This article uses a LOTO oscilloscope and a 5A current probe to test the protection curves of two common types of fuses. One is a blown type, and the other is a self-recovering type. We usually need to protect some circuits from excessive current, such as preventing users from short-circuiting the output source, or preventing users from injecting a large current into the circuit and burning it. In this case, we need to insert a fuse in series in the circuit that needs protection.
The principle of a fusible fuse is that when the current increases, the temperature of the fuse rises, and when the rated current is reached, the fuse burns out, thus cutting off the current path.
The principle of the resettable fuse (Polymeric Positive Temperature Coefficient , PPTC ) is that when the current increases to a certain level, the overcurrent causes its temperature to rise, and the resistance value increases sharply by several orders of magnitude, reducing the current in the circuit to below the safe value, thereby protecting the subsequent circuits. After the overcurrent disappears, it automatically returns to a low resistance value, eliminating the trouble of frequent replacement of current fuses.
We measured the current protection of a nominal 200mA/250V blown fuse. We used LOTO 's USB oscilloscope OSC482, with a current probe in series in the power supply loop. The internal resistance of this current probe is only 0.2 milliohms, and the fuse is connected in series. Then the current waveform in the current is monitored through the oscilloscope. In the normal state, we do not turn on the power supply, the current is 0 , and then suddenly turn on the switch, a 5A current is suddenly added to the loop, we observe the current waveform in the fuse path, and analyze the protection of the fuse.
We get the following current curve results:
It can be seen that when a 5A current is suddenly added to the circuit, the current suddenly rises from 0 to about 3.55A in 50ms , then drops to 1.25A in 260ms , and then completely blows out, and the current becomes 0. This does not correspond to the nominal 200mA at all, so when using this fuse, if your circuit cannot withstand a 3A current for several hundred milliseconds, it will still be damaged.
Next, let's take a look at the actual measurement of a 350mA resettable fuse.
I purchased the PPTC model NANOSMDC035F-2 .
Hold Current IH : Maximum current that will not cause a sudden change in resistance. Trip Current IT :
Minimum current that can cause a sudden increase in resistance, generally twice the hold current. Rated Voltage
VMAX : Maximum voltage that can be sustained at rated current without damaging the PPTC device itself.
Maximum Current IMAX : Maximum current that can be sustained at rated voltage without damaging the PPTC device itself. Typical power
Pd : Power consumed in the operating state.
Max Time to Trip Ttrip : Maximum operating time under specified current. Resistance Tolerance RMIN , RMAX : Minimum Rmin and maximum Rmax values of static resistance without power . The practical value of the device is within this range, that is , Rmin ≤ R ≤ Rmax .
For convenience, I soldered two pins of the PPTC chip . Under the same circuit environment, we get the following current curve results:
It can be seen that when a current of 5A is suddenly added to the circuit, the current suddenly rises from 0 to about 1.5A in 24ms , and then drops to about 0A in 500ms . This does not correspond to the nominal 350mA at all, so when using this fuse, if your circuit cannot withstand a 1.5A current for tens of milliseconds, it will still be damaged.
Disconnect the circuit and check the resistance of the PPTC , it is 1.7 ohms. Although it is not the maximum 1.3 ohms written in the data , it is still considered self-recovery.
Combining the results of the two tests, under the same nominal value, the protection performance of the resettable fuse is better than that of the fusible fuse, and it is more convenient.
Actual test video link:
https://www.bilibili.com/video/BV1UA411v7wC
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