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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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4th Jun 2019, 10:18 am | #1 |
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Selenium rectifiers to diodes
When valve rectifiers are replaced by silicon diodes, a resistor should be placed in series. This extends the current pulse and reduces dissipation in the transformer and reservoir capacitor and result in the correct voltage.
Old selenium rectifiers go high forward resistance and heat up and smell of garlick. Should a resistance be placed in series with a replacement diode or will a [B]NEW[B] selenium rectifier be a match for a diode? |
4th Jun 2019, 10:44 am | #2 |
Dekatron
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Re: Selenium rectifiers to diodes
Silicon diodes have very low forward resistance, though do impose a voltage drop of about 0.8V. This is negligible for HT rectification, but for low voltage (LT battery eliminators etc) it could be significant and you may get a LOWER voltage by replacing selenium rectifiers with silicon.
Personally, for HT, I would put a resistor in series with a replacement silicon diode or even a NOS selenium rectifier, of such a value that the HT voltage is about right with the mains set to specified input voltage (using a Variac). |
4th Jun 2019, 10:45 am | #3 |
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Re: Selenium rectifiers to diodes
Presumably a NEW selenium rectifier will be a drop in replacement for an OLD one. No resistor needed. A replacement of a NEW selenium rectifier by a silicon diode would require a resistor.
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4th Jun 2019, 11:04 am | #4 |
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Re: Selenium rectifiers to diodes
I think you need to clarify what is meant by "a NEW selenium rectifier".
Do you mean new as in "made last week" or new as in "not been used yet". The key thing with using selenium is that it must not get too hot. I am fairly sure the failure mode that leads to awful fumes involves them initially getting very hot. Selenium was supposed to have a self-healing aspect when it came to impulse damage. Aged selenium rectifiers I suspect are likely to have a higher resistance then brand new which could lead to thermal disaster depending on the current. You have to keep checking very carefully. |
4th Jun 2019, 11:39 am | #5 |
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Re: Selenium rectifiers to diodes
By new I meant a rectifier to new specification. I have details of a host of valve rectifiers but have not seen similar details for selenium rectifiers.
I assume there is some voltage drop or they would not have fins or a metal heat sink. Old selenium rectifiers go high resistance in the forward direction and leak a little in the reverse direction. In the few HT cases I have dealt with, I have left the selenium rectifier in place and connected a diode in parallel. No need to construct a terminal board. Check the temperature. The latest one I have looked at was a high voltage EHT stick rectifier in a scope. This was replaced by a few 1N4007 diodes in a ball point pen tube. |
4th Jun 2019, 12:00 pm | #6 |
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Re: Selenium rectifiers to diodes
They do have a significant forward resistance even when new.
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4th Jun 2019, 2:24 pm | #7 | |
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Re: Selenium rectifiers to diodes
Quote:
They made custom multi-plate rectifiers for me the restoration of the Lorenz SZ42 cipher machine at Bletchley Park. They even sprayed them the same colour as the originals (red). The main difference is that original rectifiers use steel plates, and GD manufacture them on Aluminium. These have better long term reliability, and a lower voltage drop. Craig |
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4th Jun 2019, 5:02 pm | #8 | |
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Re: Selenium rectifiers to diodes
Quote:
However, it's your choice to do it your way, and if you think you've got away with it, then fair play to you! |
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4th Jun 2019, 8:13 pm | #9 | ||
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Re: Selenium rectifiers to diodes
Quote:
Greg.
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4th Jun 2019, 10:38 pm | #10 |
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Re: Selenium rectifiers to diodes
Like the cat's whisker before, it wouldn't have been used if it hadn't been the only choice. You could say that the modern diode has to be degraded to imitate one, usually by adding internal resistance.
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5th Jun 2019, 12:28 am | #11 | ||
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Re: Selenium rectifiers to diodes
Quote:
The electrolytics will be overvoltaged by both types of diode until the rest of the valves have warmed up. If the valves are cooked due to over voltage, that brings me back to my original question of what the voltage drop of the Se diode is and whether a series resistance is desirable. Reverse leakage will show up as high temperature in the Se diode. My bodges are for my own use and I would not do the parallel trick for any equipment which is not supervised by me. I have little experience in this area, perhaps I have been lucky. I have very little time and so I go for the quickest and easiest way, I doubt if I will do many more radio repairs. |
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5th Jun 2019, 8:35 am | #12 |
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Re: Selenium rectifiers to diodes
I saw this once in an old TV where the selenium rec had been paralleled with a BY100 silicon and no series resistor. Bulging and weeping smoothers and a severely over-run line output stage was the result...….I was surprised that the smoothers hadn't actually blown violently but they were rated at 350V which must have helped. I can't remember if I actually fixed the set or wrote it off...….
I wasn't aware that new selenium rectifiers were still available? There may be some new old stock ones about but I wouldn't trust them. Besides the smell when they fail has to be witnessed to be believed....!
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5th Jun 2019, 8:42 am | #13 |
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Re: Selenium rectifiers to diodes
If the old Se rectifier is to be left in situ, just add another tuppence worth of Si in series with the parallel pair. Won't matter what state the Se job is in then.
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5th Jun 2019, 9:35 am | #14 |
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Re: Selenium rectifiers to diodes
The thing about selenium is they have a very low PIV rating so you usually find them in stacks. Add in a 1V forward voltage drop - per unit - and you can see why they drop a few volts. But they have a lowish maximum temperature rating so it's thermal problems that get them.
I have a suspicion that they are much more reliable than people think. Indeed their claim to fame was extreme long life and surge immunity. It may be that the only thing that degrades them is getting rusty (as old ones used steel plates). If you believe their P.R. then the reason for the smelly catastrophic failures was typically electrolytic capacitor failure resulting in the rectifier overheating - the one thing they cannot stand. |
5th Jun 2019, 3:56 pm | #15 |
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Re: Selenium rectifiers to diodes
The bad thing about leaving the old Se rectifier in parallel with the new diode is when the old Se rect ages more, and decides to go short-circuit.
Why leave it there, when you can easily hoick it out and fit a nice 3-way tagstrip to take your new diode and the obligatory series-resistor? [One potential issue with this: generic silicon power-rectifiers are 'slow switchers' due to charge-storage effects in the semiconductor junction - this can cause significant RF/MF-noise-generation which is something you don't want around a receiver. Modern Schottky-diodes are no more-expensive and their lack of charge-storage effects makes them a lot quieter] |
5th Jun 2019, 6:44 pm | #16 |
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Re: Selenium rectifiers to diodes
1000V Schottkys are a bit thin on the ground..... fast recovery Si does the job OK.
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5th Jun 2019, 9:52 pm | #17 | |
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Re: Selenium rectifiers to diodes
Quote:
I have also used 1N4007 without problems. Latterly several in series for a scope rectifier. The UF4007 is a fast diode which produces little noise. This may become my standard replacement. The RAS310AF will need a new mortgage but the 4007s are cheap as chips. |
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