Introduction
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You are the weakest link – goodbye!
A fuse is a designed weak link in a circuit. They are protection devices. The term fuse comes from the Latin fusus which means to melt. This gives us some idea of how they work. A fuse protects against high current. They are designed to open the circuit when the current flow exceeds a certain level. And they do this most often by melting.
You want it to break?
It may seem counterintuitive to place a device in your product that is designed to fail. In the end, it is all about safety. We must consider what would happen to our device if the fuse was not present. If the current draw exceeds design parameters, the device is no longer functioning as intended. In that situation is could become damaged, damage other equipment or it could harm people. As a designer, we don’t want to leave that outcome to the fates.
In a way, the fuse does multiple things. It alerts the user to that fact that there is a problem. It also removes the device from operation and forces the user to identify the source of the problem and fix it or to abandon the use of the product.
While a blown fuse does represent a failure, it also localizes the damage to the fuse. A properly designed system will be unharmed by a blown fuse. Once the fault is cleared and the fuse replaced, the device should return to service without becoming grotty.
Not as simple as it seems
A fuse might appear very simple but it has many requirements to function properly. A fuse cannot corrode overtime. It cannot become inoperable because of oils, dirt or other contaminants. It cannot produce gas, flames or arcs. Or rather it must contain those things from the environment.
As the current in a fuse increases, the temperature of the fuse rises. As the temperature rises, the resistance of the fuse increases which adds to the temperature of the fuse. As the fuse crosses the rated current the relationship between fuse temperature and fuse current becomes very non-linear and the fuse soon melts. When the fuse breaks, the current does not immediately stop, but an arc will bridge the now broken fuse. The length of the arc will grow until the voltage required to sustain the arc is higher than the available voltage and the arc will collapse.
A fuse may be surrounded by air or it by other materials that are intended to speed the quenching of the arc. Silica sand or other non-conductive liquids may be used. And all the while the arc should not be allowed to interact with the environment because doing so could cause an explosion if flammable gases are present.
Resettable fuses or PTC’s
A resettable fuse, also known as a PTC for positive temperature coefficient, is a device that will become active, or trip, at a certain current and then limit the current to downstream circuits. The PTC is a non-linear resistance. The change in resistance is very sharp and can be considered as a bi-stable device. That is, it has two resistances a low resistance for normal operation and a high resistance for tripped operation. The amount of current that flow after a PTC is tripped is sufficient to keep the device in the tripped state. This prevents oscillations between the two states. Ideally the device needs to be powered down and allowed to cool before it is returned to operation, at which time it will return to the low resistance state.
Use in low voltage situations
The voltage drop across a fuse is usually only a few volts. This is not a problem in high voltage operations but in hand held devices, the voltage drop can affect your circuit operation. Placing a fuse on a 5V USB line in a device that could have a fault sounds like a good idea, but selecting the fuse trip point too close to the expected operating current can cause a voltage drop of a volt or more. This is not a problem if the line is feeding a 3.3V regulator, but if you are expecting 5volts to feed a boost converter, then a drop to 4 volts can be the difference between working and not working.
Fuse manufacturers suggest that fuses be operated at no more than 75% of rated value at room temperature. In elevated temperatures, the operating condition should be derated, see the data sheet for specifications.
In any event plan on a voltage drop across a fuse and measure it early so your circuit will operate as intended and have the desired protection offered by a fuse.
Final thoughts
This newsletter is sponsored by Celtic Engineering Solutions LLC, a design engineering firm based out of West Jordan, Utah, which can be found on the web at: www.celticengineeringsolutions.com. You can find the newsletter on the company blog, LinkedIn or by subscribing. Send your emails to The Celtic Engineer at: [email protected].
