How can I safely transform 120v power to 12v for a fan?

[John Galt]

Hi there, I'm new here. I'm working on a project and I have a technical question regarding the wiring of the components.

First, is wall power AC or DC? It seems that if there is a positive and negative terminal in the wall outlet, it would be DC (like a battery). But then again my understanding is that all electric power has to flow from opposite charged terminals.

This project will use 120v wall outlet power wired in parallel to a heating element and and a 12v fan. The heating element will use 120v power but the fan, being rated at 12v, needs to transform that 120v to 12v. I've been searching for step down transformers but it seems that only high voltage transformers can be found. This makes me believe that a 'step down transformer' exists in the electronics industry, its just not referred to as such.

I was just wondering, what type of device should be used in series with the 12v fan to transform the 120v power so it doesn't destroy the fan? I've opened up a few voltage transforming electronic devices around the house and taken a look at the components but I don't know what the devices are called so I can't begin ordering them from electronics suppliers.

Any help would be appreciated.

 

[berkeman]

Hi John, Welcome to PF. I'm a little worried about you having such a basic knowledge of electricity and trying to do a project involving the AC mains. Working with AC mains power is very dangerous for you, plus, if you make the device incorrectly (not following Underwriter Labs standards), you can easily create a fire hazard or shock hazard.

What is the context of this project? Is it for school or for you own personal hobby? What is your electrical and electronics educational background? What projects have you built up to now?

Okay, I'll answer your questions anyway, but please keep in mind my concerns and warnings above. AC mains in U.S. homes generally consists of two Hot wires and one Neutral wire. The two Hot wires have 240Vrms between them (that's an AC signal with 240Vrms = 676Vpp). You can either use them as the differential power to a device (240Vrms is usally used for the higher-power devices like clothes driers), or you can use one Hot and the Neutral line to give you 120Vrms.

By code, the Neutral line is connected to Earth ground at the fuse panel, so if you measure the voltage on the Hot lead at an AC mains receptacle, you will measure about 120Vrms to Neutral and also to Ground in the plug. Depending on how far you are from the fuse panel, you might measure a small AC voltage on Neutral with respect to Ground in the receptacle.

To make a heater + fan like you describe (and meet UL safety standards), you would probably use a 3-prong power cord and a metal enclosure. The metal enclosure must be grounded to the ground lead in the power plug in a particular way in order to meet UL safety standards. You must put an appropriate fuse in series with the Hot lead as it enters your metal enclosure. After the fuse and a switch (in series with the Hot also, by code), you would put your heating element across Hot and Neutral. In parallel with the heating element, you would put the input (primary side) of a 120Vrms to 12Vrms step-down transformer, and the output of that transformer would be connected to the fan.

You can find 120Vrms --> 12Vrms step-down transformers at handy distribution houses like digikey.com or mouser.com, or even at your local Radio Shack.

I'm not kidding about the dangers involved with this project, John. Especially if you are just starting to learn about electricity and electronics. Please be very careful, and be sure to follow the National Electric Code (NEC) and UL standards whenever you work with AC mains power.


EDIT -- BTW, you can use a 2-prong power cord (ungrounded), but you must meat the UL standard requirements for "double-insulated" construction. That's more complicated, so I recommended the 3-prong power cord and grounded enclosure construction method above.

 

[Danger]

If I might make a recommendation to simplify things a bit...
Since the fan probably requires DC and would therefore need a rectifier along with the transformer, my personal approach would be to connect a standard 120V socket (such as the end of an extension cord) in parallel with the heater, then simply plug a 12VDC adaptor into it to run the fan. That would eliminate the possibility of mis-wiring the low voltage side. I cede to berkeman if he disagrees, since he's the expert, and definitely also stress the safety aspects that he pointed out.

 

[John Galt]

Berkeman, thank you for the warnings, I am not going to attempt this project until I have an OK on my wiring schematic. I will settle for no less than UL safety standard as I do not want to injure myself.

For the record, I am a hobbyist with VERY limited physics training (high school and college intro courses). Other electrical projects I've completed are multiple light bulb wiring which has been running constantly for many many KwH and a homemade temp controllable herbal vaporizer, this project is an extention of the vaporizer project. You provided a lot of useful information :) that will be better understood when I've done some research. I'm starting a book on non-commutator generators so hopefully I'll understand AC power a bit better after the read.

Danger, I have the option of deciding whether to use an AC or DC fan, both are rated at 12v. I want to use the most cost effective fan but I understand the easiest one to self-wire can also be the best choice. My original plan was to do exactly as you said, find a company that supplies 12vdc adapters and wire that in series with the 12vdc fan. I just can't find them anywhere. Do you know a good place to start looking??

Is my best (safest) bet to go ahead and go with the 120vac through the heating element and then wire a parallel 12vdc fan in series with a 120vac to 12vdc adapter??


Edit* I just found a directory to suppliers of this sort of device. They might carry what I need, if not, you guys know of a good source; internet or not?

Thanks guys.

 

[berkeman]

Digikey.com and Mouser.com are good sources for everything from discrete components to power supplies to fans to, well, just about everything.

You also have the option of using a 120Vac fan, as long as you observe all the AC Mains safety stuff that I mentioned before.

 

[NoTime]

You also have the option of using a 120Vac fan, as long as you observe all the AC Mains safety stuff that I mentioned before.

I certainly second berkeman's safety concerns.

There is also another option.
With the proper choice of heater and/or fan.
You can use the heater as a droping resistor feeding
a bridge rectifier to feed a 12v DC fan. Saves a transformer.

Also you will want a thermal cutout for the heater, so you don't get a catastrophic melt down.