Home Repair forum - Re: Is Knob-and-Tube *Always* Dangerous? OT & holler butt crap

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Re: Is Knob-and-Tube *Always* Dangerous? OT & holler butt crap

Tom Horne, Electrician

2006-07-28, 5:25 pm

z wrote:
> Tekkie® wrote:
>
Snip
> New electric blankets pair the wiring in opposite directions, so once gain there's negligible net field. (I got an old one I've always meant to try feeding with a rectified and somewhat filtered supply but I notice nobody else has tried this for some re
ason; any comments?)

The controls for the electric blanket will not handle DC. Switching for
DC loads must be much more robust then that for AC.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

Chris Lewis

2006-07-28, 5:25 pm

According to Tom Horne, Electrician <hornetd@mindspring.com>:

> The controls for the electric blanket will not handle DC. Switching for
> DC loads must be much more robust then that for AC.

Depends on how the controls work. I thought they were (still)
usually variable resistors with thermal cutoffs to act as
cutoff timers. That'd work just as well on DC.

If, on the other hand, they're using SCRs or Triacs and the like,
you'd have to change the controller.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

Mark Lloyd

2006-07-28, 5:25 pm

On Fri, 28 Jul 2006 19:44:09 -0000, clewis@nortelnetworks.com (Chris
Lewis) wrote:

>According to Tom Horne, Electrician <hornetd@mindspring.com>:
>
>
>Depends on how the controls work. I thought they were (still)
>usually variable resistors with thermal cutoffs to act as
>cutoff timers. That'd work just as well on DC.
>

A variable resistor here would be very inefficient.

>If, on the other hand, they're using SCRs or Triacs and the like,
>you'd have to change the controller.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Democracy is two wolves and a lamb voting on what
to have for lunch. Liberty is a well armed lamb
contesting the vote." - Benjamin Franklin

Tom Horne, Electrician

2006-07-29, 8:25 pm

Chris Lewis wrote:
> According to Tom Horne, Electrician <hornetd@mindspring.com>:
>
>
> Depends on how the controls work. I thought they were (still)
> usually variable resistors with thermal cutoffs to act as
> cutoff timers. That'd work just as well on DC.
>
> If, on the other hand, they're using SCRs or Triacs and the like,
> you'd have to change the controller.

No matter what the control is the actual switching contacts will be
damaged if DC is applied to them because of arcing at the contacts. So
unless you are talking about a variable resister that never applies full
voltage to a cold circuit the control contacts will destroy them selves.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

Chris Lewis

2006-07-31, 1:25 pm

According to Tom Horne, Electrician <hornetd@mindspring.com>:
> Chris Lewis wrote:
>
> No matter what the control is the actual switching contacts will be
> damaged if DC is applied to them because of arcing at the contacts. So
> unless you are talking about a variable resister that never applies full
> voltage to a cold circuit the control contacts will destroy them selves.

Point taken - if it uses mechanical switching. However, the wattage of
these devices is quite low, and it wouldn't be surprising to me to
find that the contacts are sufficient for such current levels with
both AC and DC.

Yes, DC is harder on switch contacts than AC. But at these power
levels, it's not that big a difference.

Of course, it voids the UL/CSA approval (unless explicitly stated
otherwise) it's probably best to not try it anyway. People tend
to get rather intimate with their heating blankets, so problems
tend to have nastier consequences.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

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