Variac question

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

 

[George Herold]

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.

 

[default]

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.

 

[default]

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

 

[Jim Thompson]

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 | |
| Voice480)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.

 

[Tim Wescott]

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!

 

[M Philbrook]

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

 

[Tim Wescott]

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!

 

[John Fields]

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

 

On Sun, 16 Apr 2017 17:34:08 -0500, John Fields
<jfields@austininstruments.com> 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

---
When the hum happens, is the transformer being fed by the VARIAC
loaded?

John Fields

Yeah. The transformer in this case is in a battery charger. I also,
just to see, plugged my regular non- isloated variac into the isolated
one. BIG hum. I have been too busy the last couple weeks to take the
isolated one apart but I'll let everyone know what it looks like
inside later this week.
Eric

 

[Phil Allison]

Tim Wescott wrote:

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.

** If the OP's unit does use a triac, the likely cause is triac misfiring leading to "half waving" of the AC voltage.

Half waving is more likely with inductive loads ( lagging phase angle) and two transformers in series qualifies for that.

Only happens with pulse firing rather than continuous or hard firing.


..... Phil

 

Tim Wescott wrote:

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.

Humming noises involve using too little copper on the load side (skimping). I'd try increase the diameter of the white or grey wires there.

 

[rickman]

On 4/17/2017 12:42 PM, etpm@whidbey.com wrote:

On Sun, 16 Apr 2017 17:34:08 -0500, John Fields
jfields@austininstruments.com> 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

---
When the hum happens, is the transformer being fed by the VARIAC
loaded?

John Fields

Yeah. The transformer in this case is in a battery charger. I also,
just to see, plugged my regular non- isloated variac into the isolated
one. BIG hum. I have been too busy the last couple weeks to take the
isolated one apart but I'll let everyone know what it looks like
inside later this week.

I'm not sure you need to take it apart. If it is similar to an
autotransformer but with an isolated primary, it will have a largish
knob in the center of the transformer that is obviously turning a wiper
across the coil. If it is a potentiometer driving a triac type circuit
the knob will likely be a small knob or slider. If the distinction in
your case is not clear, then I guess you will need to take it apart.

--

Rick C

 

On Mon, 1 May 2017 21:21:34 -0400, rickman <gnuarm@gmail.com> wrote:

On 4/17/2017 12:42 PM, etpm@whidbey.com wrote:
On Sun, 16 Apr 2017 17:34:08 -0500, John Fields
jfields@austininstruments.com> 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

---
When the hum happens, is the transformer being fed by the VARIAC
loaded?

John Fields

Yeah. The transformer in this case is in a battery charger. I also,
just to see, plugged my regular non- isloated variac into the isolated
one. BIG hum. I have been too busy the last couple weeks to take the
isolated one apart but I'll let everyone know what it looks like
inside later this week.

I'm not sure you need to take it apart. If it is similar to an
autotransformer but with an isolated primary, it will have a largish
knob in the center of the transformer that is obviously turning a wiper
across the coil. If it is a potentiometer driving a triac type circuit
the knob will likely be a small knob or slider. If the distinction in
your case is not clear, then I guess you will need to take it apart.

Took it apart last night. As you said it has the isolated primary. No
semi-conductors inside, just wires and wipers.
Eric

 

>"Took it apart last night. As you said it has the isolated primary. No
semi-conductors inside, just wires and wipers.
Eric "

If you actually have an isolated variac hold on to it. Most of them are simple autotransformers. They work but hot ground is still hot. An isolated one allows you to do more, like work on the primary side of a power supply without floating the scope.

With all these old amps around here I am pretty sure I could dig up something that would put out 120 VAC isolated, but being the lazy asshole I am I just float the scope.

Variacs are actually losing their usefulness. Some modern SMPSes won't start if the voltage comes up slow. There are actually some circuits that will self destruct if brought up slow on a variac. But I certainly would not throw it out.

 

On Wed, 3 May 2017 10:10:11 -0700 (PDT), jurb6006@gmail.com wrote:

"Took it apart last night. As you said it has the isolated primary. No
semi-conductors inside, just wires and wipers.
Eric "

If you actually have an isolated variac hold on to it. Most of them are simple autotransformers. They work but hot ground is still hot. An isolated one allows you to do more, like work on the primary side of a power supply without floating the scope.

With all these old amps around here I am pretty sure I could dig up something that would put out 120 VAC isolated, but being the lazy asshole I am I just float the scope.

Variacs are actually losing their usefulness. Some modern SMPSes won't start if the voltage comes up slow. There are actually some circuits that will self destruct if brought up slow on a variac. But I certainly would not throw it out.

It sez it's isolated. And I am not going to get rid of it. I have
found uses for both of my variacs over the years. One function they
serve is to remnd me to pick things up. Hitting one with your little
toe doesn't have as near as much effect on the variac as it does on
your toe.
Eric