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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 4th Feb 2020, 11:31 am   #1
GMB
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Default Substitute for spark gap valve?

I am wondering what to do about replacing a GD550W spark gap valve?
It was rated at 500-630V and the one I have is looking very degraded. They do not have a long life quoted in the specification, so not surprising.

What concerns me is the peak current. Not sure what it will be but maybe large as it is being used to fire a capacitor (0.5uF) into a step-up transformer with quite a low primary resistance.
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Old 4th Feb 2020, 12:00 pm   #2
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Default Re: Substitute for spark gap valve?

I wonder about a Gas Discharge Tube such as used in PSU protection?

I have seen them for around 300V but assume they are also available with higher striking Voltages. One thing that might go against them is I'm not sure they can withstand repeated action - striking Voltage degrades after each hit. But that might be when they are called upon to pass very high current when catching transients.
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Old 4th Feb 2020, 12:34 pm   #3
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Default Re: Substitute for spark gap valve?

The snag seems to be that they have a life of about 500, not 40,000,000 as the original part did.
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Old 5th Feb 2020, 1:24 pm   #4
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Default Re: Substitute for spark gap valve?

Have you simulated or measured the peak current, and the repetition rate?

Perhaps an SCR with a trigger circuit that turns gate on when anode voltage reaches a certain level? You'd have to simulate the pulse and do some current, dissipation and Tj estimation. Also depends on how the current pulse extinguishes itself and resets, and the off-time for cooling. Certainly cheap and available with 1kV+ rating.
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Old 5th Feb 2020, 1:30 pm   #5
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Default Re: Substitute for spark gap valve?

Quote:
Originally Posted by GMB View Post

What concerns me is the peak current. Not sure what it will be but maybe large as it is being used to fire a capacitor (0.5uF) into a step-up transformer with quite a low primary resistance.
What’s the function of the circuit ? Does the valve trigger in a fault condition or repetitively and intentionally?

What happens to the high voltage subsequently developed in the LC circuit once the valve conducts ?
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Old 5th Feb 2020, 2:05 pm   #6
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Default Re: Substitute for spark gap valve?

It is from a vintage electric fence energizer!

I would like it to work and I had seen it was using early transistors in the circuit so guessed it would easy to fix and modify too. I did not bargain for an unobtainable valve being used however!

The circuit is brilliantly simple and efficient, designed to run for about a year on a lantern battery (I think). The transistor part is an inverter, not quite traced the circuit yet, but looks like a simple blocking oscillator with a big pot-core. This gradually charges up the 0.5uF capacitor until the valve fires the charge into the final step-up transformer (which may be faulty too).

So it is self re-setting as the input feed will determine the firing rate and I expect it to be quite low. These things usually fire every few seconds or so.

I was about to measure the inductance of the transformer but the PP9's in the bridge are flat so I must find some more (not so hard these days - coming back into fashion for some reason).
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Old 5th Feb 2020, 2:44 pm   #7
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Default Re: Substitute for spark gap valve?

I built a fence charger in the late 60's. Car battery and an inverter (blocking osc designed to stop once the capacitor was charged. Low current timer using a neon bulb dumping a charged capacitor into an SCR gate. The housing and output transformer were from an old mechanically switched unit.

The SCR was a surplus unmarked one from Jim Fish, an epoxy block on a bit of angle ally like some of those rectifier bridges of the period.

It worked for many years.

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Old 5th Feb 2020, 3:02 pm   #8
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Default Re: Substitute for spark gap valve?

I have also designed a fence charger which has (after a little field development) distinguished itself by working for the last 25 years without a failure.

Powered from a redundant car battery and fed via reverse polarity protection and an 8V regulator, a free running blocking oscillator charges a 1uF capacitor to 500V. A discharge tube fed from a divider chain triggers a glass passivated SCR which dumps the charge into the primary of a boiler igniter transformer.

The discharge tube was the subject of development work. Neon tubes tended to be unreliable, either due to electrode sputtering or a failure to strike reliably in the dark.

The fix was to use the innards of a fluorescent lamp starter. The trick is to choose a starter which fires reliably at the chosen trigger voltage, but quenched before the discharge causes the bimetal contacts to close. As these tubes contain tritium, dark striking is good.

A 120W tube starter fits the bill perfectly and strikes every time reliably. Another good left-field application for these tubes is in the Philips G8 TV power supply protection circuit. Here, the tube contacts do close on over-voltage, triggering the protection circuit.

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Old 5th Feb 2020, 3:27 pm   #9
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Default Re: Substitute for spark gap valve?

Quote:
Originally Posted by GMB View Post
It is from a vintage electric fence energizer!


The circuit is brilliantly simple and efficient, designed to run for about a year on a lantern battery (I think). The transistor part is an inverter, not quite traced the circuit yet, but looks like a simple blocking oscillator with a big pot-core. This gradually charges up the 0.5uF capacitor until the valve fires the charge into the final step-up transformer (which may be faulty too).
These things usually fire every few seconds or so.

