Variable voltage regulator: How do I.....?

[zebra50]

Hi!

In a circuit like this:

how do I calculate what resistors I need to get a given voltage? I want to build a switch into a tube heater supply which lets me dial in a regulated 5, 5.5, 6V etc. I know I can make it work with a 5k trim pot, but a switch would be useful to get back to exactly the same place.

I could do it by trial and error, but I'd rather learn how to calculate it.

[Diagram is cut from the G7 schematic - so thanks/apologies in advance to Jakob]

Cheers!
Stewart :thumb:

 

[NewYorkDave]

Read the LM317 datasheet. It tells you how.

 

[zebra50]

:thumb:
Thanks NYDave. I'll go do the reading. I feel a bit daft for not trying that first.
Stewart
:thumb:

 

[Mark Burnley]

Well,

You can grab the datasheet, connect a pot and tweak and measure, or use this online calculator thats' pretty handy!

Mark

 

[Mark Burnley]

...and don't forget to use a make-before-break switch. With the LM317, if the Adj pin becomes disconnected from 0V for a second (e.g. when switching between ranges with a break-before-make switch) the Output pin rises to just below the Vin voltage!!

Mark

 

[Gus]

A simple thing might be a switch setup like a gain switch the total string will be one end of the voltage range and connected to the wiper and the others would be at the different contacts. Even if break before make the total voltage can never be any thing different than the total sum of the resistors in the calculation for the voltage because that end is connected to the wiper and the adjust pin.

 

[PRR]

> get back to exactly the same place.

Tube heaters are NOT that fussy. Put in a pot and label it "Hot--Cold". Works well enough on the kitchen stove....

 

[Gus]

When you are going for the best out of a tube in a microphone circuit. The heaters can be fussy for noise, grid charge etc.

 

[SSLtech]

[quote author="PRR"]Tube heaters are NOT that fussy.[/quote]
[quote author="Gus"]The heaters can be fussy for noise, grid charge etc.[/quote]
He's calling you out!

...and you two are probably close enough for a quick jackets-off round of fisticuffs in a neutral parking lot, before the evening is out...

:razz:

Keef

 

[Gus]

No I am not. I agree most tube stuff is not a big deal.

Tube microphones do seem to be a case IMO that fil voltage and cathode temp make a difference.

 

[SSLtech]

:wink:

No prob Gus, -I just wanted to use the same response in two threads!

http://www.groupdiy.com/index.php?topic=4476&start=0

Keith :razz:

 

[zebra50]

Thanks all.

It IS for a microphone PSU. I want to see if I can really hear the difference in repeated tests where I switch back and forwards between different heater voltages. Am just trying to prove something to myself really. I have pots on my existing supplies.

Mark, that calculator is pretty cool. With R1 set to I need 1k5 + 100R steps up to 1k9 to dial in between 5.2V and 6.3V.

Gus, thanks for the string idea - I was thinking of something like that.

Cheers!
Stewart