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Old 21st Feb 2017, 8:54 am   #1
trsomian
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Default Metal HT rectifiers

I am trying to get an AVO all wave oscillator going again, but I have a specific question about metal rectifiers for HT. The unit appears to be a varnished cardboard tube about 4 to 5 inches long, and maybe half an inch diameter. It seems to have a forward voltage drop of about 40 to 50V.

I realise that inside these are a stack of low voltage rectifiers, so they are less efficient than a 1N4004, but is 40V plus normal.

Also, how do these things normally die, increase in reverse leakage, of decrease in forward conductivity, or both of these
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Old 21st Feb 2017, 9:15 am   #2
Nuvistor
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Default Re: Metal HT rectifiers

If it's a selenium rectifier the forward resistance increases reducing the output voltage.
There are other types though, whichever the silicon diode will have much less of a voltage drop.
Easiest way to tell if it's faulty is compare the HT with the manual if you have one, unless the load is too great a low voltage would indicate a failed rectifier.
You will require a series resistor of suitable size if you use a silicon diode as replacement.
Frank
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Old 21st Feb 2017, 11:07 am   #3
David G4EBT
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Default Re: Metal HT rectifiers

It sounds very much like a 'Sentercel' stick rectifier.

Some time ago I restored a Heathkit 'C3U' capacitance bridge which uses a Sentercel 'K8 - 25'.

The AC going into the rectifier from the transformer was fine, but instead of 150V DC coming out of it, as I recall it was down at about 55V. I disgorged the contents and fitted a silicon rectifier with a series resistor to drop the Voltage to the desired 150V. I then checked the extent to which the H.T. was overvoltage, and checked the current that was flowing. Divided the current into the voltage to work out the value of series resistor required to drop the excess voltage. I don't recall the figures, but I can see from the attached pic that it looks like a 100 Ohm resistor.

(Sentercel made many other styles of rectifiers by the way).

Hope that's of interest.
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Old 21st Feb 2017, 11:28 am   #4
GMB
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Default Re: Metal HT rectifiers

Isn't that a 1k resistor?
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Old 21st Feb 2017, 11:37 am   #5
David G4EBT
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Default Re: Metal HT rectifiers

Ooops, well spotted - yes, of course! Makes much more sense too. As I said, I can't recall the actual figures, but if say I had to drop 30V, a 1K resistor suggests 30mA current being drawn, which sounds about right.
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Last edited by David G4EBT; 21st Feb 2017 at 11:42 am.
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Old 21st Feb 2017, 12:35 pm   #6
John M0GLN
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Default Re: Metal HT rectifiers

As I had my AVO all wave oscillator apart to measure the HT voltage for your other enquiry I have just measured the total HT current taken when working and it's just under 3mA CW and 6mA when modulated.

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