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RE: Cheap DC Filament PS

Posted by coffee-phil (A) on June 5, 2017 at 14:28:35

In Reply to: RE: Cheap DC Filament PS posted by Triode_Kingdom on June 5, 2017 at 08:52:23:

Hi Triode Kingdom,

It is true the grossly oversizing will cut into the advantage of the switcher.

I sure did not want to throw a damper on this idea. There may not even be a problem. First off if the switcher just current limits at its rated current as opposed to shutting down and restarting again the voltage across the tube heaters will rise steadily with no drama. Even my 500 ma PS would eventually (tens of seconds) light the 150 ma heater. For each successive attempt the tube heater would be a bit hotter and drop a bit more voltage. If I wasn't watching the voltage rise with a scope I would have been unaware that the PS was having to make several repeated attempts to start. Actually it may have been fine. I just didn't like it.

Even if your switcher shuts down and retries as mine as opposed to current limiting there are solutions. The most obvious would be to have a series resistor limiting the current at the supply's rating and switching it out in a few seconds as Chip647 suggested.
An other approach would be to follow the switcher with a low drop linear regulator with current limiting. The switcher output would be set to the desired voltage plus the drop out voltage of the LDO linear. Linear Technology makes a variety of LDO linears.

Phil

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