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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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21st Feb 2017, 8:54 am | #1 |
Hexode
Join Date: Oct 2010
Location: Box End, Beds. UK.
Posts: 271
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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 |
21st Feb 2017, 9:15 am | #2 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
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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 |
21st Feb 2017, 11:07 am | #3 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
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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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David. BVWS Member. G-QRP Club member 1339. |
21st Feb 2017, 11:28 am | #4 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
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Re: Metal HT rectifiers
Isn't that a 1k resistor?
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21st Feb 2017, 11:37 am | #5 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
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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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David. BVWS Member. G-QRP Club member 1339. Last edited by David G4EBT; 21st Feb 2017 at 11:42 am. |
21st Feb 2017, 12:35 pm | #6 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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.
John |