Variac question

Started by April 5, 2017
All,
I have two variacs, one is the typical type that does not isolate the
load from the mains. The other one I have isolates the load. When I
use the isolated one to power an inductive load, like a transformer,
the load really hums. The non-isolated variac does not do this.
Anybody know?
Thanks,
Eric
On Wednesday, April 5, 2017 at 11:13:08 AM UTC-4, et...@whidbey.com wrote:
> All, > I have two variacs, one is the typical type that does not isolate the > load from the mains. The other one I have isolates the load. When I > use the isolated one to power an inductive load, like a transformer, > the load really hums. The non-isolated variac does not do this. > Anybody know? > Thanks, > Eric
How does the isolated one work? (I've never heard of one.) George H.
On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote:

>All, >I have two variacs, one is the typical type that does not isolate the >load from the mains. The other one I have isolates the load. When I >use the isolated one to power an inductive load, like a transformer, >the load really hums. The non-isolated variac does not do this. >Anybody know? >Thanks, >Eric
There's no reason I can think of why that might happen, unless you don't really have an isolated variac and instead have a triac/SCR circuit on the primary of a transformer that provides the actual isolation. Some Navy gear I worked on had that (back in the 70's) and it was noisy.
On Wed, 5 Apr 2017 16:21:23 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Wednesday, April 5, 2017 at 11:13:08 AM UTC-4, et...@whidbey.com wrote: >> All, >> I have two variacs, one is the typical type that does not isolate the >> load from the mains. The other one I have isolates the load. When I >> use the isolated one to power an inductive load, like a transformer, >> the load really hums. The non-isolated variac does not do this. >> Anybody know? >> Thanks, >> Eric > >How does the isolated one work? >(I've never heard of one.) > >George H.
They do make them. Primary winding is close to the core, and the variable tapped winding is on the outside and isolated - bulkier than the usual auto-transformer style of variac, but otherwise identical. They can also be step up or step down at max output voltage.
On Wed, 05 Apr 2017 20:20:15 -0400, default <default@defaulter.net>
wrote:

>On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote: > >>All, >>I have two variacs, one is the typical type that does not isolate the >>load from the mains. The other one I have isolates the load. When I >>use the isolated one to power an inductive load, like a transformer, >>the load really hums. The non-isolated variac does not do this. >>Anybody know? >>Thanks, >>Eric > > > >There's no reason I can think of why that might happen, unless you >don't really have an isolated variac and instead have a triac/SCR >circuit on the primary of a transformer that provides the actual >isolation. > >Some Navy gear I worked on had that (back in the 70's) and it was >noisy.
I never took the thing apart. I bought it used so I don't have a schematic of the insides. Looks like I should be taking it apart. That's OK, I love looking under the hood. Thanks, Eric
On Wed, 05 Apr 2017 20:23:48 -0400, default <default@defaulter.net>
wrote:

>On Wed, 5 Apr 2017 16:21:23 -0700 (PDT), George Herold ><gherold@teachspin.com> wrote: > >>On Wednesday, April 5, 2017 at 11:13:08 AM UTC-4, et...@whidbey.com wrote: >>> All, >>> I have two variacs, one is the typical type that does not isolate the >>> load from the mains. The other one I have isolates the load. When I >>> use the isolated one to power an inductive load, like a transformer, >>> the load really hums. The non-isolated variac does not do this. >>> Anybody know? >>> Thanks, >>> Eric >> >>How does the isolated one work? >>(I've never heard of one.) >> >>George H. > >They do make them. Primary winding is close to the core, and the >variable tapped winding is on the outside and isolated - bulkier than >the usual auto-transformer style of variac, but otherwise identical. >They can also be step up or step down at max output voltage.
IIRC AutoTransformers can do about 2X the VA of a regular transformer for the same core size. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
On Wed, 05 Apr 2017 17:44:33 -0700, etpm wrote:

