How A Scale Is Protected With and Without a Fuse

//How A Scale Is Protected With and Without a Fuse

How A Scale Is Protected With and Without a Fuse

Question

An inexpensive scale I bought doesn’t have an external fuse. Should I be concerned?

Answer

As we all know, using electricity carries inherent dangers, so it is important that the electronic equipment we supply is designed to minimize potential hazards. Recent news items about the safety of some of China’s exports illustrate how important this is, and how some unsafe, inexpensive, foreign manufactured products can easily find their way into domestic markets.

You can never assume a product you’re not familiar with is safe; always check a new piece of equipment for proper protective features.  For the end user, unprotected equipment can injure people or destroy property if it develops a fault condition. For the manufacturer and distributor, this can mean liability problems as well as damage to reputation.

The most familiar electrical feature used to protect people and property, the equipment itself, and the power line, is to build a replaceable fuse into the equipment. We all know what a fuse holder looks like, and its presence tells us the device has protection.

With a replaceable fuse, an over current condition like a short, heats the metal link inside which melts. This breaks the flow of electricity; disaster is averted.

But the marketplace always demands more features and benefits for less money. This drives manufacturers to look for ways to lower the cost of what they make and sell, so that black knurled knob on the back of equipment may be the first thing to go. Luckily there are other ways besides a replaceable one-shot fuse to protect a piece of equipment.

One of the interesting aspects of those little black external AC adapters that come with a lot of equipment is that their internal transformer is wound with a smaller diameter wire than a similar transformer built into a piece of equipment. This gives it two advantages.

First, as more current is pulled through the winding, the thinner wire heats up more quickly than thicker wire. As the wire heats up, its resistance also goes up, which limits the flow of current.  So the adapter has a certain amount of inherent current limiting capability.

Second, any wire will melt and open up if enough current is pulled through it. The thin wire of a wall wart is no exception, and it will melt and break, or open, when the current going through it gets above a certain value. We’ve all experienced an adapter that has no output, and what most likely happened is too much juice was pulled through it. The adapter has now become a fuse; a used fuse, at that.

Thin wire in AC adapters is a deliberate design feature to provide current limiting and fusing protections. Personally, I would not rely on these as the sole protection – sometimes a device designed to be “the first thing to go” turns out to be more robust than the sacrificial lamb it is intended to be.

It is also important to realize that a device with an internal battery carries the same hazards and should be properly protected. There is a lot of energy stored inside a battery, and if released all at once (like with a short circuit), a lot of damage can occur.

For equipment with an internal power supply, there are also discrete safety components that can be designed into circuits, the re-settable fuse being a popular one. It is soldered on to the scale’s main circuit board, the same as resistors, capacitors and other components. When the current being pulled through it exceeds a certain point, it opens like a fuse, but will reset itself after the over current condition is cleared.

This component is particularly handy for the end user, as its ability to reset itself when the fault condition is cleared removes the need to keep a supply of various values of replacement fuses handy. Unfortunately these aren’t visible on the outside of the scale, so you have to ask about them, examine the circuit board, or check its schematic to see if one is there.

There are other board level components that can be used, as well.

Unfortunately there are a few foreign manufacturers who just leave all the protection devices out; this is dangerous for everyone.  Very rarely are imports subjected to electrical safety inspections. Make it your duty to find out if the scale has any safety devices built in if none are visible. Uncle Sam is not necessarily going to do it for you. The seller, distributor or manufacturer should be able to tell you what protections are built into the scale. Robust safety protections should give the scale a long, useful, uneventful life; a lack of safety protections should give anyone cause for concern. If you’re not sure, ask!

By | 2017-05-09T11:21:25+00:00 May 9th, 2017|Uncategorized|Comments Off on How A Scale Is Protected With and Without a Fuse

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