[sci.electronics] AC-DC at wall current voltage

weverka@boulder.colorado.edu (Robert T. Weverka) (10/06/90)

Looking for assistance
I want to convert an AC electric blanket to DC.
The controller is a bimettalic thermastat that makes contact when cold.
When contact is made the 60 Hz wall voltage is dropped accross the blanket
resistance of 100 ohms heating the bed, and the same 60 Hz wall voltage is
dropped accross a 47000 ohm resistor in the controller (presumably to heat the
controller).
  I think I have to keep the voltage up so that the blanket still runs at
about 100 Watts.  This means I need moderate efficiency and can't use a step
down transformer.
  My thought was to put an AC-DC converter between the controller and the
blanket.  I can get the full wave bridge, but a 10,000 microfarad capacitor
with a rating of 200 Volts (recall 110 volt is the RMS wall current) is hard
to find and may be bigger than I'm bargaining for.  Is it dangerous to rectify
wall current without a transformer for isolation?  If so, is it possible to
find a one to one transformer ratio?
  In addition to the above questions I would appreciate advice on either:
     1) where do I get the capacitor of 10,000 microF 200 V rating  or
     2) an alternate means of efficient wall current to DC conversion.
My electrical background consists of repeated browsing of Horowitz and Hill
yet I can't find what I want there.

Thanks in advance,  -Ted            email  weverka@boulder.colorado.edu

PS.  Yes the desire to convert the blanket is motivated by the cancer scare
of 60Hz magnetic fields.  Yes I know that the evidence is not conclusive 
(for a review see 9/90 IEEE Spectrum), but this is a better safe than sorry
type situation to be installed in my kid's bed.

siegman@sierra.STANFORD.EDU (siegman) (10/07/90)

In article <27666@boulder.Colorado.EDU> weverka@sashimi.Colorado.EDU (Robert T. Weverka) writes:
>
>I want to convert an AC electric blanket to DC.
>
> . . .
>
>PS.  Yes the desire to convert the blanket is motivated by the cancer scare
>of 60Hz magnetic fields.  Yes I know that the evidence is not conclusive 
>(for a review see 9/90 IEEE Spectrum), but this is a better safe than sorry
>type situation to be installed in my kid's bed.

The chances of YOUR doing something wrong in a home-brew operation
like this, leading to electrocution, fire, what have you, seem to me
enormously greater than any risk from the EM fields in the blanket.
Who knows whether the thermostatic and other controls in the blanket
depend on the voltage being ac instead of dc, and so on?  All in all,
a thoroughly bad idea.

(And by the way, how do we know that the DC magnetic fields from a DC
blanket don't do harm?)

wb8foz@mthvax.cs.miami.edu (David Lesher) (10/07/90)

{plan to rectify ac-->dc with cap. filter}

>  This is a BAD idea. If you size the capacitor large enough to get
>rid of the ripple you will be running the blanket at the AC peak
>voltage not 110. This will mean it will be running at about twice
>its rated power. The controller will probably still work, but the 
>resistance wire in the blankets will get hot too quickly. You are
>asking for a fire! Please don't do this for safety reasons!!!

Sorry, I disagree. 

You only charge the cap up to peak if the load is not there.
With a load, the cap cannot stay at peak.  What you are implying
is that you somehow get double the energy from the wall while
drawing the same current.

On the other hand, many thermostats are rated AC only, and lack
the spacing to break a capacitive DC load. 

Maybe what he really wants to do is make a blanket with many
feet of embedded plastic tubing, and run warm water thru it.
Use a small pump and external heater for the water.

-- 
A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu 
& no one will talk to a host that's close............(305) 255-RTFM
Unless the host (that isn't close)......................pob 570-335
is busy, hung or dead....................................33257-0335

chuck@mitlns.mit.edu (10/07/90)

-Message-Text-Follows-
In article <27666@boulder.Colorado.EDU>, weverka@boulder.colorado.edu (Robert T. Weverka) writes...

> 
>Looking for assistance
>I want to convert an AC electric blanket to DC.
>The controller is a bimettalic thermastat that makes contact when cold.
>When contact is made the 60 Hz wall voltage is dropped accross the blanket
>resistance of 100 ohms heating the bed, and the same 60 Hz wall voltage is
>dropped accross a 47000 ohm resistor in the controller (presumably to heat the
>controller).
>  I think I have to keep the voltage up so that the blanket still runs at
>about 100 Watts.  This means I need moderate efficiency and can't use a step
>down transformer.

