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How to Power an AC device using DC- from an outlet Answered

I have a heating pad (Sunbeam, the kind of pad used for muscle aches etc.) that plugs into the wall (110v AC). It says on the pad that it draws 42watts. For various reasons, I'd like to have the heat generated by a DC source rather than AC.

I'm thinking maybe there is a way for me to plug some kind of converter into the wall (AC to DC) and then plug the heating pad into this which will still result in heat. I have no idea if this is possible. The pad has a switch for three different heat settings.

Can I do this? How?

Thanks for any suggestions!

26 Replies

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gopy97 (author)2016-08-04

i want to operate ac device with dc voltage? is it possible? if yes how can i make?

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Toga_Dan (author)gopy972016-08-05

heat: easy. Yes.

Light bulb: yes.

Motor with brushes: yes.

Induction motor : I don't think so.

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Downunder35m (author)gopy972016-08-04

Please give no details at all so everyone here leave a few hundred comments with possible options...

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Patrik (author)2008-05-18

Are you sure it doesn't already include a transformer and AC-DC converter? Most electric equipment that are meant to be in direct human contact (whether it's an electrical blanket or an electrical razor) use low-voltage DC out of safety considerations. Wouldn't want to spill your beer on that heating pad while you're wearing it, right?

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Goodhart (author)Patrik2008-05-19

Yeah, most of the ones I have seen are not *shiver* isolated with a transformer, and also *double shiver* not protected by a GFCI device. Maybe the newer ones are though...

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user

No transformer. If you go to your local Walgreen/Rite-Aide you'll find these heating pads. They simply plug into the wall.

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user

The vast majority of Walwarts I'm seeing on Google appear to be made for simple electronic devices, with low (mA) output, nor do any of them have a two pronged socket. I'm thinking I might have to use one with higher Amp output (I found one - at 4Amp) and modify it to accept a two pronged plug (maybe from Radioshack). Your simple two word comment reflects exactly the direction I was thinking, BTW. PKM, what do you think?

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NachoMahma (author)Patrik2008-05-18

. Most heating pads I've seen run on 110VAC (I'll assume 220 in other parts of the world). All the ones I've seen have the elements sealed (sonic welding?) in a heavy plastic bag to keep out moisture. . Not something I'd buy at a garage sale, but reasonably safe if used/maintained properly and thrown out when called for.

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PKM (author)2008-05-18

Firstly, as is quite clear from your comments below it seems you don't know very much about electricity. I strongly advise against any experimentation with mains electricity if you don't fully understand it, it's incredibly unwise.

That said, I have no idea why you want to run this heating pad off DC. From the point of view of energy efficiency I believe it won't gain you anything, if you have safety concerns about the device I suggest you dispose of it and get a replacement, and if you want to use it for heat in further experiments I don't see why AC or DC makes a difference.

The device is designed to run on mains AC, so unless it is very simple its controls (the selector switch) will probably not work properly if not fed mains AC of the right voltage. You can convert AC to DC with simple devices, but I have no idea why you want to do this, it probably won't work the way you may expect, may be dangerous to attempt and also may get you in trouble with your electricity supplier. The metering system they use makes assumptions about the way your power is used, which drawing DC from the wall may not follow.

So, in summary (or TL;DR if you like) I have no idea why you want to run this heating pad on DC, and without more information about what you are trying to achieve can't give any sensible advice other than don't.

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user

Dear PKM: I appreciate the safety concerns, but why do you care for the intellectual reasons for my doing it? It's no big deal, just want to see if it can be done. Here's what I want, plain and simple: I want the pad to generate heat, but from a DC source. Real simple. If you believe generating the same amount of heat from a DC source is dangerous- let me know (and why). The pad itself is in fine condition. I opened up the 3-switch warmth control switch, and it is regulated by what appear to be resistors. I'm not trying to do anything sophisticated electronically. I simply want to plug the pad into a DC source to generate roughly the same amount of heat. Either this can be done or not. If so, how?

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user

I didn't mean the intellectual reasons, I just wondered if there was any ulterior motive behind wanting to run it off DC. In school I was known for asking innocent-sounding questions that just happened to be a key part of some hideously dangerous scheme, usually involving something exploding, so I'm a little wary of unusual but simple-sounding requests for information. OK- so. The heating element will have a certain resistance, which will generate a certain amount of heat when fed 110V, whether that be AC or DC. With the stock heating element, halving the voltage will halve the current and so produce a quarter of the heat output. The simplest answer to your question, then, is feeding the stock element 110V DC should produce the desired effect. I know of no commercially available devices that produce 110V DC, but it is possible to produce with a bridge rectifier (Google around if you are unfamiliar with these) and a large-valued capacitor. As for the controls... I suspect it is not just resistance, as this would mean a significant proportion of the heat generated would be generated inside the controller at low settings. I would suspect the pad may have several elements of different resistances which the controller switches between, but without having the thing to look at I can't be sure. Does this answer your question a little better?

