What does this symbol represent? - Page 1
Forum section: electronics / beginners

author: redgear
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:58:28 am
content: Which component does the circled symbol represent?

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 04:59:33 am
content: That's a neon lamp or gas discharge surge arrestor.

author: redgear
author_rank: Frequent Contributor
timestamp: September 26, 2019, 05:05:53 am
content: Quote from: james_s on September 26, 2019, 04:59:33 amThanks! But how do I confirm which one is that?

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 05:16:02 am
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: September 26, 2019, 05:17:05 am
content: 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??

author: AVGresponding
author_rank: Super Contributor
timestamp: September 26, 2019, 05:23:32 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 05:25:01 am
content: If it's a neon indicator it's gonna need a series resistor, otherwise it will act just like a gas discharge surge arrestor.

author: redgear
author_rank: Frequent Contributor
timestamp: September 26, 2019, 05:33:05 am
content: 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

author: redgear
author_rank: Frequent Contributor
timestamp: September 26, 2019, 08:07:18 am
content: Just checked it from the gerbers, it is a spark gap.Do I need one?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2019, 10:56:58 am
content: Quote from: redgear on September 26, 2019, 08:07:18 amFor 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

author: ArthurDent
author_rank: Super Contributor
timestamp: September 26, 2019, 03:34:43 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 03:43:15 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 26, 2019, 04:03:42 pm
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 26, 2019, 04:22:15 pm
content: 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

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 27, 2019, 12:17:12 am
content: HmmmmAfter 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.

author: redgear
author_rank: Frequent Contributor
timestamp: September 27, 2019, 06:02:22 am
content: Quote from: james_s on September 26, 2019, 03:43:15 pmThis schematic is a Reference Design from Ti for their BQ76930 fuel gauge. Guess, a professional should have designed it

author: redgear
author_rank: Frequent Contributor
timestamp: September 27, 2019, 06:16:11 am
content: Quote from: T3sl4co1l on September 26, 2019, 10:56:58 amD6 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.

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2019, 06:18:53 pm
content: Quote from: redgear on September 27, 2019, 06:02:22 amThat'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.

author: iMo
author_rank: Super Contributor
timestamp: September 27, 2019, 07:02:13 pm
content: http://www.ti.com/lit/ds/sluscd0b/sluscd0b.pdf11.1.4 - esd pcb spark gapsFig. 7 and 8

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2019, 08:20:52 pm
content: 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

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2019, 09:40:08 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2019, 10:22:22 pm
content: 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

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2019, 10:31:04 pm
content: 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.

author: Circlotron
author_rank: Super Contributor
timestamp: September 27, 2019, 11:19:55 pm
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2019, 11:40:40 am
content: 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

author: redgear
author_rank: Frequent Contributor
timestamp: October 16, 2019, 06:38:02 am
content: I had contacted Ti about this and this was their replyQuoteNow, which one should I use? I guess spark gap would be the worst option. Should I use a Flyback Diode or the TVS or is there a better alternative?Thanks