I was about to measure the inductance of the transformer but the PP9's in the bridge are flat so I must find some more (not so hard these days - coming back into fashion for some reason).


Nice ! I love reliable old circuits like the one you describe.

It sounds like a SIDAC would work nicely. Easily determined triggering and ideal for low repetition rates.
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Old 5th Feb 2020, 5:30 pm   #10
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Default Re: Substitute for spark gap valve?

RS Components sell Switching Spark Gaps.

https://uk.rs-online.com/web/c/passi...ng-spark-gaps/

There is not a great choice of voltages. Perhaps a 230V one could be used for a gentle fence. I do not know whether two in series would work, possibly with a high value resistor in parallel with each one to balance the voltages.

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Old 5th Feb 2020, 5:52 pm   #11
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Default Re: Substitute for spark gap valve?

The snag is their life, quoted at 4M operations, means they would not last the year out. The original part had a quoted life of 40M operations so more like a decade. This seems to be the problem with this technology.

Or am I misunderstanding the spec. sheet?
I mean, what are they for?

Last edited by GMB; 5th Feb 2020 at 5:57 pm.
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Old 5th Feb 2020, 6:38 pm   #12
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Default Re: Substitute for spark gap valve?

A photo, or are they like the ones on RS?
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Old 5th Feb 2020, 6:59 pm   #13
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Default Re: Substitute for spark gap valve?

Quote:
Originally Posted by GMB View Post
The snag is their life, quoted at 4M operations, means they would not last the year out. The original part had a quoted life of 40M operations so more like a decade. This seems to be the problem with this technology.
The specified life is with a Discharge capacitance of 2.2 µF. With your 0.5 µF the life might be about 4.4 times longer. As the minimum order is 5 they could be paralleled so that as the spark of voltage of one increases another takes over. The smaller discharge capacitor and 5 in parallel might extend the life to 88M operations.

Just an idea; no guarantees.

David
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Old 5th Feb 2020, 7:02 pm   #14
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Default Re: Substitute for spark gap valve?

This is what a GD550W looks like. About 2 inches long.
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Old 5th Feb 2020, 7:12 pm   #15
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Default Re: Substitute for spark gap valve?

About 2x the size of a fluorescent lamp starter. Using a glass passivated SCR to handle the main discharge current makes a light duty discharge tube practically everlasting. You need good quality components in the field (literally). I applied automotive build and component selection standards.

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Old 5th Feb 2020, 7:39 pm   #16
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Default Re: Substitute for spark gap valve?

When I built mine I used a TO18 sensitive gate thyristor driving an industrial nut and bolt thyristor. The inverter was from a throw away camera flash.
I used a VDR from an old computer power supply for the triggering.
It has worked for 30 years keeping unwanted animals off seedling beds.
The circuit works with almost any ignition coil.

A later circuit I have tried is an SMPSU chip with the oscillator frequency reduced to a few hundred hertz fed with a start-up resistor to the power pin.
This drives a modern ECU type ignition coil directly with a FET without an inverter.
It produces a short burst of pulses each cycle.
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Old 5th Feb 2020, 9:26 pm   #17
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Default Re: Substitute for spark gap valve?

0.5 uF is a very small discharge capacitor in electric fence terms. A BT151 - 800R would be able for the currents involved. As they are normally used with discharge capacitors up to 20 uF charged to approximately 600V .

If needed I could find a example trigger circuit for the BT151.

A beefier thyristor used in electric fences is a BTW92-800R


Be aware though that electric fences are high voltage, high current devices. Commercially made output transformers produce approximately 7 KV and have a very low output impedance. For safety the width of the output pulse must be no longer than 0.3 mS. Anything longer than this and they can be lethal!
So be careful if modifying one!

Frank
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Old 6th Feb 2020, 12:42 am   #18
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Default Re: Substitute for spark gap valve?

I found references to IEC60335-2-76 as the standard for this, which may just specify <8J into 500 ohms as the requirement (but the standard costs a lot of money - which always annoys me).

The nice thing about this kind of design is that it is intrinsically safe because the pulse energy is well-defined. But my concern was that as the firing tube degrades the energy will go up - but the capacitor rating seems to be the same as the maximum firing voltage so presumably the capacitor breaks down if something makes it all go too high.

Actually I just found in a better experiment, that the firing valve appears to be working albeit 30V over its maximum rating, inspite of looking dreadful inside. But a modern firing system might be better.

If anyone has access to IEC60335-2-76 could they please read it and say what else is required?
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Old 6th Feb 2020, 9:05 am   #19
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Default Re: Substitute for spark gap valve?

If you could make sure that the repaired electric fence does not create electromagnetic interference, it would be appreciated by local listeners and radio hams.
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Old 6th Feb 2020, 2:56 pm   #20
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Default Re: Substitute for spark gap valve?

It will be tested to modern CE standards before deployment.

Personally I worry more about the 2 Chinese ones in use nearby plus the 132kV pylon and the two lower voltage overheads too, plus two racks of assorted EE mobile transceivers and PSUs and a rack of microwave links.

So a great spot for ham field days.

M0GXB
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