What problems should be paid attention to when selecting PPTC self-recovery fuse? Explain as follows.
I. normal working voltage of PPTC self-recovery fuse.
Analysis: First of all, we need to know where the PPTC self-recovery fuse is used in the product (such as input, output, signal terminal, control terminal, etc.), and determine the voltage range that the PPTC self-recovery fuse bears after being protected at this position (in the circuit where the PPTC self-recovery fuse is connected in series, when the current of the circuit is abnormal, the PPTC self-recovery fuse is within a certain time range). When jumping from low resistance to high resistance, preventing the flow of abnormally large current and protecting the subsequent circuit from being damaged by large current, almost all the voltage of the circuit is attached to the PPTC self-recovery fuse. (This is the basic condition of choice.
Second, the normal working current of the product.
Analysis: In order to fully understand the normal working current of PPTC self-recovery fuse, if you don't know enough about it, it will lead to wrong selection, so you should pay attention to two points.
(1) pay special attention to, in the early stage of understanding the normal working current of products, it is often not particularly accurate. Because electronic engineers often naturally and habitually expand parameters when choosing equipment models. For example, the actual measured value is 0.42A, but it is habitually set at 0.5A, which may be expanded to 0.8A for the sake of safety. This is the usual habit of electronic engineers when choosing PPTC self-recovery fuse.
(2) For some products, when the current is not constant, the maximum current of the product is often the normal working current, so it is necessary to know the following points at this time.
A, the current of the product working for a long time.
B. Duration of maximum current.
C. repetition frequency of maximum current (time interval).
When the maximum peak current of the product is not large (less than 2 times), the duration is not long (1~2 seconds), and the repetition time interval is long (more than 20 seconds), the normal current of PTC thermistor can be elected as the normal working current value.
The maximum peak current of the product should be elected as the normal working current value because the maximum current of the product lasts for a long time (over 30 seconds).
The maximum current duration of the product is not very long, but the repetition time interval is relatively short (less than 5 seconds), so the maximum current value less than the product should be elected as the normal working current.
The correctness of current selection is related to whether the fuse protection effect of collector is the best.
Third, understand the environmental temperature range of products.
Analysis: To know the working temperature range of PPTC recovery fuse, whether it works in a high temperature environment, or whether there is a heating source (such as power equipment, radiator, etc.) beside PPTC self-recovery fuse, check the ambient temperature and current discount rate table of PPTC self-recovery fuse according to the environmental temperature used by the product, and calculate according to the following formula:
Ih= normal operating current/discount rate of ambient temperature and current.
Through the calculation results, check the product manual and find the closest model.
IV. Action time (time required for protection).
Some products will put forward some requirements for running time, and the resistance range of fuses should be determined through experiments according to the requirements of products. Before that, we should know the fault current of the product (that is, the current value that the product needs to protect). In the above determined models, the PPTC self-recovery fuse with the smallest resistance value is elected for experiment, and the inimum resistance value of PPTC self-recovery fuse is determined.
