when the current is not constant, the maximum current of the product is often taken as the normal working current, so what you need to know at this time are:

  A. The current that the product works for a long time (usually);

  B. Duration of maximum current;

  C. The repetition frequency (time interval) of the maximum current.

  For the maximum peak current of the product is not very large (less than 2 times), the duration is not very long (1 ~ 2 seconds), when the repetition interval is relatively long (more than 20 seconds), you can choose the PTC thermistor usually The current is taken as the normal working current value.

  When the maximum current duration of the product is relatively long (greater than 30 seconds), the maximum peak current of the product should be selected as the normal operating current value.

  For the maximum current duration of the product is not very long, but the repetition interval is relatively short (less than 5 seconds), we should choose the maximum current value that is less than and close to the product as the normal operating current.

  The correctness of the current selection is related to whether the current collector fuse protection can achieve the best effect.

 

3. Understand the ambient temperature range used by the product

  To understand the operating temperature range of PPTC resettable fuse, whether it is working in a high temperature environment, or whether there is a heat source (such as power devices, heat sinks, etc.) next to PPTC, according to the ambient temperature of the product, check the PPTC ambient temperature The reduction rate table with current is calculated according to the following formula:

  Ih = normal operating current / reduction rate of ambient temperature and current

  Through the calculation results, consult the product manual to find the closest model.

 

5. Action time (time required for protection)

  Some products will put forward some requirements on the operation time. According to the requirements of the products, the resistance range of the self-recovery fuse should be determined through experiments. Before this, you should know the fault current of the product (that is, the current value that the product needs to protect). Among the above-identified models, select the PPTC resettable fuse with the smallest resistance value for experiment to determine the minimum resistance value of the PPTC resettable fuse