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From: chuck@mitlns.mit.edu
Newsgroups: sci.electronics
Subject: Re: AC-DC at wall current voltage
Message-ID: <1990Oct6.193138.6583@athena.mit.edu>
Date: 6 Oct 90 22:04:14 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: M.I.T. Lab for Nuclear Science
Lines: 71

-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