Converting a household space heater to DC

[Garbus]

Hello,

Basically, I use a portable electric space heater to keep my room warm during the winter, and it works quite well, except it makes a very loud hum that keeps me awake, so my options right now are poor sleep due to hum or having my room drop to about 12 C over night if I turn the heater off. For a number of reasons I can't use my electric baseboards, otherwise this would be a non issue.

Now, I assume the awful humming is because of the 60Hz AC power going through the heater (its a simple convection heater, so no fan or anything to cause noise), so my line of thinking is to install a bridge rectifier between the wall socket and the heater, thus turning the AC into DC and hopefully eliminating the hum. Theres no electronics on this heater, as far as I can tell, there's just a power control that switches more or less banks of heating elements on, so it should work on DC right?

If I get a proper spec rectifier that can handle the voltage current, should this work to eliminate the hum? Are there any issues I am overlooking with turning AC into DC?

Thanks!

 

[FactChecker]

I hope you don't spend a lot on a rectifier that can handle that load and find out that it hums.

 

[Asymptotic]

Can you provide heater make and model number?

Have you attempted to determine exactly where the sound is coming from? The noise source could be as simple as a loose section of sheet metal guarding.

 

[DaveE]

It will probably be ok. You will get 120Hz ripple in the DC but with much lower amplitude than the original 60Hz. I'm assuming you mean a full wave rectifier, of course. Make sure you put the rectifier after any switches, fuses etc. those parts probably can't handle switching DC current.
Also pay attention to the heat sink requirements of the rectifier, the current rating is usually a proxy for operating temperature, or they have specified the temperature or heat sink requirements for the current rating. Diodes usually fail from getting to hot from the current, not from the instantaneous current value. Recognize that the place you may want to mount the rectifier might already be hot from the heating element thus requiring a better heat sink than you might expect.
However, I wonder if it wouldn't be cheaper and easier to buy a different heater.
[edit] No! not between the wall socket and the heater, between the heater element and everything else. Or, replace the switches and fuses with DC rated parts.

 

[tech99]

Be careful about insurance cover/liability if you modify it.

 

[tech99]

I think your present hum is at 120 Hz, so the rectifier will make no difference.
If we suppose the hum is caused by the wire expanding and contracting over the cycle, then it receives two maxima per cycle, giving 120 Hz.
On the other hand, if we assume the hum comes from the magnetic attraction between wires, then it will be maximum at current peaks. As both wires involved in this have the same alternating current, there are two maxima per cycle. Further, the pull is dependent on current squared.
If you reduce the current by half, then the thermal variations will be only a quarter. The magnetic variations depend on current squared, so will also be a quarter. If the sound amplitude is a quarter, then that is a sixteenth acoustic power. This is 12dB, so it will sound about half as loud.

 

[anorlunda]

I like @Asymptotic's suggestion. You have many 60 Hz appliances that don't make an annoying hum. The difference between those that do and those that don't is something vibrating, probably something loose.

I also use a 60 Hz space heater. It makes no hum that I can hear.

 

[Garbus]

Perhaps then it might be best to just buy a new heater then. Ah well.

 

[Tom.G]

Perhaps then it might be best to just buy a new heater then. Ah well.

Or unplug it and tap it or shake it to find the offending piece; then, if applicable, bend the offending piece to shut it up. If you can't find the problem, take the beast apart and see what you can find; after all, it's no good the way it is and maybe you'll learn something for the next time.

Cheers,
Tom