> On Wed, 05 Apr 2017 20:20:15 -0400, default <default@defaulter.net> > wrote: > >>On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote: >> >>>All, >>>I have two variacs, one is the typical type that does not isolate the >>>load from the mains. The other one I have isolates the load. When I use >>>the isolated one to power an inductive load, like a transformer, the >>>load really hums. The non-isolated variac does not do this. Anybody >>>know? >>>Thanks, >>>Eric >> >> >> >>There's no reason I can think of why that might happen, unless you don't >>really have an isolated variac and instead have a triac/SCR circuit on >>the primary of a transformer that provides the actual isolation. >> >>Some Navy gear I worked on had that (back in the 70's) and it was noisy. > I never took the thing apart. I bought it used so I don't have a > schematic of the insides. Looks like I should be taking it apart. That's > OK, I love looking under the hood. > Thanks, > Eric
I'm curious now -- report back if you find out. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
In article <37adnekLGLZ7OnvFnZ2dnUU7-VudnZ2d@giganews.com>, 
seemywebsite@myfooter.really says...
> > On Wed, 05 Apr 2017 17:44:33 -0700, etpm wrote: > > > On Wed, 05 Apr 2017 20:20:15 -0400, default <default@defaulter.net> > > wrote: > > > >>On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote: > >> > >>>All, > >>>I have two variacs, one is the typical type that does not isolate the > >>>load from the mains. The other one I have isolates the load. When I use > >>>the isolated one to power an inductive load, like a transformer, the > >>>load really hums. The non-isolated variac does not do this. Anybody > >>>know? > >>>Thanks, > >>>Eric > >> > >> > >> > >>There's no reason I can think of why that might happen, unless you don't > >>really have an isolated variac and instead have a triac/SCR circuit on > >>the primary of a transformer that provides the actual isolation. > >> > >>Some Navy gear I worked on had that (back in the 70's) and it was noisy. > > I never took the thing apart. I bought it used so I don't have a > > schematic of the insides. Looks like I should be taking it apart. That's > > OK, I love looking under the hood. > > Thanks, > > Eric > > I'm curious now -- report back if you find out.
could be a saturation issue
On Thu, 06 Apr 2017 17:06:34 -0400, M Philbrook wrote:

> In article <37adnekLGLZ7OnvFnZ2dnUU7-VudnZ2d@giganews.com>, > seemywebsite@myfooter.really says... >> >> On Wed, 05 Apr 2017 17:44:33 -0700, etpm wrote: >> >> > On Wed, 05 Apr 2017 20:20:15 -0400, default <default@defaulter.net> >> > wrote: >> > >> >>On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote: >> >> >> >>>All, >> >>>I have two variacs, one is the typical type that does not isolate >> >>>the load from the mains. The other one I have isolates the load. >> >>>When I use the isolated one to power an inductive load, like a >> >>>transformer, the load really hums. The non-isolated variac does not >> >>>do this. Anybody know? >> >>>Thanks, >> >>>Eric >> >> >> >> >> >> >> >>There's no reason I can think of why that might happen, unless you >> >>don't really have an isolated variac and instead have a triac/SCR >> >>circuit on the primary of a transformer that provides the actual >> >>isolation. >> >> >> >>Some Navy gear I worked on had that (back in the 70's) and it was >> >>noisy. >> > I never took the thing apart. I bought it used so I don't have a >> > schematic of the insides. Looks like I should be taking it apart. >> > That's OK, I love looking under the hood. >> > Thanks, >> > Eric >> >> I'm curious now -- report back if you find out. > > could be a saturation issue
I was thinking that, but triacs sound like a better candidate. It seems like the saturation would have to be BAD to make a follow-on device noisy. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On Wed, 05 Apr 2017 08:14:30 -0700, etpm@whidbey.com wrote:

>All, >I have two variacs, one is the typical type that does not isolate the >load from the mains. The other one I have isolates the load. When I >use the isolated one to power an inductive load, like a transformer, >the load really hums. The non-isolated variac does not do this. >Anybody know? >Thanks, >Eric
--- When the hum happens, is the transformer being fed by the VARIAC loaded? John Fields