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