How to pick the right gas discharge tube (GDT)

Thread Starter

RichW

Joined Nov 8, 2019
24
Hi,

I am wondering if anyone here can offer some advice. I need a gas discharge tube to protect against high voltage spikes. I am using this supplier:

https://uk.rs-online.com/web/c/pass...tection-components/gas-discharge-tubes/?sra=p

But I know very little about these components. For example, some in the data sheet might give a protection for surge 2000 A (8/20 μsec) or 1KV/µS. Do I choose based on max current or max voltage? Do I just choose the one with the fastest response to be on the safe side? What's the difference between DC spark over voltage, impulse spark over voltage and maximum voltage rating? How do i choose the "suppressor type"? I'm getting really lost here.

I am estimating that the voltage spikes are occurring in the kV range.

Any advice would be appreciated.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
hi Rich,
What is the equipment you are trying to protect and what is the source of these HV spikes.?
E
Hi,

Thanks for the reply. It is an amplifier working at ~20-100 kHz. I was using it at only 100 W (it can go to 2000 W) and the voltage spike from the connected device fried the MOSFET's. So now I am trying to install a GDT but no idea on these.
 

Hymie

Joined Mar 30, 2018
1,002
GDTs are normally used on incoming mains circuits to limit the transient voltages on the mains supply degrading the performance of the equipment. These transient voltages can reach as high as 4kV.

An important consideration in the use of GDTs is once they ‘fire’ they conduct until the current flow drops below a holding level; for this reason an MOV/varistor of suitable breakdown voltage is placed in series (with the GDT) otherwise the GDT may place a permanent short circuit on the circuit you are trying to protect.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
GDTs are normally used on incoming mains circuits to limit the transient voltages on the mains supply degrading the performance of the equipment. These transient voltages can reach as high as 4kV.

An important consideration in the use of GDTs is once they ‘fire’ they conduct until the current flow drops below a holding level; for this reason an MOV/varistor of suitable breakdown voltage is placed in series (with the GDT) otherwise the GDT may place a permanent short circuit on the circuit you are trying to protect.
Thanks Hymie. So the GDT should be chosen based on the voltage/current of the supply so it isn't constantly "firing up".

If my device connected to my amp is already voltage dependant i.e. it is a load, the do I still need MOV/varistor?

Back to the OP, the picture is starting to become clearer. So, DC / impulse spark over voltage must be the voltage for activation of the GDP - although I don't know the difference here. Why are some of the GDT's very high current but low voltage, and vice versa?

Inside our amplifier there is MOSFET with max Drain-Source voltage is 133V, so the voltage spike must go through that. So, I want a GDT with activation <133 V but max voltage rating in the kV region. Does that sound correct?
 

Hymie

Joined Mar 30, 2018
1,002
Let’s say within the amp you have a 133Vdc rail, on which you wish to minimise noise spikes – the MOV/varistor should have a rated voltage of at least 133Vdc, to avoid a short circuit once the GDT fires.

Your problem is that GDT breakdown voltages start at around 100V, so having such an arrangement of an MOV in series with a GDT would not prevent rail voltage spikes reaching well over 200V. There are better methods for filtering DC power rails than using an MOV/GDT combination.
 

Janis59

Joined Aug 21, 2017
1,356
That seems a kind of neon bulbs igniting at voltage marked in it name. Thus - 1000V means they will ignite at 1 kV and consujme quite large the current to damp any resonant processes in the surrounding.So, my experience with that is about femtoampere-scale electrostatic amplifiers - in the input of 3 fA and 60 fA opamps we like to solder the CG75L - that is wonderful detail.
 

MisterBill2

Joined Jan 23, 2018
9,785
I am guessing that it is at the output of the amplifier that the destructive voltage spike appears, although it is not really clear on that point. But my advice will be based on that guess.
First, you need to know what the maximum voltage the amplifier will be delivering during the intended operation, and select a protection device with a trigger voltage (firing voltage) above that level. This will prevent unintended protective action.. Second, you need to know the impedance of the part of the amplifier circuit that is producing the voltage spikes. This will determine the current capacity required for the protection device. Probably you will also need to have an idea as to the rise time of the spikes that you are protecting against, so that you can select a protection device that is fast enough.
It may be that none of the GDT devices will be suitable, in which case you may need to use a clamp circuit, which can be as simple as a suitable diode connected to a low impedance voltage source, back biased to a voltage that you want to protect against exceeding. That diode would be biased off during normal operation but as a voltage spike developed the diode would become forward biased and conduct the spike to be safely absorbed by the voltage spike.
The benefitof using a clamp circuit is that it always switches off when the transient voltage ends.
 
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