I was wondering if anyone could help me out with this. I need to design a circuit that would somehow cut off a signal if there is a continuous +180V RMS hot short applied to it. The trick is that this signal cannot be altered and i can't use any components that would require power (i.e. opamps). Once the hot short is removed, the signal needs to function again, so you can't use fuses. I was thinking of somehow using FETs that would open if the gate voltage went above some level. Not sure how exactly to implement it thouhg.

Anyone have any idea?

Thanks a lot!

 

Oh sorry, I guess "hot short" is a slang term. What I meant was if a signal wire or pin is somehow shorted to 180VRMS power.

 

You need to tell us EXACTLY what you want, and why you want it, it sounds pretty obscure?.

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Yes, without a circuit drawing and details about the signal it's hard to recommend a protection method.

Lefty

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Measurement changes behavior

 

I think he's talking about protecting a DC ELV (Extra Low Voltage) circuit from being shorted to the mains.

This isn't as hard as it sounds.

On the ELV circuit:

• Use a crowbar consisting of a high current rated SCR across the +V and return terminals
• Connect a high current rectifer in reverse parallel with the +V and return rails.
• Bond the return connection to protective earth at the source.
• Ensure the power supple has adequate over current protection.

At the mains side:

• Ensure that it's protected with an approprietly sized circuit breaker.
• Install an RCD/GFI to cut off the power in the event of a leakage current in excess of 30mA.

Now, if the mains ever does get shorted to the DC bus then the mains supply should be cut off.

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He mentioned that the victim could be a signal wire.

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Ron (aka Rube)

 

I was thinking of using a IGBT (commonly used in tesla coil's as a HV spark gap) and some how power the input with the 180V.

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MechTronics

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Lol A Switch

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your question is quite obscure. maybe you want to say that the design should be such that the circuit would open if 180 volts is applied, otherwise it should work as it is?????????

 

Thanks a lot everyone for the suggestions!

Let me clarify my problem a little bit more... This is for a Fuel Quantity processor box for an airplane which interfaces directly with the fuel tanks. FAA has very strict requirements about protecting the circuitry going to the fuel tanks for obvious reasons. Along with the fuel sensor wires that go to the tank, there are about 30-40 more wires (including power) that are bundled together with with them. Thus, it is theoretically possible for a power wire to somehow make contact with one of the fuel tank wires, and therefore cause an explosion. This is why I need to disconnect this signal before this happens.

Like I said earlier, I have a requirement that each signal needs to be able to function again normally once this "hot short" is removed, which means I can't just use a fuse. The circuit needs to stay disconnected during overvoltage, and then reconnect again once overvoltage is gone. Also, I can't be loading down these signals too much.

I am trying to use depletion mode n-channel FETS, that will open if there is an overvoltage at the gate. Not very successful so far lol

 

Thanks Hero! I am going to try this.

 

Oh, you're talking about signal lines, I was thinking about power, implementing my safety measures are still a good idea but the overvoltage protection really needs to be designed in to the system.

Protecting signal lines agains 180V spikes isn't that difficult, take a PIC for example, just connect each IO pin to +V and 0V via a diode and connect a 1M resistor in series with the external device. The procedure is similar for op-amps, connect the pins to +V 0V and -V via diodes and make sure the input resistors are high enough to prevent high currents from flowing throught them.

If the overvoltage protection is an afterthought, then zeners, MOVs, SCR crowbars and gas discharge tubes are your friends but they're not fool proof and if they gat damaged you might not know they've blown, it's best to check them as often as you can.

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This is good advice, with the caveat that megohms times picofarads = microseconds, and megohms times microamps = volts.

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Ron (aka Rube)