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Registered Member #5323
Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104
I want to protect a capacitor from ever seeing more than its 10kV input rating and am considering using a BOURNS Gas Discharge Tube for protection (image and specs below). The DC BREAKOVER VOLTAGE is listed as 8.64kV and the IMPULSE SPARKOVER VOLTAGE is listed at 10kV. Question: What is the difference between these two specifications and which should I pay most attention to if I want to protect my cap from ever seeing more than 10kV?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
depends upon the power source,
for a low current source the breakover voltage is the primary parameter, as soon as the voltage reaches the breakover voltage the gdt will start to conduct and discharge the capacitor,
IF the power source could impulsively supply 50 kA then the voltage could briefly rise to 10 kV
since I doubt that you have a high current source charging the capacitor it would not exceed c8.64 kV
Note; once the gdt has fired it will stay conducting as long as current is supplied to it so a secondary cutoff may be required to prevent fire/explosion etc.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I see from the Bourns data sheet that the "C" in the part number of your Gas Discharage Tube signifies a tolerance of ± 20%.
The voltage rating of capacitors is as subject to some degree of tolerance as any other component, so you'll see that in a worst case of 20% higher breakdown voltage of the spark gap, and not much negative tolerance on your capacitor voltage rating, could see your device fail. In any event, the margin is too small to be considered as good engineering practice.
You'll also see from the Bourns data sheet that the parameters of their Gas Discharage Tubes in this class change very quickly after a low number of discharges. You can't be running it like a sort of Zener diode, and have it clip off peaks every cycle. It's got to be a safety device of last resort, so it's a good choice to have in reserve when less expensive, less drastic, methods of preventing over-voltage have failed.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
PM, please tell me you have backups of your data sheets on something immutable such as Bluray? would hate to hear that you got Crypto'd etc. I have access to some archiving systems here..
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
a gdt (incidentally, where 4HVers use GDT to mean 'Gate Drive Transformer, the rest of the electronic engineering world uses GDT for 'Gas Discharge Tube') will not 'clip' waveforms like a zener or TVS, once a gdt has 'fired' it will stay conducting down to a very low voltage, 20V in this case. So just about ALL of the capacitor stored energy will be dissipated/discharged, rapidly.
The capacitor, gdt and total loop inductance will behave somewhat like the primary circuit of a sgtc So add resistance in series with the gdt to limit the peak discharge current (e.g. R > (Vpk/Ipk) and 'dampen' the oscillation. the resistance must also be low enough to limit the power source (e.g. R < (Vpk/Isupply)
Proud Mary made a good point about voltage tolerances, but the gdt is used as a method of last resort, that is, it should only operate infrequently under fault (overvoltage) conditions. You need to find the peak voltage that your capacitor can tolerate from the datasheet, probably the 'test' voltage which is usually 1.5x to 2x nominal peak voltage, then choose a gdt that has a voltage tolerance lower than the peak voltage AND higher than your peak expected operating voltage.
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Gas Discharage Tube specification question