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PKM (author)PKM2008-05-19

I just saw your other comment below. If the controller is actually as simple as you describe, then you could probably plug the entire thing, not just the element, into a 110V DC source and use it exactly as normal. A lower voltage DC source would produce much less heat, though, as halving the voltage will quarter the power. My final note would be that, if you are thinking about electricity theoretically, 110V AC is not actually 110 volts, it's more like +155V to -155V but referring to it as 110 is an engineering convention to make power calculations simpler- look up "root mean squared power" for more details, it's beyond the scope of a simple explanation here.

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Goodhart (author)PKM2008-05-19

110V AC is not actually 110 volts, it's more like +155V to -155V

IIRC, most homes are fed a 220 (+-) v line in which the full phase is split into two (again approx) 110 lines. The range is normally (again IIRC) 110-125 (+ or -) v on each "portion" or phase.

Now, I could be talking apples to your oranges, as I am not as familiar as I should be with the Mains line, in my area.

Anyways, I agree with the "it matters little to the resistive element" whether it sees AC or DC; if not "resistive enough" (if it somehow became less resistive) it will be considered a short.

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PKM (author)2008-05-18

Well I thought I expressed my feelings in the comment below- I have no idea why you want to power the thing on DC from the wall. Patrik raises a good point- low voltage DC is better for heating elements so the controls of the pad may already be converting the 110V AC from the wall into low-voltage DC for the heating element, so it wouldn't use a wall-wart but would still internally use low-voltage DC. I have to ask, why do you actually want to run the thing from a DC supply in the first place?

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As I mentioned above in response to Patrik, there is no transformer/inverter/rectifier. The switch is extremely simple, with a single resistor on each wire controlling the amount of current. Nothing else. As mentioned, the reasons have nothing to do with electronics, but my desire to see if it can be done. Now that Patrik mentions it, one good reason would be to enhance safety.

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Unfortunately, if the heating pad is made to run at that AC voltage, running it at a lower DC voltage will simply generate far less heat.

I guess you could split the heating element in two, so you get two half-length elements that you can run at half the voltage. 55VDC is still nothing to sneeze at, though. (Not to mention that you are making it *less* safe by tapping into the well-insulated heating element halfway...)

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tech-king (author)2008-05-18

lets start with some basic math. 42 watts divided by 12 volts makes 3.5 amps. if you use a standard car battery and a DC-AC inverter, you get 225ah divided by 3.5 or 64.2. now factoring a 10% efficiency loss from the inverter would give you the total time this device could be run off a car battery without the battery being charged. you could buy a dc-ac inverter at Canadian tire and use that to run the bad off dc.

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NachoMahma (author)tech-king2008-05-18

. Why go to all that trouble? Why not just plug a 1:1 isolation transformer (for safety) into the mains and use a rectifier bridge? You'd lose a few volts, but should still be able to get ~35W of heat. If ripple is a problem, add a filter.

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user

Dear NachoMahma: Thanks for the reply. However, I'm not an electrician, could you please translate what you just wrote? (i.e. 1:1 Isolation transformer? Mains? Rectifier bridge?)

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user

. If you don't understand it, you don't need to be doing it. See PKM's comments about safety.
. If you decide to go ahead anyway (I won't help you, but Google is your friend), the heating element doesn't care if it's getting AC or DC - AC voltage is measured in RMS which gives the same heating effect as the same volts DC when applied to a resistive load. The control is most likely a simple mechanical switch that will wear slightly faster with DC (more arcing when you break the circuit), but should work very well if you leave it in one position. If the control has any electronics, it probably won't work on DC without modifications.

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Wow. A little bit on the condescending side Nacho. I didn't realize that one needs to be an electrician to ask for advice from you. I know a fair amount of electronics, but don't do it regularly. If you're not going to be helpful, then don't give advice that only electricians can understand (and an electrician would likely not need the advice in the first place).

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I'm following you, sort of,Watts divided by Volts gives you Amps = 3.5. And a standard 12V battery has 225 amp hours (ah), so 225ah divided by 3.5Amps tells me how many hours I could run the pad off of the battery (using an inverter lowers efficiency so approx 10% less). But why are we even talking about a car battery, since I want to run it off an outlet? Why suggest a DC to AC inverter when the key Q is how do I run the pad off of DC from an AC electrical outlet?

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LinuxH4x0r (author)2008-05-18

You can buy an inverter. These can be plugged into a 12 volt dc (car or car battery) power source and will supply you with ac power. You can buy one for as little as $10, or as much as $150 for the more powerful ones. Just make sure it is rated for more than you will need to use.

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user

(I have no need for a battery) I'm trying to run the pad on DC from an AC outlet. How do I do this?

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