  Yes, you want to supply a DC voltage equal to the RMS outlet voltage
in the range of 110-120 volts.


>  My thought was to put an AC-DC converter between the controller and the
>blanket.  I can get the full wave bridge, but a 10,000 microfarad capacitor
>with a rating of 200 Volts (recall 110 volt is the RMS wall current) is hard
>to find and may be bigger than I'm bargaining for.  Is it dangerous to rectify
>wall current without a transformer for isolation?  If so, is it possible to
>find a one to one transformer ratio?

  An isolation transformer makes equipment much much safer since the 
power will no longer conduct to ground if the secondary is not grounded. 
It can also provide some limited surge protection. However rectifying
AC doesn't make it more dangerous from a electrical perspective, I don't
know if there might be a medical reason for DC to be worse than AC.

>  In addition to the above questions I would appreciate advice on either:
>     1) where do I get the capacitor of 10,000 microF 200 V rating  or

  This is a BAD idea. If you size the capacitor large enough to get
rid of the ripple you will be running the balnket at the AC peak
voltage not 110. This will mean it will be running at about twice
its rated power. The controller will probably still work, but the 
resistance wire in the blankets will get hot too quickly. You are
asking for a fire! Please don't do this for saftey reasons!!!

>     2) an alternate means of efficient wall current to DC conversion.
>My electrical background consists of repeated browsing of Horowitz and Hill
>yet I can't find what I want there.
> 

  Given that your disire is to INCREASE the saftey of your kid, and that
while inconclusive in any case the risk of an electric blanket is small
much less than driving in a car for instance. Then ANY lack of forsight
in the design of the blanket power supply is likely to be far more 
dangerous. I think given your objectives your only option is to not
use an eletric blanket or buy something designed by professionals.

  FLAMES please note:  I don't think there is anything unsafe about
electronics hacking at home. I do a lot of it. But I don't think that,
it is as safe as an Electric blanket. That is there MAY be a 1/100,000
chance of getting cancer from an electric blanket used for years. This
is a very small probability. Although I think home electronics hacking
is safe, I think more than 1/100,000 electronics hackers accidently kill 
or seriously hurt themselves in their liftimes. This is particularly true
for something used by a child who doesn't know enough about electricty
to be safe if something goes wrong.


> 
>PS.  Yes the desire to convert the blanket is motivated by the cancer scare
>of 60Hz magnetic fields.  Yes I know that the evidence is not conclusive 
>(for a review see 9/90 IEEE Spectrum), but this is a better safe than sorry
>type situation to be installed in my kid's bed.

				Chuck@mitlns.mit.edu

larry@kitty.UUCP (Larry Lippman) (10/07/90)

In article <1990Oct6.220317.18337@mthvax.cs.miami.edu>, wb8foz@mthvax.cs.miami.edu (David Lesher) writes:
> Maybe what he really wants to do is make a blanket with many
> feet of embedded plastic tubing, and run warm water thru it.
> Use a small pump and external heater for the water.

	This type of blanket using water circulation is already available.
Not only is there heating, but cooling is also provided.  A typical
manufacturer is Aquamatic, which produces various models under the
tradename of "K-Thermia".

	No danger of electric shock or magnetic fields with one of these
suckers!

	Available at your local hospital supply distributor, starting
price around $ 4K.  Explosion-proof models for use in environments
containing ether or cyclopropane are avilable at extra cost.

	:-)

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635                  {utzoo, uunet}!/      \aerion!larry

wolff@duteca (wolff) (10/08/90)

siegman@sierra.STANFORD.EDU (siegman) writes:

>In article <27666@boulder.Colorado.EDU> weverka@sashimi.Colorado.EDU (Robert T. Weverka) writes:
>>
>>I want to convert an AC electric blanket to DC.
>>

>The chances of YOUR doing something wrong in a home-brew operation
>like this, leading to electrocution, fire, what have you, seem to me
>enormously greater than any risk from the EM fields in the blanket.

I agree completely....

>(And by the way, how do we know that the DC magnetic fields from a DC
>blanket don't do harm?)

Here in holland a few days ago a newspaper reported reduced balding effects
from a hairdryer cap which was equipped with several probes that were fed from
a simple 9v battery. This was tested on balding men where one
group was subjected to the haircaps only, while another group the 9v batteries
were indeed hooked up. The balding process promptly stopped in the group of
men that were subjected to the electric DC fields for (if I remember correctly)
about half an hour a week.
Extrapolating a 200v DC field might cause enormous hairgrow on your child :-)!

