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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Jul 2020, 8:33 pm   #1
dazzlevision
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Default Small glass cased surge arrester

I have been scavenging components from a switch mode power supply board from a Philips time lapse VHS VCR and came across this small device. It is open circuit on a DMM’s Ohms range, but when tested on my multi-voltage Megger, it fires at around 500V and an arc is visible through the glass. Never come across such a device before, but I expect this VCR is semi-industrial. The symbol below the device is silk screened on the PCB.

Does anyone know who makes this component?

There is very comprehensive mains filtering and surge protection on the “hot” side of the chopper transformer, including a pair of 100nF class X capacitors, whose values have reduced over the years to 1 and 1.5nF, as you wound expect from their self-healing construction. There is also a disc VDR and bifilar filter choke.
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Old 15th Jul 2020, 12:35 am   #2
Refugee
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Default Re: Small glass cased surge arrester

Green... Blue... Brown...
560 volts.
They are sold by most of the big component suppliers in various sizes depending on how much energy you need them to absorb.
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Old 15th Jul 2020, 12:55 am   #3
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Default Re: Small glass cased surge arrester

Anyone know of a part number and/or datasheet/manufacturer ?
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Old 15th Jul 2020, 1:18 am   #4
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Default Re: Small glass cased surge arrester

Modern ones are in tough ceramic packages for safety in the event of gross overloads.
Here is a few for you to look at.
https://uk.farnell.com/w/c/circuit-p...e=axial-leaded
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Old 15th Jul 2020, 6:14 am   #5
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Default Re: Small glass cased surge arrester

You can even get push-pull dual ones for use on balanced lines.

GDTs are often found in telephones and in line interface sections of electronic phone exchanges.

One of those components that are all around us, unseen.

You can even get them in surface-mount rectangular packages that look like capacitors in a whiter shade of ceramic.

You do see a flash of blue-ish light through the ceramic when they do their job. And doing their job progressively changes their characteristics, so you design them to operate under exceptional circumstances. Use them to take the high voltage sting out of a lightning-induced surge and o reduce the peak that other, more precise transient protection measures have to handle downstream of the GDT.

Compared to hulking great TVS diodes, GDTs have low capacitance so they can go on RF lines. I tried one on the antenna port of a transceiver, but soon found that it burned out quite spectacularly if transmitted into a bad VSWR. There wasn't any margin between the size of transient I wanted to clip and the peaks in normal operation, and GDTs are very fast.

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Old 15th Jul 2020, 7:53 am   #6
Craig Sawyers
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Default Re: Small glass cased surge arrester

And a lot cheaper than bidirectional TVS diodes https://www.mouser.co.uk/datasheet/2...df-1840051.pdf the 430V version of which is the thick end of £100 a pop.

The only advantage of a TVS diode is that it is not life-limited in the same way as a GDT is, if you can cope with 10uA leakage current. But a GDT is two orders of magnitude cheaper.

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Old 15th Jul 2020, 8:42 am   #7
dazzlevision
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Default Re: Small glass cased surge arrester

Thank you for all your responses.

What intrigued me about this particular component, was to see it (for the first time) in a mass produced piece of kit and in a glass encapsulation.

I've encountered many forms of spark gap/GDT/varistor/semiconductor transient suppressor during my career in railway signal and telecommunications (where transient suppression was essential!), but not in domestic equipment - and in this particular encapsulation.

I see that the same component is actually fitted in one of my domestic VHS VCRs - a Philips VR900, with a virtually identical SMPSU.

The only similar component that I have found in an internet search is on the RS Components website:https://uk.rs-online.com/web/p/gas-d...B&gclsrc=aw.ds
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Old 15th Jul 2020, 9:47 am   #8
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Default Re: Small glass cased surge arrester

I've quite often noticed these things on the aerial input circuit of older car radios, particularly Clarion- at first glance they look like a typical Ge diode-type package but the PCB shows the spark-gap symbol and any diagram encountered confirms this. In this case presumably lower than 560V striking for static protection! Sure I've occasionally seen them on VCR SMPSU primary side too, again Japanese origin. Maybe use in a Philips device suggests Matsushita sourcing?
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