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Certain electrical designs require permanently disabling hardware functionality on the fly. Sending an overcurrent to a weak fuse may be a method for accomplishing this, breaking the circuit in a particular region of a device. My questions are:

  1. What are the potential risks caused by this process, assuming a fairly weak fuse?
  2. Are there alternatives to this method for disabling functionality permanently?
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  • \$\begingroup\$ How permanent is permanent? If it's a discrete fuse, it can be replaced... \$\endgroup\$ – HikeOnPast Aug 16 '12 at 19:30
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    \$\begingroup\$ Watch out. This method for disabling a disposable device by deliberately blowing a fuse has been patented some time in 2000s (not too long ago). Sorry, I can't find a pat# . In the patent, the method was geared towards preventing reuse of disposable medical applicators. \$\endgroup\$ – Nick Alexeev Aug 16 '12 at 20:05
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    \$\begingroup\$ I'm curious: If one is able to send an overcurrent (to blow a fuse) on the fly, then can't one also instead turn off a load switch in the same place, or even switch a transistor OFF ? I mean, are there specific advantages to this proposed fuse method ? \$\endgroup\$ – boardbite Aug 25 '12 at 2:31
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Assuming that by "permanent" you mean "without physical repair", what you describe is reminiscent of a crowbar circuit: short the bus, blow the fuse. I'm not aware of any safer way of doing what I understand you to want. An SCR is probably a good choice.

Risks include detonation of the device being used as the crowbar, if it absorbs too much energy from the short before the fuse opens. To avoid that, check the \$I^{2}T\$ rating of the fuse and of the device. If the \$I^{2}T\$ rating of the device is higher, it should not detonate before the fuse fails. You may need to use high-speed fuses, which may be difficult to find, depending on the voltages and currents involved.

Another risk is accidentally triggering the crowbar early. How you deal with that depends on the context, and how irritating a false trigger is compared to a missed trigger.

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We don't know much about your application, but assuming that it has some level of intelligence, writing flag bits to EEPROM is a proven way to inhibit functionality based on certain criteria. As long as you're not working on a game console or other application where the effort to reverse engineer the protection scheme has a financial payoff, it may be "permanent enough".

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  • \$\begingroup\$ Nice approach but unfortunately, this process must be as irreversible as possible \$\endgroup\$ – Daniel Li Aug 29 '12 at 15:29
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Assuming that the device uses EEPROM/Flash, One way to disable to device is to program a key in EEPROM initially and checking for the key every time. Once the security condition is triggered, erase the EEPROM and/or the program flash.

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