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In Reply to: location of relay for time delay posted by alex on September 11, 2005 at 19:05:34:
Definitely not between bridge and smoothing capacitor. The surge current when the relay closes will most likely weld the contacts.Between secondary and bridge - I would not do this for the same reason.
Between line and primary? Surge there too, not only the surge charging the smoothing capacitor but also the surge due to inductance of the power transformer.
So what can be done? I have found that a time delay relay set to 60 seconds (or whatever you feel is appropriate) that turns on a zero-crossing solid state relay (no contacts) in the transformer primary circuit has worked flawlessly even though the power transformers (2) are toroids. The smoothing capacitors are 75000uF operating at 180 volts and the surge current is high, so be careful when you chose your recifiers. The SS relay is rated at 25A continuous. I don't remember the surge rating.
There is (as always) a complication. A solid state relay introduces a switching transient occuring when the AC voltage goes through zero volts. This may get into the amplifier; it certainly got into mine, causing a buzz. The way to avoid this is to use yet another(!) conventional relay that shorts the SS relay 'contacts'. Then no buzz. I think/hope that the inertia of the mechanical relay will allow the SS relay operate first, so that the SS relay takes the surge and the regular relay just shorts the SS relay 'contacts'. Clearly this not a cheap approach but for a circlotron I deemed it the best solution.But there is a completely different and cheaper approach involving a resistor between the bridge and the smoothing capacitor. The resistor reduces the surge and it causes a lower, kinder, voltage than normal operation be applied to the capacitor and tubes during warmup. A time delay relay is used to short the resistor after whatever time you want. The result is that the usual B+ is applied to the tubes after that are ready to accept the full voltage.
Bill
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Follow Ups
- Re: location of relay for time delay - pickle 08:40:47 09/15/05 (0)
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