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I have a power amp that is essentially functions like a benchtop supply, and you can connect banana plugs into it. I would like to protect it against ESD, but also protect the amplifier. People in our manufacturing floor are stupid and sometimes connect things like batteries to the supply up backwards, I really can't prevent that, and if they do they will blow the diode right out. Is there a good way to protect the diode also? These are the ways I could protect it that I don't necessarily like:

  1. I could fuse it but there would be no way to know if the fuse was blown and then you still have no diode and no protection and nobody on our floor would replace it.
  2. Resistor in series, I don't like this idea because it makes the diode less able to sink ESD and protect the input. 8kv (human body model) across a 1ohm resistor is 8000V
  3. Placing a resistor anywhere from the amp to the terminal will be bad, too much current.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • 2
    \$\begingroup\$ With the reasons given for not installing a fuse, could you instead install a resettable circuit breaker? This would both give an indication when tripped & be readily reset without any component replacement. \$\endgroup\$ – Robherc KV5ROB Jan 27 '16 at 23:30
  • \$\begingroup\$ Why do you want to shoot at your power supply, and with what caliber bullet? \$\endgroup\$ – Passerby Jan 28 '16 at 9:36
  • \$\begingroup\$ Fuses are difficult because you put them in a product and if they don't shut the product off nobody will ever know it existed except for me. Resetable fuses are nice and I've used them in the past, I put them in and the software engineers never implemented all of the functionality and didn't do any error detection with the overcurrent signals, so I don't know if I'll use them again. Electronic fuses would probably not work for ESD protection. If the inputs were protected, then they would get burned out. If the inputs weren't protected, then the internal mosfet would. \$\endgroup\$ – Voltage Spike Feb 4 '16 at 0:02
  • \$\begingroup\$ Passerby, I plan on shooting 15kv bullets at the inputs with my ESD gun (its even calibrated). \$\endgroup\$ – Voltage Spike Feb 4 '16 at 0:03
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If you use bidirectional TVS diodes instead of unidirectional TVS diodes and spec them above the possible battery that could be connected this would protect your output.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ [Recall that we are asked to to protect against people with lapses of judgement, not just natural phenomena like ESD.] A battery connected to the output can inject lots of current and ultimately defeat this protection. Let's assume that a battery is +4.2V, the output is set to +1V, the Vdd is +5V. The OpAmp will try to sink current, because it "wants" to bring the output voltage down to +1V. In the meantime, the TVS will not start conducting, because +4.2V it's below their threshold. Battery can source lots of current, the OpAmp will try to sink it all and toast in the process. \$\endgroup\$ – Nick Alexeev Feb 13 '16 at 23:13
  • \$\begingroup\$ @NickAlexeev The OP's concern was the TVS diode. Specifically that the diode would get forward voltaged by a battery and burn out. Because the OP did not ask how to protect the OpAmp from over current then we have to assume that he has already figure it out. \$\endgroup\$ – vini_i Feb 13 '16 at 23:25
  • \$\begingroup\$ I'm not going to assume. The O.P. is asking "Is there a good way to protect the diode also [from a battery to the output]?" Your protection scheme will be defeated when faced with a battery having voltage greater than TVS rating. TVS usually fail short (as opposed to open). \$\endgroup\$ – Nick Alexeev Feb 13 '16 at 23:32
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Fuse is the usual way to do it.
I could be 1-time metal fuse. It could be a resettable PTC fuse, which resets itself after some time. It could be a circuit breaker, which @Robherc commented about above.

Add a Zener with decent power rating in series with the TVS, and add a fuse in series with the output (downstream of the TVS and Zener). TVS is good at catching fast spikes such as ESD, while a Zener diode is better at handling prolonged overvoltage. If the output is getting abused, there will be large current flowing into or out of the output stage. This current would trip the fuse, which would disconnect the output stage.

output protection consisting of Zener diode, TVS, fuse

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