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The circuit diagram of a board I'm working on has parts labelled as "sacrificial components". These components seem to be pairs of probe points connected via a capacitor, and connected to nothing else.

What are these "sacrificial components"? What are their purpose?

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    \$\begingroup\$ They are offerings to the circuit gods in exchange for keeping the magic smoke in the rest of the components. Usually one virgin transistor will suffice... \$\endgroup\$ – W5VO Jul 7 '12 at 1:19
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To elaborate on W5VO's comment about offering to the gods. +1 by the way.

Sacrificial for protection

In my experience sacrificial component implies that the part will take some kind of damage and get destroyed in order to prevent some more precious part of the circuit from taking damage. Usually, a sacrificial part is designed so that it's easy to replace. One example, would be a common AGU fuse.

Another example. A certain instrument needs to measure an input with an expensive A/D converter. The input arrives via connector, which is exposed to the outside world. Harm can come through the connector (ESD, overvoltage, reverse polarity). A sacrificial OpAmp buffer in a socketed DIP package can be added between the connector and A/D.

http://en.wikipedia.org/wiki/Sacrificial_device

On the other hand, that all doesn't make a lot of sense in the context of O.P., in which sacrificial parts are not connected to anything. How would harm come to them? A snippet of your schematic and even a portion of the PCB layout would help understand your context better.

Sacrificial for fabrication

During fabrication* sacrificial mean that something is destroyed in the process of making the product without becoming a part of the product. Sacrificial material is a part of the fabrication process. Simple example: when you want to drill a hole, you might put a piece of wood on the other side of your part, so that the drill bit doesn't over-penetrate into something important.

* of anything, not just electronics.

May be, this is your case. May be, test points are used for some mechanical purpose. EDA package demands that they have to be connected to something (anything), so they are connected to the dummy capacitor.

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They're parts which have a weakness that will cause them to go first in case of something goes wrong in the circuit.

The fuse is the archetype of a sacrificial component. The most common use of a fuse is at the power supply stage of a device. Connecting the wrong voltage, like 230 V AC instead of 115 V, will blow the fuse, and prevent (much) damage further on. The fuse may also protect against further damage if the circuit goes haywire. When I worked with Philips Audio we used TE5 fuses inside circuits for protection.

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There are sockets for TE5 fuses, but we usually soldered them, to prevent DIYers to replace them. It also helped that (until now :-)) TE5s are not directly recognizable as being fuses. When a fuse inside a circuit blows that's often because another component failed. Just replacing the fuse may cause other components to go before the new fuse blows. And so on. That's why just replacing the fuse is not always a good idea.

A fusible resistor differs from the fuse in that it also has the function of a passive component.

"In the conventional wirewound resistor, ceramic rod at the resistor's core acts as a heat sink for the wire element. This can delay fusing, resulting in high temperatures to fragment the coating and ionize the air near the fusing spot. If ionization occurs close to the cap edge and at a voltage peak in the mains cycle, it can initiate a momentary flashover outside the component body, releasing far more energy than is required to fuse the wire element. Although the opening of the circuit is safe for most applications, it can be with a “bang” with splattering of cement coating. This is un-safe operation and not desirable."

(from this datasheet)

The difference between a sacrificial component and a normal one is that the sacrificials are designed to fail in a highly predictable way. A fuse is not just a too thin wire, it's a more sophisticated component than it looks.

I can't think of other sacrificial components right now. I'm not sure I would call Nick's opamp buffer one. Like I said sacrificial components should be designed to fail predictably, and an opamp isn't. Nobody can guarantee it will stop an input overvoltage from proliferating through the circuit.

MOVs and other overvoltage protectors aren't sacrificial either because they're not designed to fail when an overvoltage occurs. On the contrary, they will happily (well...) take current peaks of thousands of amperes without failing. They're protection devices, but not sacrificial.

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  • \$\begingroup\$ Couple examples: The small resistor in an RC-snubber is a sacrificial component in my opinion. If the capacitor short circuits at the end of its lifetime, the resistor burns instead of other components blowing up. With a bit of luck the circuit will even continue to work. Also I read about a zener diode on the gate of a MOSFET to protect the microcontroller output when the FET's fails. \$\endgroup\$ – jippie Jul 7 '12 at 8:03
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    \$\begingroup\$ @jippie: True for the Z-Diode, but in an RC-snubber, you want the dampening effect of the resistor on some inductive-capacitive (LC) circuit prone to ringing, and the capacitor is there to prevent DC flowing through the resistor, causing huge power losses. \$\endgroup\$ – zebonaut Jul 7 '12 at 8:22
  • \$\begingroup\$ True about the damping effect, I think the resistor has both functions. \$\endgroup\$ – jippie Jul 7 '12 at 8:26
  • \$\begingroup\$ @jippie - if the resistor also serves to protect it should be a fusible. See the quotation I added in my answer. Note that it has to fail predictably. \$\endgroup\$ – stevenvh Jul 7 '12 at 8:33

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