Goodbye 3AG fuse, we'll miss you

Fuses are an essential part of many system designs, and we’ve come to depend on them since the earliest days of electricity. The basic concept of using a fusible link – which self-heats due to current flow and then opens to cut off current flow if there is an overcurrent condition – is simple, reliable, clear cut, and unambiguous. Fuses protect subcircuits against localized faults in a device like a power regulator and implements system and user protection as mandated by regulatory standards.

Among the most widely-used fuse body sizes is the 3AG size, measuring 6.3×32 mm, which is available in standard ratings from 100 mA to 15 A, in fast-acting, slow-blow, precision, and time-delay versions. It usually has a clear glass enclosure (Figure 1).

The glass-body 3AG fuse is among the most widely-used in older consumer products as well as older and current chassis-based instrumentation and equipment. Source: Codrey Electronics
Figure 1. The glass-body 3AG fuse is among the most widely-used
in older consumer products as well as older and current
chassis-based instrumentation and equipment.
Source: Codrey Electronics

Shatter-proof ceramic ones are also available, but one of the handy features of the glass-body version is that you can tell at a glance if it has been blown (Figure 2).

Состояние предохранителя 3AG в стеклянном корпусе легко проверить.
Figure 2. It’s easy to check the status of the glass-body 3AG fuse.
Source: Codrey Electronics

Among the many attributes of the 3AG fuse is that it fits into a fuseholder socket and can be removed, inspected, and replaced without tools in a few seconds. Of course, that convenience can lead users to sometimes change it without first finding out why it need replacing: was it due to an eternal surge or because of an internal short circuit?

Этот держатель предохранителя привинчивается к монтажной поверхности, а провода присоединяются либо с помощью скользящих контактов, либо припаиваются.
Figure 3. This fuseholder is designed to be screwed to a mounting
surface, and the wires are either connected using slip-on
contacts or soldered.
Source: Keystone Electronics via Digi-Key

The 3AG fuse is supported by a large number of fuseholders, including PCB surface-mount units, discrete-wire models (Figure 3), as well as the very popular panel-mount version often used on the back of a chassis (Figure 4). There are even RFI-shielded holders with a captive cap for applications where you don’t want to lose the cap, which is part of the fuse circuit path (Figure 5).

The panel-mount fuseholder is widely used on chassis, most often on the back panel; this spectrograph project uses one fuse and holder for each of its three independent power rails. Source: University of California Observatories
Figure 4. The panel-mount fuseholder is widely used on chassis, most often on the back panel;
this spectrograph project uses one fuse and holder for each of its three independent
power rails. Source: University of California Observatories
 
For rugged applications, this metal 3AG holder has a captive cap, which also maintains chassis EMI/RFI shielding integrity. Source: eBay
Figure 5. For rugged applications, this metal 3AG holder has a captive cap, which also
maintains chassis EMI/RFI shielding integrity. Source: eBay

Of course, fuses come in many other form factors such as the blade-type widely used in automobiles, where they aren’t the only circuit-protection component in use. Among the other well-known circuit-protection devices are:

  • Metal oxide varistor (MOV)
  • Positive temperature coefficient (PTC) thermistor
  • Transient voltage suppression (TVS) diode
  • Gas discharge tube (GDT)
  • Polymer PTC resettable fuse

Like fuses, each of these has a well-defined and appropriate role in providing circuit protection, yet the basic circuit-breaking fusible-link retains its position in many designs due to its combination of consistency, direct action, and irreversibility. In fact, many designs use one or more of the above for highly-localized protection, plus a thermal-link fuse as a system-level cutoff if things really go the wrong way. In this sense, the classic fuse acts as a backup and offers extra insurance.

The classic fusible link design represented but not limited to the 3AG style is not physically compatible with many of today’s compact product units. These products don’t have the room for that fuse, and they don’t have a way for the user to get in there and change the fuse, nor would that be a good idea in many cases.

Now consider a rechargeable Li-ion battery pack with its requisite battery management system (BMS): if things really get out of control, having a fuse to terminate current flow is a desirable extra layer of protection. Furthermore, it’s a good idea to figure out what happened and why before you replace the fuse and re-initiate the current flow.

To meet the size needs and non-replaceable preferences while retaining the virtues of a “hard” fuse and circuit break, vendors are now offering surface-mount fuses as tiny, PCB-friendly components that provide the same level of current-cutoff circuitry as the classic fusible link of the 3AG. They do this not by further miniaturization of the traditional thermal element, but instead they use a variety of innovative structures and technologies.

For example, Bourns has the SinglFuse product portfolio, which comprises seven different fuse-construction technologies: thin-film sputtering, thin-film PCB, ceramic multilayer, ceramic cavity laminate, wire core, ceramic tube, and ceramic cube (Figure 6).

Создание предохранителей для поверхностного монтажа, способных работать в широком диапазоне токов и отвечающих требованиям регулирующих органов, требует использования различных базовых технологий.
Figure 6. It takes a variety of underlying technologies to create surface-mount, regulatory-approved
fuses that can handle currents across a wide range. Source: Bourns via Digi-Key

The package sizes of these thermal devices range from an almost invisible 0402 – 0.040×0.020 inch or 1.0×0.5 mm – at the lower current ranges to 3812 – 0.150×0.100 inch or 3.81×2.54 mm – at current ratings of 62 mA to 100 A. Despite their diminutive size, they are UL/CSA/IEC approved, and some models are also AEC-Q200 qualified for automotive use.

I’ll be honest: I’m going to miss the widespread use of the 3AG fuse. I know it’s not going away since it still is the best fit for many application scenarios. However, I will also have to be on the lookout for fuses that look like any other surface-mount device, and it won’t be easy to identify or replace them when there’s a problem.

Plus, there’s something satisfying at a visceral level about pulling a 3AG fuse from the holder to check it and replace it if needed. Maybe it’s a “caveman/fire/take action” thing? Nonetheless, times and technologies need change, even for humble devices such as fuses.

EDN

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