What does this symbol represent?

[redgear]

Which component does the circled symbol represent?

[james_s]

That's a neon lamp or gas discharge surge arrestor.

[redgear]

That's a neon lamp or gas discharge surge arrestor.

Thanks! But how do I confirm which one is that?

[james_s]

By looking at the device, or examining the BOM, or by understanding what it is you are trying to build and selecting the appropriate component for the job. It is being used as a surge suppressor, whether it's a purpose built device or a neon lamp used as one I can't say just from the schematic.

[Psi]

Looks like it's going to a battery pack.

If it was a Gas Discharge Surge arrestor it would explode if it ever arcs over and shorts the battery.

So it's probably a neon.

Or it may just be a symbol they picked to show the battery connector??

[AVGresponding]

By taking it apart and looking at it...

99.9% it's a neon indicator lamp acting as an over voltage indicator, looking at the rest of the circuit. The zeners are the limiters, and neons tend to not start conducting until past 60 or 70 volts, then conduct down to 30 or so, so it's only going to strike if the output exceeds the stated voltage limit.

[james_s]

If it's a neon indicator it's gonna need a series resistor, otherwise it will act just like a gas discharge surge arrestor.

[redgear]

Yep, it is a battery pack.

I don't think it is a neon lamp. Maybe they picked some symbol to represent the battery connectors like Psi told

[redgear]

Just checked it from the gerbers, it is a spark gap.
Do I need one?

[T3sl4co1l]

Just checked it from the gerbers, it is a spark gap.
Do I need one?

For what?  Lightning protection?

I don't see how a spark gap could ever activate when there's two diodes right nearby.  Diodes that are themselves redundant -- did the designer not know that TVSs are regular diodes in the forward direction?

Tim

[ArthurDent]

It would make sense that the symbol is a 2-contact connector or 2 terminals for measuring the voltage at this point where the output is labeled to be 0-60 volts.

[james_s]

A bit more context would help. Where did the schematic come from? Is it a professionally designed product or some hobby circuit found online? It's possible someone used whatever random symbol they found or didn't really know what they were doing.

[schmitt trigger]

Thanks Arthur for the table.
While I knew most of the component designators from heart, it is good to know that there is an IPC standard for them.

[ArthurDent]

Here is some additional information from a related thread started by redgear. 

https://www.eevblog.com/forum/beginners/how-do-i-size-these-components/msg2708946/#msg2708946

[schmitt trigger]

Hmmmm
After reviewing the list, what would be a good letter designation for a jumper?
I use JP. Although from the list it would be a W.

[redgear]

A bit more context would help. Where did the schematic come from? Is it a professionally designed product or some hobby circuit found online? It's possible someone used whatever random symbol they found or didn't really know what they were doing.

This schematic is a Reference Design from Ti for their BQ76930 fuel gauge. Guess, a professional should have designed it

[redgear]

I don't see how a spark gap could ever activate when there's two diodes right nearby.  Diodes that are themselves redundant -- did the designer not know that TVSs are regular diodes in the forward direction?

Tim

D6 is a flyback diode, its purpose would be to clamp PACK+ near PACK-.  It would be a pulse current when the discharge FET opens under load, have a fast response, and stand off the normal operation voltage of the battery plus any transient.

[james_s]

A bit more context would help. Where did the schematic come from? Is it a professionally designed product or some hobby circuit found online? It's possible someone used whatever random symbol they found or didn't really know what they were doing.

This schematic is a Reference Design from Ti for their BQ76930 fuel gauge. Guess, a professional should have designed it

That's why I asked, unless I missed something it was never mentioned what this was or who designed it, we are not psychic.

My best guess is that it's a spark gap or gas discharge device intended to protect against ESD.

[iMo]

http://www.ti.com/lit/ds/sluscd0b/sluscd0b.pdf

11.1.4 - esd pcb spark gaps
Fig. 7 and 8

[T3sl4co1l]

Wow, that's dumb.

Well, par for the course with app notes.

FYI, spark gaps don't fire until huge voltages, among other terribly messy things.  But a TVS (even a small one like a regular zener diode*) handles ESD just fine, so there's no point in putting a spark gap AND zener in there.  The spark gap will never fire.  Just stick with the one that's better anyway.

*They're not usually rated for it, so it's hard to say how much they will actually handle (8, 15, 30kV?).  The modern equivalents are small TVS singles or arrays, available in impressively small packages and with low capacitance**, specified for ESD and surge.  Some are CSPs (chip scale packages) smaller than 1N47xx dies, so it can't be too bad.  There may be optimizations in TVS dies, their actual construction, that improve survival at high power levels, that are lacking in zeners, I'm not sure.

**If they're a few pF, they're probably a diode in series with a zener, which is messier (the zener has to be charged up to the signal's peak voltage, which won't help with, say, high speed data signals without error correction), but usually okay.

Tim

[james_s]

I'm certainly not an expert on this matter, but from what I remembered, ESD pulses can have extremely fast rise times that are difficult to clamp with solid state devices, but a spark gap should in theory be very fast. Static electricity can certainly reach potentials that will fire a spark gap, I mean I've had it zap from my finger to an object almost an inch away. So maybe there is some value in having both depending on circumstances? I can at least understand why an engineer thought there might be value.

[T3sl4co1l]

Thyristor devices aren't fast enough for ESD, right.

Avalanche devices (TVS, zener, MOV..) are essentially instantaneous.

It's actually worse because spark devices have an ionization period that's voltage dependent.  Like I said, "terrible messy things".  It might break over (at ESD speeds) at 2 to 8kV, but only be rated for like 500-1000V and for that matter only meet regulations at 100V (not that that matters for a signal line).

You'll never get enough voltage for long enough, to fire both, if you use both.  Not without dozens of cm of trace length between them.

Tim

[james_s]

Hm interesting, well as one of the analog wizards around here I'm going to trust your judgement on the matter. I read an article at some point that talked about the difficulties in suppressing ESD given the unusually fast rise times and high potentials but I don't remember where I saw that.

Either way I think it can be agreed that the spark gap is not really necessary and it may well be that the engineer that designed the charger did not have a great deal of expertise on the ESD matter and was just doing what seemed like a good idea to them at the time. I often have fallen into the trap of assuming that design choices in commercial products were all done for a good reason however that is not always the case.

[Circlotron]

If it was a neon or other gas filled device, shouldn’t it have a dot inside the blue circle to show it was gas filled?

[T3sl4co1l]

Yes, and other electrode shapes may apply, though the two circles is pretty standard.  I suppose an air gap doesn't really need to specify gas in the envelope, but there may not be an envelope as such (which seems to be the case here), in which case... a pair of arrows and no envelope would do well enough, really.

Tim