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Old 7th Jan 2011, 4:30 pm   #1
GMB
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Default Filament protection

Having just encountered a set with all filaments blown, this raises the issue of protection schemes.

Can anyone say with any confidence what the absolute maximum voltage for a 1.4V (and 2V) battery valve might be expected to be?

Has anyone got a working crow-bar solution to protect them?
It looks a bit hard to do something that will both limit the voltage to something survivable while at the same time not starting to conduct at normal working voltage.
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Old 7th Jan 2011, 4:52 pm   #2
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Default Re: Filament protection

If I were starting this, I think I'd use an LM10 (precision op-amp and reference) driving a beefy power transistor (or even an SCR). The LM10 will operate down to 1.1V so will make an excellent precision shunt reference.

A much simpler solution is to operate from a healthy alkaline D cell. Will absorb overvoltages such as slipping test prods from HT, etc.

I've had 1.6V short-term (few seconds) on my Dx96 valves, no apparent ham done, but not to be recommended. And 2.3V on my 2V valves. But I tend to treat all these filaments as though they are as fragile as early tr*ns*st*rs.
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Old 7th Jan 2011, 4:54 pm   #3
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Default Re: Filament protection

In parallel fed filaments fed either from a dry cell or perhaps a 2v cyclon cell there should not really be an issue.
The problem can arise though with series fed filaments, or with mains derived power supplies.
In a battery set that uses 1.4v filament valves in parallel, I use 3 IN4001 diodes in series, this will clamp at around 1.7-1.8v, a 150ma fuse is fitted to the filament supply and once this voltage is reached the fuse blows, you could of course use 2 IN4001 and 2 Schottky diodes in series to get a clam volts of 1.6v.
Series connected valves offer the simple clamp of using a zener diode of approx 1 volt above the operating voltage. A 1watt zener should be used and a suitable low rated fuse in the supply feed.
There are more elaborate ways of course but I feel the simpler the circuit the less trouble you get.
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Old 7th Jan 2011, 5:27 pm   #4
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Default Re: Filament protection

The problem arises from complex connectors and/or accidents during testing. Somehow (not by me) the whole RX section has blown filaments - and I don't want that repeated.

The cell impedance idea doesn't work so well with complex military sets that like to switch the LT to different sections according to what is going on - although this has saved some of the valves which were offline at the time of whatever happened to it.

It wasn't clear to me whether 3 Si diodes would be OK as at high current the voltage drop pushes up to 1V a piece. Actually, a detailed look at the specifications suggests that just 2 diodes might work so long as the LT isn't much above 1.4V. But I guess it comes down to the race between the valves and the fuse.
Have you dared to test your 3 diode solution?

The LM10 looks interesting. I didn't realise it could work at such low voltage.
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Old 7th Jan 2011, 5:57 pm   #5
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Default Re: Filament protection

If you want to get a little more complex you could always use a comparator - set the ref voltage at the max you would allow and then feed the other input from the actual supply and drive a relay from the output to switch off the supply to the filaments when it's tripped. Only disadvantage is that you would also need a supply for the comparator.
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Old 7th Jan 2011, 7:45 pm   #6
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Default Re: Filament protection

And put another diode across the three (two) 'tother way round in case of reverse polarity overvolts.
 
Old 7th Jan 2011, 8:07 pm   #7
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Default Re: Filament protection

... yes, that's a good point which I may well have forgotton.
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Old 7th Jan 2011, 8:12 pm   #8
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Default Re: Filament protection

I only checked the protection without the valves and with 1.8v it blew the fuse.
It might be an idea though to select the IN4001's you are using for the lowest forward voltage, as they can vary a bit.
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Old 8th Jan 2011, 1:21 am   #9
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Default Re: Filament protection

Quote:
Have you dared to test your 3 diode solution?
Could you test your circuits using leds or torch bulbs as dummy "filaments"?
Maybe "grain of wheat" bulbs?
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