						Roger Wolff 

chuck@mitlns.mit.edu (10/08/90)

-Message-Text-Follows-
In article <1990Oct6.220317.18337@mthvax.cs.miami.edu>, wb8foz@mthvax.cs.miami.edu (David Lesher) writes...
> 
>{plan to rectify ac-->dc with cap. filter}
> 
>>  This is a BAD idea. If you size the capacitor large enough to get
>>rid of the ripple you will be running the blanket at the AC peak
>>voltage not 110. This will mean it will be running at about twice
>>its rated power. The controller will probably still work, but the 
>>resistance wire in the blankets will get hot too quickly. You are
>>asking for a fire! Please don't do this for safety reasons!!!
> 
>Sorry, I disagree. 
> 
>You only charge the cap up to peak if the load is not there.
>With a load, the cap cannot stay at peak.  What you are implying
>is that you somehow get double the energy from the wall while
>drawing the same current.
> 

    Not at all. Unless there is some current limiting in the
supply, such as winding resitance to limit the current, then
you will get 20 or 30 amps at the peak to charge the cap and
zero the rest of the time. Since he is hooked up to a very
large transformer with likely 100 amp service into the house
he has to draw a lot of current to lower the peak voltage.

   If the cap is near peak sometime during the cycle then there
are 2 possibilites. If it is small compared to the load
and he will get lots of ripple, and a lower average voltage.
If he has lots of ripple then he has a large AC component to
his power, and he hasn't eliminated those 60hz fields he wanted
to get rid of. If on the other hand the cap is large enough
to sustain the voltage with little drop for one half cycle
then it will always be near the peak voltage. The power equation
still works becuase his RMS voltage will be 1.4 times the
aveage line voltage, and thus the RMS current = Vrms/R
will also be 1.4 times as high.

  (1.4 is short hand for square root of 2)

   (sqrt2*V0)*(sqrt(2)*I0)= 2*V0*I0 == twice the power.

					Chuck@mitlns.mit.edu

jon_sree@world.std.com (Jon Sreekanth) (10/22/90)

In article <1990Oct7.214431.25748@athena.mit.edu> chuck@mitlns.mit.edu writes:

   In article <1990Oct6.220317.18337@mthvax.cs.miami.edu>, wb8foz@mthvax.cs.miami.edu (David Lesher) writes...
   > 
   >{plan to rectify ac-->dc with cap. filter}
   > 
   >>  This is a BAD idea. If you size the capacitor large enough to get
   >>rid of the ripple you will be running the blanket at the AC peak
   >>voltage not 110. This will mean it will be running at about twice


>       Not at all. Unless there is some current limiting in the
>   supply, such as winding resitance to limit the current, then
>   you will get 20 or 30 amps at the peak to charge the cap and
 
>      (sqrt2*V0)*(sqrt(2)*I0)= 2*V0*I0 == twice the power.
>
>					   Chuck@mitlns.mit.OBedu

Rectifying line AC and putting a capacitor in parallel will in fact 
increase the average power delivered to a resistive load. For any size
of capacitor and resistive load, it's easy enough to calculate the
power delivered by integrating the (square of the) voltage waveform.
The area above the sine wave is the extra boost. 

          ..          .. 
       .     .*    .     .*
     .        . * *        . *
   .           . .          .   *
  ._____________.____________.

      ... is full wave rectified waveform, without any cap
      *** is with a capacitor (not very large compared to load resistance)

About seven years ago, back in India, I used this principle to make a
power booster for a photographic enlarger lamp. Line regulation was lousy,
and the bulb would frequently be too dim. Rather than spend the bucks for
a real autotransformer based regulator, I rigged up a bridge with 4 * 1N4001's
and a salvaged capacitor (100 u or so, from an old valve radio, probably), 
this weird looking DC waveform went through a series transistor and some 
feedback circuit to provide a reasonably constant brightness. 

(It never worked too well, by the way, I must have omitted something. The 
series transistor kept blowing; and that got expensive :-)

/ Jon Sreekanth

billn@hpcvaac.cv.hp.com (bill nelson) (11/01/90)

(Robert T. Weverka) /  8:45 am  Oct  6, 1990 / writes:

>PS.  Yes the desire to convert the blanket is motivated by the cancer scare
>of 60Hz magnetic fields.  Yes I know that the evidence is not conclusive 
>(for a review see 9/90 IEEE Spectrum), but this is a better safe than sorry
>type situation to be installed in my kid's bed.

If you really want safety for your kid - get rid of the electric blanket.
No one has ever been electrocuted by a plain blanket.
 
Bill