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In Reply to: RE: Thank you gusser (NT) posted by rcollege on May 02, 2017 at 17:16:44
One potential issue with delay relays has to do with the relatively small contacts. For longest life, it's a good idea to use a larger relay to turn on the HV transformer. I'm getting ready to add delays to my own SETs, and have decided to go even a step further. The delay relay will power a main relay, and that one will be wired so it latches electrically. In addition, one set of contacts will be wired to disconnect the delay relay. So, once the amplifier is fully operational, the delay relay will power down and cool off. Not only will its contacts only need to pass a few mA to activate the HV, its heater will only be used for a minute or so each time the amplifier is turned on.
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Buy Chinese. Bury freedom.
Follow Ups:
You said
"One potential issue with delay relays has to do with the relatively small contacts"
Correct that also includes SS time delay relays.
You said
"The delay relay will power a main relay, and that one will be wired so it latches electrically. In addition, one set of contacts will be wired to disconnect the delay relay. So, once the amplifier is fully operational, the delay relay will power down and cool off. Not only will its contacts only need to pass a few mA to activate the HV, its heater will only be used for a minute or so each time the amplifier is turned on"
The disconnection of the delay relay is simply overkill I've run a similar scenario using 24V/30A relay activated from SS delay mains relay / timer for 10 yrs without fault. You can safely derate the DC current rating by 50% into a inductive load ie transformer and still be well under the DC rating. Relay coil DCR for 12-24V are under 1k ohm so the current requirement are small. But install a diode - reverse biased across relay coil to absorb back EMF on turn on / off. Seriously this configuration has been used in cross bar / step by step electro-mechanical telco exchanges for generations....bullet proof.
Johno
Disconnecting the relay heater after it closes has the additional advantage of resetting the timer. This means that the timing cycle will occur on a brief power interruption as well as cold turn-on.
"...the timing cycle will occur on a brief power interruption as well as cold turn-on."
Exactly right. The main relay will drop out instantly in the event of a power failure, then the delay relay cycles again. This is necessary in order to be sure the DHT filaments are always at full temperature when HV is applied.
About Johno's comments, none of the glass delay relays in my stash have contacts rated anywhere near 30A. Most are probably 3A, and that's probably a non-inductive rating, perhaps DC. I've also noticed costs rising for these parts, and I wonder if replacements will be easy to find in 10 or 20 years. Call it overkill if you want, but all things considered, I don't regret taking the necessary steps to see that these last as long as possible.
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Buy Chinese. Bury freedom.
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