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In Reply to: RE: ideas on variable filament supplies? posted by vinnie2 on July 22, 2015 at 03:55:08
In the primary side, current is low. A low power pot or rheostat would work fine there. It is common practice in high-power radio transmitters to adjust filament voltage, where filament current can be 20-30 amps. A low current adjustment in the 120VAC primary side is the way to do it.
Follow Ups:
Do I just hook up one pot on one leg of the primary? Does it matter which leg?
Yes, in one leg of the primary as shown. You may need to determine how much resistance is needed by experimenting.
What I found on the web for using a pot for a variable resistor shows one of the AC wires to terminal 3 and terminals 2 and 1 connected to the one of the primary wires of the filament transformer. A bit different than what you show. Does it sound right to you?
Edits: 07/23/15
Only two terminals are needed on the pot. You could connect the third one to the wiper (center)terminal, that would not matter.
Well it looks like a 20 to 25k pot (using just two terminals) is just about right for a 5v tansformer. It gives me a nice smooth adjustment range from about 0.9 to 5.7 vac with no load. May need different values for other transformers, but probably not for the common ones.
Now I am sitting here trying to figure out why I would not want to install one of these little dudes on every filament suppy in every piece of equipment I have or ever plan to build. Seems to me a person could dial in the exact filament voltage needed while the amp was fired up. No more trying different resistors clipped to the filaments and then never getting it quite right anyway.
It also means a person could use any filament transformer that they wanted as long as it was at least as much as what they needed. Opens up a lot of options for using surplus and hamfest finds. Am I missing something or is this really cool?
Thanks for the tip Al, I can see lots of use for this one.
Edits: 07/23/15 07/23/15 07/23/15 07/23/15
Offhand, 20-25K sounds way too high, bit I have not done the math. It really depends on how much current is being drawn from the AC line.
One thing for sure-it will not boost voltage. If you have a 5V Xfmr, 5V is the most you will get when the pot is fully closed. You could use a 6V Xfmr, and dial it back slightly for 5V.
You were right about it being way too high a value; that was because I did not have a load on the transformer. Duh! With the tube filament connected into the circuit we need to get down to about 2.5k for the pot value to get a useful range of voltage. Trouble is then you get a hot pot after running it a few minutes. Going to have to break down and do a little math and figure what rating I need and theh shop around for a rheostat with the proper specs. It may not be the universal solution I first thought it could be, but it might still work for giving me an adjustable filament supply on my test mule bread board. Hopefully this will still be a simpler, cheaper solution than a $60 variac.
Edits: 07/24/15 07/24/15
Roger on that. Quite often you will find 5 and 6 volt filament transformers cheap at hamfests, and you could dial them back to anything from 5 volts on down. Very useful for some of the dht triodes that have the odd filament votages like the 26 and 27 tubes I am trying to work with now that have filament voltages of 1.5 and 2.5 v.
I tried several different values of pots, starting with a 100 ohms and working up. Didn't get any really useful range until I got up to 20k. Not sure if it will heat up too much; that is the next thing I want to try. I will let one run for a while under a filament load and see how it does.
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