Time delay relay hair tearing
I thought it was the perfect design. SO confident of the way it would work, I began building this Power Supply for the 4CX250B linear.
Because you are supposed to wait 2 minutes from the time the amp is turned on til you apply plate voltage to the tubes (allowing the filaments to heat properly) I designed up this collage of relays that would give me the 120 second delay.
I ordered a 120 second relay from Ebay and it came in in perfect shape. Sure enough, about 2 minutes after you apply power to it, the contacts OPEN. OH, yeah, that was a problem. No Normally Open relays available (unless I sprang for the BIG BUCKS and bought one brand new). So I was happy to pay about $10 for this one and in it came
Tonight I clip leaded it all up.
The Time delay Relay, with it's normally Closed contacts, would open at 120 seconds, which, in turn, would release voltage to another Normally closed relay which closes when power is removed. 2 Normally CLosed's equals a Normally open. Sure seemed like it should work!
The 2nd set of contacts keyed another relay which closed when the 2nd relay was released from power.
However, a Time Delay relay doesn't just have the contacts open and close.
They release slowly, so there is a whole bunch of sparking and chattering that happens. And, when you have 2 more relays downstream from it, this CHattering goes INSANE!!!!!!!!!!!!!!!!!!
All the relays operate on 115VAC
The final relay has contacts that apply power to the High Voltage transformer. But, oh, there is ONE MORE relay in the mess!!!
Since you can't just apply 115 VAC to the transformer immediately, you have to go through some series resistors first, for maybe 5 seconds, then those resistors are shorted out by the next relay and, VOILA, you have High Voltage!!!
OH, but the CLINCHER is that if you turn off the time delay relay, and then repower it before it has time to adequately cool, then that time is subtracted from the 120 seconds. So, if it goes on and off and on, the relay is energized immediatley. NO TIME DELAY unless the relay is adequately cooled, another 2 minutes later. So you really need a 2nd 2 minute relay to time the timer!!!!!!!!!!! YIKES!!!!!!!!!!! THIS IS GETTING MESSY!!!!!!!!!!
So now, the Time Delay relay is out the window. $10 flushed down the drain.
The N/C Contacter relay is out the window. I need a Normally open one now.
It is back to the drawing board. Maybe I can design a 555 timer circuit to handle the 2 minutes, and a 2nd 555 to handle the 5 second 2nd delay.
What I am sweating now is keeping RF out of the 555 circuit! One whisp of RF can reset more timers and flip relays that don't want to be flipped, and it will be a NIGHTMARE!!!!!
So, back to the drawing board. Maybe I can use the 120 second time delay relay to run a fan or something......
Last edited by WA6MHZ; 06-10-2009 at 12:42 AM.
Maybe you could use your time delay relay to activate an electronic relay (SSR) instead of a mechanical one. This would probably help the debounce of the contacts.
Although the chatter is usually caused by the relay turning on and off because of improper wiring, wrong voltage or ac instead of dc etc.
In my line of work, we use time delay relays for preventing compressor short-cycling. You can adjust the delay, and as a bonus, if you remove power, it restarts the delay at zero. They cost around $25 or so. Control voltage input from 19-240vac. Maybe this would work a little better for you.
Put a big sign on the plate switch which cautions you to wait 2 minutes before activating. If you put a shield over the switch (which are very cheap) this will be an additional reminder.
This may work for you: On an amplifier using a 4CX250FG (28V filament, rather then 6V) I used 2 Square D 9050JCK11V14 time delay relays. The coil of the relay is 24V, which is supplied from the filament transformer. This also provides an interlock to confirm that there is filament power.
Originally Posted by WA6MHZ
The first 9050JCK11V14 time delay relay provides 5 minutes of filament preheat, then operates. This activates a larger Square D 8910DPA33 contactor to provide AC power to the HV transformer primary. In my case the transformer is too big, requiring the contactor.
The second 9050JCK11V14 time delay relay provides control for another D 8910DPA33 contactor for soft start for the HV, again, the size of the transformer required the contactor to short out the soft start resistors. A smaller transformer likely could be switched directly by the time delay relay.
If you have a bit of D-C available, it is very easy to make a multi-minute delay relay. Just feed the relay with a 2n7000 enhancement FET with an R-C in the input. It can be cycled immediately by shorting the cap. I use 10-100 uf tantalums.
TOM K8ERV Montrose Colo
Lets back up a minute.
I know you don't want to draw full plate current before these tubes are at the proper cathode temperature.
I don't seen anything in the datasheet that says plate voltage cannot be applied.
A properly designed amplifier will have the tube/s cut off during stand by. This can be accomplished by ether negative bias on G1 or removing voltage from G2. Ether case the tube is not drawing plate current and this simplifies the start up operation.
Only two things you have to do during warm up.
1. inhibit T/R relay operation.
2. Cut off the tube.
This is simple, no need to complicate things.
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I have a QRO Tech amp that uses a 555 timer for the 2 min. warmup for the 4cx800a's - simple circuit and no problems. Think about it - during the timing phase there is no RF - that's what the timer is there for! Once it's latched, it seems to stay latched - there is no external latch in the design (from what I remember) - just a transistor driving a relay. If you really want, you can setup an external latch as well I guess...
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Put the $10 relay on E-bay.when youv'e sold it for 20,buy yourself an egg-timer and a book.By the time your'e through the book ur ready to tx.