Voltage Barrier

[Opticalworm]

Hello everyone,

I'm trying to design a voltage barrier using a quick blow fuse and a TVS Diode (See attachment). Does anyone know what sort of parameters should I be looking for on quick blow fuses and TVS diodes?

From What I gathered, Under normal condition the TVS diode is inactive when the supply voltage is less then the breakdown voltage of the diode (Vrb) and If the supply voltage goes above the diode's Vrb then it becomes active and starts the conduct, which in turns, short circuits the supply to ground. As long as the fused is selected so that it blows before diode reaches the peak current (Ipp), for a set time, then the Diode should be safe.

What you think?

Btw,the design is for an Intrinsically safe certification (class ia)

[Simon]

look up TVS diode on wikipedia, it has a nice explanation of all of the voltages that make up a TVS's spec (yes there are a few and I neb=ver remember the right names)

[tyblu]

...I neb=ver remember...

"b" and "=" are not right beside each other on the keyboard. wtf mate. Was that a "vb" finger mash, then a <backspace> miss, hitting "=" instead, followed by an instinctive <enter>, sealing the deal? I want a play-by-play.  

[Simon]

yea I think you figured it out, obviously I didn't 

[Opticalworm]

look up TVS diode on wikipedia, it has a nice explanation of all of the voltages that make up a TVS's spec

Thank you Simon, but I've already been wikipedia about TVS diode unfortunatly its don't really go into as much detail as I would have hoped.

Here a better source by microsemi about TVS diodes. http://www.microsemi.com/micnotes/125.pdf.

I don't think my original question was clear enough. Sorry. I was hoping that someone knows how to match a fuse with a TVS (or zener) diode in such way that the fuse will blow before the diode, which in turn, protects the connected circuit from over voltage.

[Simon]

do you actually need the TVS for speed or will a zenner do the job ? the thing is you need to find the maximum safe current for the TVS for enough time to blow the fuse, you need to find out how fast the proposed fuse blows

[Time]

Your TVS is going to be much faster than your fuse, even when using a fast break fuse.  You can expect ns or sub ns conduction times from a tvs.

[Simon]

well a TVS is used more to supress voltage spikes (won't last long enough to take the fuse out) I'd suggest a zenner if your just monitoring a steady voltage that may get out of hand

[Zero999]

Use a nice chunky SCR and a zener to make a good crowbar.

[Opticalworm]

well a TVS is used more to supress voltage spikes (won't last long enough to take the fuse out) I'd suggest a zenner if your just monitoring a steady voltage that may get out of han

It doesn't have to be a TVS diode or even changing the circuit to a Crowbar setup using a thyristor. I'm mostly having trouble rating the component to failure mode.

[FreeThinker]

I think you should take a look at the spec of your fuse in particular it's currant protection curve. This will tell you its 'Rupturing' capacity ie 2x capacity for 20 seconds or 100 x capacity for 10ms etc this will give you an idea of its fault handling and from that you can see how well your Tvs will perform (which one will blow up first  )

[Frant]

What you think?

Btw,the design is for an Intrinsically safe certification (class ia)

I think you should read the standard (IEC 60079-11 or a corresponding national standard applicable in your country) very thoroughly. You should also gather some additional literature from the Internet, with practical examples of IS implementation. The device you want to design is called "Zener barrier".

[Neilm]

One thing to remember is the input to Vin.  When the fuse ruptures there will by definition be current flowing. You don't say what order of magnitude the currents or voltages are, but if there is resonably long wire on Vin it will have some inductance. When the fuse ruptures the voltage on the Vin side of the fuse could rise quite high and (possilbly) flash over so take care when laying the circuit out.

Neil

[Zero999]

It depends on your source impedance. Your source likely has some current limiting so it just has to survive long enough under the max current your source can deliver.

I've done that before and the simplest thing to do is get some quick blow fuses test it; see what blows first.

I agree the maximum short circuit current can be a problem, especially if you're using a large lead acid battery. A low value resistor or even a choke can help to limit it to a safe level.

[williefleete]

for this kinda thing a zener wont work. the diode would blow before the fuse does
for things like this you will want a crowbar circuit which as the name implies is equivalent to putting a crowbar across the power rails
if its DC use an SCR that can handle the current across the power rails and gate it using a voltage comparater which will monitor the input voltage compared to a zener or other reference, when it goes over trigger the SCR and boom the fuse (and probably the SCR) will blow