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Old 9th November 2006, 04:42 PM   #1
nickds1 is offline nickds1  England
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Default Heater current monitoring...

This may be a rubbish thought, but has anyone considered using the Zetex ZXCT1021 or similar to sense high-side current for a heater supply? If this is differentiated, you could stop the HT until you knew for sure that the heaters were up to temperature... no more "lets wait a random time interval that might be ok"...

Just a thought
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Old 9th November 2006, 04:51 PM   #2
anatech is offline anatech  Canada
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Hi nickds1,
Most heaters have 11 sec. as a warm up time. If you gave the circuit 20 sec before the B+ was applied you would be pretty safe I'd think. If the filiment suply had failed you are still safe because the heaters will not be warming up at all. Running the timer off the heater supply would ensure no B+ was applied ever until the heaters had power for at least 20 sec, or whatever time you wanted.

Now for a wrinkle. If you did want to sense current, a heater will have zero current until power is applied. Then it will draw a much higher current until it warms up where the current will fall to it's normal level. You would need to implement a window detector to do it this way.

-Chris
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Old 9th November 2006, 08:22 PM   #3
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nickds1,

Using a CCS or constant current source for the DC heater
and you solve this point...

You can use the pass resistor to measure if there is current or not on this heater...

I discarded the idea, because stripping the cathode of a broken tube is "no object"

So I did put 20 second delay for the B+ using the 18 volts out of the bridge
{with a little transistor and a RC on base to switch the 12 relay}
used for the "compound regulator" {LM317} providing some limitation at startup
and let adjust the heater voltage ...

Regards.

Alain.
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