EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: pedroteck on January 12, 2015, 05:18:48 pm

Title: What is this electronic symbol ?
Post by: pedroteck on January 12, 2015, 05:18:48 pm

Any one familiar with this PCB marking " SG " , i am confused weather its a cap or an oscillator , its connected directly to voltage test probe input of a bench DMM btw.

Title: Re: What is this electronic symbol ?
Post by: c4757p on January 12, 2015, 05:24:14 pm

Spark gap, for high voltage overload protection.

Title: Re: What is this electronic symbol ?
Post by: T3sl4co1l on January 12, 2015, 05:35:18 pm

FWIW, the symbol (two plates in a circle) is also sometimes used for normally-open relay contacts on ladder diagrams, or, I suppose, a capacitor that's special in some way (perhaps sealed in a vacuum, hence the circle emphasizing the envelope).  There's usually a solid dot inside the circle, denoting a gas filling (gas discharge tubes (GDTs), neon lights, etc.).  Alternatives to the straight lines include round, triangle/arrow, or 'D' shaped electrodes, emphasizing the nature or intent of the discharge (from neon glow discharge to high current spark gaps).

The "SG" designation and use definitely suggest at least an air type spark gap, in your case.

Tim

Title: Re: What is this electronic symbol ?
Post by: pedroteck on January 12, 2015, 05:55:21 pm

Thanks a lot .

I see now why its near the voltage input , it acts kind of  like a MOV .

Title: Re: What is this electronic symbol ?
Post by: Dr. Frank on January 12, 2015, 09:49:50 pm

The symbol origins from a neon discharge bulb, which also ignited on a pre-determined voltage. Can be found in old FLUKE DCV calibrators, or in HP neon bulb choppers.

Same effect as a SG, maybe more precise, but for sure more expensive today.

Frank

Title: Re: What is this electronic symbol ?
Post by: pedroteck on January 17, 2015, 02:11:09 am

Quote from: Dr. Frank on January 12, 2015, 09:49:50 pm

The symbol origins from a neon discharge bulb, which also ignited on a pre-determined voltage. Can be found in old FLUKE DCV calibrators, or in HP neon bulb choppers.

Same effect as a SG, maybe more precise, but for sure more expensive today.

Frank

Maybe you have a point Dr.frank since this component belongs to an old bench dmm near the AC/DC voltage input .

The problem is that i have lost it and need to replace it with something but not quite sure with what  , if it was acting like a MOV i wold replace it with that but i need to be sure .

I remember that the component was in a plastic package ( like a small sized polyster cap) with black colour and a encircled T marking on top of it .

Like that but a little bigger ( i hope this will help more) :

Title: Re: What is this electronic symbol ?
Post by: pedroteck on January 18, 2015, 09:24:37 pm

Here is a close up view of the inside of the component case :