DIY Mains Regulation - Oops, I forgot to factor in heat dissipation

[Paul Birkeland]

Like this.

[Paul Birkeland]

PJ also gave another good hint that you might want to investigate.  If you have a variac and an AC voltmeter built onto it, then you can also accomplish this.  The variac would need to be able to have a slight step-up, but that's not all that uncommon.

This one would appear to be up to the task, though I'd probably put a meter on the output with better resolution.

https://www.ebay.com/itm/Variac-Transformer-Variable-AC-Voltage-Regulator-3000w-Metered-0-130V-30Amp/392133114170?epid=27023253779&hash=item5b4cf4793a&_trkparms=ispr%3D1&enc=AQAEAAADIKvsXIZtBqdkfsZsMtzFbFsbX3WcW5fmB%2Fx7ZbaZTyexgFkxnWBYvPtlHgxByoGxTAB6Y5eKmoU7GwHfGOFYTlvj1WiKfE%2B8NgKcj4MdwsDpWWgS6EFQJdy2ACJfhn7PL4nnCA8vWp2M1paYbuw%2FPjcXVOvg3XYrvF%2Br312vICZLIEIxH1fjlQAn4j4gTHOdBSJUqJDWTheJT81lvgUts18t%2BOeY7qDyP8XvQzysrioB0aBVn2poOpJKrpDmG7WXyG%2BM543WZ4vCq0JJsIYFSw0iW33D1z0NczaWIwa9v1PeWluppQm6E91vqpcJw7HUcfhdrRfd%2BnHrLE6NfzrbY%2F%2F4Qsa8zI0w%2B51C0%2BtdupmveFn7GA0JwCxt9YygR%2Fy02QV3KCSfNGwmGgLPERjegQZFwD9iIDxEmzT3%2BhF%2BEkFXenOLIN5ff6ANYDhJkPmfBQw3tjXOMLn8mKbWeNwKXJDpi6mauUl7PwJEffhthW1o1ryW4VXKmvD2ujgfyl6ss5gGgD4GC80FaPC%2Bsar%2BW3MEQ7y5Pz6GUv%2FHmnwwLHSZLXB3XfJh36CdAjg45eNrOJ%2FhFusrGQ93Vh0avBIi9gwOQ8KrZlAJD%2B5yWW4J1nr0pmpVSKp2TWg4eUhU%2Fo36Txo088hg0M4fcYaLQozc9v%2BGXb800RfBC9G6pFapST%2Fl8QXCRIwzj3gZL%2BVdphdh76TO3dbgJwAEexDuC%2FC04vh19jbJXroG%2FOtx4sp7pUIXaMjqNom3aUEtQercf6bk2jf9PbVDmQH6xGLC%2F0YKY8wcwBdN5Co6UilFgcTOUNPC0pf7S77jGzOvk86AF0vJm%2Bb8BZoIK9mfJIoluKQUs%2FRDeQ6DK3lz4s%2F6FdsOzKWHBr9%2BvJnNMQYD7yotFNLmcl2vH01NbHquuqz3vrYliVnj7hr4WG6Ln38qZB1nJrV0fjreEmKGX%2FT97KXgowc5ikLL8GggsfodgGnF0V4clt4WV0lL401JNRf9LJwLOopQftsvUATfi2YfREIMvgmFytiDbROG1ZDxgdFAODOHpoQm9GYeJxyYVb%2F02l2XDn6H&checksum=392133114170a9fd7eb316444104a8385098298f4384

[Deke609]

Like this.

Aha! Both the AT and "bucking" transformer primaries connected to mains, with the secondary of bucking connected in series but out of phase with the "primary/secondary" of the AT (not sure what to call it since there's only one winding).

Last question for the day (I promise!): in your diagram the series connection of the bucking secondary to the AT is made at the end of the AT winding. In my case, this would correspond to the 125 VAC tap, which I didn't plan on using. My plan was to connect the mains to 105, 110 or 115 VAC (switchable) and take the output from the 115 VAC. So I should connect the bucking secondary at the 115 VAC tap, right?

many thanks again, Derek

[Deke609]

PJ also gave another good hint that you might want to investigate.  If you have a variac and an AC voltmeter built onto it, then you can also accomplish this.  The variac would need to be able to have a slight step-up, but that's not all that uncommon.

This one would appear to be up to the task, though I'd probably put a meter on the output with better resolution.

I do have a variac that goes to 130+VAC - it's what i've been using to raise my mains voltage going into the BeePre and Kaiju.  But the panel voltmeter is tiny, front facing (so impossible to see when the variac is on the floor) and off by about 5 VAC.  But I also have a couple of Murata digital panel AC voltmeters that are accurate to 0.1VAC. I could add one to a variac easily enough - but I had gathered from various online forums that variacs are "garbage"/"noisy" for regulating the voltage going into audio gear - I figured it had something to with the brushes not making good/clean contact - dunno. But I've promised not ask any  more questions for the day, so I will save them for tomorrow (or maybe 12:01 am since technically that's a new day  )

cheers and thanks again PB, Derek

[Deke609]

I took a quick look at a historic General Radio Variac manual: https://www.ietlabs.com/pdf/Manuals/GR/M2,%20M5%20,%20M10,%20and%20M20%20Variac%20Autotransformers.pdf

The info and wiring diagrams in the attached extract from the manual are interesting. I'll need to study up on how adding the step/buck achieves the stated advantages.  The second diagram is particularly interesting to me - it's similar to PB's suggestion, except that the step/buck transformer isn't connected at all to the common/neutral wire.

Hmm ... don;t know what to make of this yet.  More study needed.

[Paul Birkeland]

Last question for the day (I promise!): in your diagram the series connection of the bucking secondary to the AT is made at the end of the AT winding. In my case, this would correspond to the 125 VAC tap, which I didn't plan on using. My plan was to connect the mains to 105, 110 or 115 VAC (switchable) and take the output from the 115 VAC. So I should connect the bucking secondary at the 115 VAC tap, right?

Since you are intending to go up, then yes, I would have the buck/boost transformer connected at the 115V connection.  You could wire up a switch to go up or down 2.5V, though just having a booster winding will give you what you need IMO. 

[Deke609]

Many thanks PB. Yes, I'm thinking of putting the connection to the buck/boost on a switch (on/off), which in turn will throw the "on" selection to a second switch for choosing between in-phase +2.5V or out-of-phase -2.5V. Which is actually a lot more useful than my imagined diode trimmers, which would have only dropped voltage.

Many thanks to you, PJ and John for helping me with this. This new approach is even better than what I had hoped to achieve with my original plan.

cheers, Derek