The following article is compliments of MOTOR magazine
Eye on Electronics
Fuse systems used to be a simple topic. When an electrical accessory didn't work, you checked the fuse panel, pulled a red plastic fuse out and put a new one in its place. If the fuse blew again, you tried to find out what was causing it to blow.
But as the electrical complexity of the automobile has grown, so, too, has the sophistication of the fuses system. According to a couple of recent SAE papers written by the engineers at Littlefuse, there's a lot more to know today regarding fuses.
First things first. A fuse is a thermal device. In other words, as current travels through the metal element of the fuse, it heats up. If the amount of current is too high, the temperature will rise to the melting point of the fuse metal and the circuit will become open.
The line that "fuses are successful when they fail" has been around a long time. It's true, too, because a defective fuse is one that doesn't open up when an excess-current situation is experienced. Fuses are designed to protect wire harnesses from being destroyed by the heat caused when excess current flows due to a fully or partially shorted load.
There are many devices in an automotive electrical system that do not draw a steady, consistent level of current during normal operation. And here is where things start to get more involved. One of the characteristics of both solenoids and motors, for instance, is that they have a high inrush of current. When first turned on, they look like a short circuit to the electrical system. This current may be two or three times more than the normal operating current of the circuit.
This brings the element of time into the equation. The degree of heat to which the fuse element is exposed depends on both the amount of current and the time it's present. Fuses can be designed to withstand that high inrush current, and at the same time react to lower levels that are present for longer periods of time.
Normally, the average automotive fuse sees hundreds or even thousands of inrush current cycles during the life of the vehicle. If you think about it, this is a lot like the thermal cycle tests they do on electrical assemblies, where pulses of heat-causing expansion are followed by cooling periods during which the metal contracts.
As a fuse thermal cycles due to these current pulses, a certain amount of metal fatigue occurs. Because fatigue alters the metallurgy of the fuse material, the amount of heat for a given current surge increases. The number one cause of nuisance fuse failures is attributed to this "wear-out factor".
I don't know about you, but the idea that a fuse could be weakened over time by normal current pulses was news to me. I've always wondered why some of the fuses I've replaced seem to blow for no apparent reason. It's nice to know that at least some of those cars I've been working on didn't have a random, intermittent short waiting to turn into a comeback.
Traditionally, fuses have been located as close as possible to the source of power. This is where they're most effective in protecting all the wiring in a given circuit regardless of where a fault actually occurs. This requirement is what has lead to the underdash fuse block as the main power distribution center for all the accessories in a car.
There are some disadvantages to this arrangement. One is that the fuse and the wiring to the various accessories have to be large enough to carry the load of those accessories. If each device had its own fuse, it would be necessary to run only enough wire to accommodate its own current needs. With shrinking car sizes and the need for lighter-weight vehicles, however, there's a push to reduce the size and the weight of the wire harnesses. One way to do this is to move the fuse out near the assemblies being protected.
There are other ways to protect the wiring system and the components attached to it besides fuses. The problem with the standard blade fuse is that, while it is cheap, it's also a one-time device that has to be replaced every time it blows. For a wiper system that's jammed by snow and ice, it would be nice to have a circuit breaker that could either be reset or that would automatically reset itself.
Circuit breakers use a piece of bimetal as a part of the circuit. When the current passing through the bimetal intensifies, the bimetal flexes, opening the contacts of the circuit breaker and tripping the circuit electrically.
There are three different types of circuit breakers currently in use - cycling breakers, remote-reset breakers and manual-reset breakers. The cycling breaker depends on the cooling of the bimetal to automatically close the circuit. If the overcurrent situation still exists, the bimetal reheats and the circuit opens up again.
The cycling breaker is what you typically find in the headlight system of a vehicle. The opening and closing rate is carefully chosen to protect the wiring, while at the same time allowing enough "on" time for the driver to get the car off the road should the failure occur at night. Power seats are another device often protected by this type of breaker.
Remote-reset breakers do not reset until power is removed from the circuit. These units are built by wrapping resistive wire around the bimetal and connecting it to either side of the contact points. Should the circuit open up, enough current will flow through the wire to keep the bimetal warm, preventing the resetting of the circuit. Switching off the power or removing the battery cables allows the bimetal to cool by depriving the heater wire of its needed current.
Manual-reset breakers are like the ones you're used to seeing in electrical panels. When they kick out, you have to manually move a reset lever or push a reset button. Internally, the manual-reset breaker works by positioning a piece of sprint-loaded insulating material between the contacts when they separate due to the over-heating of the bimetal. When you reset the breaker, you move the insulation away from the contacts.
There are newer types of fuses showing up in automotive applications, as well. One of these is called a PTC element. It works similar to a coolant temperature sensor, except that it has a positive temperature co-efficient, meaning its resistance increases the hotter it gets.
There are two variations of PTCs, depending on how they're made. The ceramic PTC uses a disc of semiconductive ceramic. It can be places in a circuit in such a way that current flows through it. If the current gets too high, the resistance resets and the current level falls off. For a ceramic PTC to work properly, the current must be reduced to zero for the device to return to its low-resistance state. This puts it into the same category as the remote-reset breaker. The main disadvantage of the ceramic PTC is that its resistance never quite reaches zero, even when it's cool.
Polymeric PTCs are made of a high-density polyethylene plastic loaded with a conductive material such as carbon black. When the device overheats due to excess current, the plastic holds the carbon black together less tightly, allowing fewer electrical paths through the carbon. When everything cools down, most of the paths will reestablish, allowing a return to pretty much the same resistance as before.
Typically, PTC devices have been used to protect such devices as the power window motor and lock motor or solenoid. These devices can be jammed by material such as ice and snow.
An item you might be seeing more of, if not for the fact that they're so expensive, is the electronic fuse. These devices typically contain a MOSFET-type transistor to handle the current to the load. Internal to the MOSFET is a temperature-sensing diode. If the diode detects that the transistor is overheating, it can cause the transistor to shut down until the temperature cools off. Typically, the smart power fuse will "reconnect" when the temperature falls about 25°C.
According to the guys from Littlefuse, one of the things you will be seeing in the future is more fuses located in more areas. This may mean that the main fuse panel will be divided up and smaller panels put in such places as under the hood, inside the doors or under the instrument panel. Self-fusing assemblies that detect when they're drawing more current than allowed and shut themselves off are here now and likely to become more widespread as their cost falls.
From a service standpoint, the thing to be aware of is that there are more fuses to be concerned with now than just the ones in the fuse panel. Some circuit breakers need to be manually reset; others need to have power removed from them to be reset. Like never before, having a good knowledge of the particular system on which you're working will be the difference between making money and making enemies.