Signal line with and without surge protection. The surge energy is reduced by an order of magnitude with a small Gas Discharge Tube.

· · Web · 1 · 2 · 7

In comparison, the same signal data line, with a small Thyristor Surge Suppressor for protection. Its response time was a lot quicker than the Gas Discharge Tube, the large 200 V surge was clamped to just 100 V within microseconds.

But the speed the TSS got destroyed by excess power dissipation was just as fast... It immediated failed with an open circuit, and after that the full 100 V surge went into the load. Tubes are more robust than semiconductors, news at 11... (To be fair, it was only a small data-line TSS, large power-line TSS chips can often compete with GDT).

Modern fast-acting, low-current SMD fuses are remarkable. This is the same surge, same data line, but with a fuse installed. It interrupted the surge within just 100 microseconds! It was so fast that my oscilloscope's timebase setting didn't even capture that properly.

Of course, a fuse in itself is useless, if the load is high impedance, or if the surge is a 8/20 spike, it won't do anything. But it looks pretty promising for limiting the long-term energy dissipation inside protective devices for a constant overvoltage surge.

The problem of testing surge protection circuits is that the test is destructive. You need to remove and solder SMD parts onto the board non-stop to keep the test going... You only get one chance at a time, and if didn't set your oscilloscope's trigger properly before you fire the surge... :blobcatknife:

Overshoot and ringing artifacts when a fuse disconnected the protected circuit in a surge. Ironically it made the input surge to overshoot, boosted it from 200 to 250 volts, and arguably made the surge more destructive.

I think what was happening is that the pulse capacitor and the wiring's parasitic inductance formed an LC resonator. The instantaneous current was 10+ A because of protective clamping to ground, then the fuse suddenly interrupted it. This was enough to excite even the tiny wiring inductance.

The second experiment showed the Thyristor Surge Suppressor attempted to clamp 200 volts to ground, but eventually exploded due to excess power dissipation, and the remaining 100-volt surge to enter the load.

But when paired with a fuse, TSS's performance is just phenomenal. It clamped a 200-volt surge down to a tiny 26-volt, 500-nanosecond bump. Magic!

It's just magical to see a fuse to blow in 750 nanoseconds. But I guess it's what happens when you pass a current over 100x of the rating across it.

Tested again with a fuse and just a regular TVS diode chip instead of a telecom TSS chip. The result is quite similar, 200-volt surge is clamped to just 28 V within 500 nanoseconds.

Conclusions: TVS diodes can be effective even for a long duration surge, as long as you use a really fast fuse to limit the total energy before the TVS explodes and fails open. The TVS itself can still be destroyed and fail short, but both are replaced in a repair.

I suspect the thermal coefficient of the fuse's resistance also provided an unintentional form of series current limiting during a surge, which is great news.

High voltage power supply - "Who needs a bleeder resistor when you can use just a screwdriver to discharge it manually with a bang?" (famous last words).

Redid the "blown fuse" test with a proper timebase. 200-volt surge, 50-ohm load, fast-acting 250 mA, 1206 SMD thin-film fuse (Littelfuse 466). The fuse opened within 68 microseconds after the surge. This was my last fuse...

MFW I fired the surge generator, probably destroyed the chip-under-test within milliseconds, and then noticed my oscilloscope was in STOP mode... :blobfacepalm:

The difference between a Gas Discharge Tube and a semiconductor surge protector - if you see sparks from the tube, that means it just started working, but if you see sparks from the semiconductor, that means it just stopped working...

They should make a Gas Discharge Tube surge protector with a Keepalive electrode... This should either dramatically improve the surge response time of a GDT, or make the turn-on voltage even lower than 60 V.

Though, nobody in the real world wants such a Frankenstein device that keeps drawing meaningless power other than me...

> It is also not a good practice to hold a GDT in its glow region as this will significantly reduce the life expectancy of the device. In this condition, significant heat can be developed on the electrodes that can damage the special emission coatings and cause premature failure of the tube. - Bourns application note

Apparently I'm not the first one to come up with this thought, and this is actually a pretty bad idea... :blobcatgiggle:

Just repeated the same surge test on a 250 mA polyfuse. The fuse didn't do anything to the surge, as if there was no fuse. When repeated again with a gas tube connected in series, the fuse arced itself spectacularly, with a bright flash and a loud bang. ⚡💥

Conclusions: A real fuse, even as small as 1206 SMD, can interrupt a 200 V surge within 750 nanosecond, but a similar polyfuse is useless for surge protection - its slow response is only useful for steady-state overcurrent, and its max voltage and current ratings are just miserably low.

BTW, it was funny that researchers abused the original Littelfuse to make the earliest thermistor-type, DC-substitution RF & microwave power sensor in WW2. You basically bias the fuse with DC very close to its rated current, couple RF power into the fuse, and put it in a Wheatstone bridge. Any incoming RF power causes a decrease of DC power to keep the bridge in balance.

It was really fragile and could be burned out easily by excess power, even a transient (the sensor was literally just a fuse!) The response time and impedance matching was also bad, requiring adjusting an RF tuners to find the maximum point for each measurement.

I need to try replicating it someday with Littelfuse's modern 0402 SMD parts...

One problem responsible for the bad impedance matching in this makeshift "Littelfuse" sensor was parasitic inductance. Some World War 2 researchers went as far as removing the fuse glass enclosure, extracting its filament, and soldering it directly in a circuit to improve its RF performance - Fun (???) RF hacks you can learn in MIT Rad Lab books.

With modern SMD fuses, this problem no longer exists. I should be able to get better results. BTW, funny to see the company still exists and keeps making the best fuses after nearly a century... Which is why usually one combined the polyfuse with either a real fuse or at least a circuit breaker.

Some exceptions exist - e.g. Snap Circuits battery holders have only a polyfuse - but given a short of three AA batteries is usually not even dangerous (and the wires and parts usually also can take the abuse around 10A for a short time just fine) I guess its primary purpose is to prolong battery life when the kids invariably will short them, and that's what the polyfuse is good at.

In fact, the polyfuse is NOT rated low enough to prevent a transistor from exploding when its base and emitter are the only things in the circuit. Not surprising, because at such low rating the motors would not turn. But that's why they put the transistors in a surrounding plastic case. Addendum: so why would one even want a polyfuse and not just a fuse? The polyfuse is cheaper to "reset" and serves mainly to catch user error, not defects.

E.g. polyfuses are often used on USB sockets (or power lines upstream from there) that are frequently accidentally shorted by inserting the plug the wrong way and pushing. Given the max power output is bounded anyway, the somewhat slow response is fine. USB hubs also tend to use them to guard against users plugging in too much power draw. The usual pattern though is a surge current already limited to short time safe amounts.

OTOH there is a reason why your breaker panel doesn't just have polyfuses. Shorted mains can easily reach 1 kA, which must be interrupted quite quickly to be safe. So why not fuse plus polyfuse? Well, there is also some "educational" value in having to go down and flip the breaker back on or replacing a fuse to avoid going above the rated current in the future... I learned this one as a kid. Had a large box - 500V 2.2μF capacitor or something like that. Had lots of experience of charging a 1000μF capacitor with a 9V battery then shorting it for nice little sparks. So what does stupid me do?

Well: power strip, resistor, alligator clips and a diode, and the capacitor is nicely charged to about 380V.

Now what does one do with a charged capacitor? Well... I ended up shorting it on a large steel plate I had there. Made quite a bang. Parents immediately jumped in and asked if all was OK, but luckily I had the power strip and stuff already neatly disconnected and put away, so they never knew I was playing with mains. So yeah, "had a bit more power in that capacitor than I thought, but all is OK".

And this is how I learned what the 2 does in E = 1/2 C U².

Sign in to participate in the conversation

cybrespace: the social hub of the information superhighway jack in to the mastodon fediverse today and surf the dataflow through our cybrepunk, slightly glitchy web portal support us on patreon or liberapay!