12VDC fans in 120VAC mains ?

[Externet]

Hi all.
Many times I have built circuits to turn on LEDS with 120VAC directly, no transformer/adapter.

Phase--------Capacitor-----------LEDS------------neutral

Where "LEDS" is a couple of counterparalleled leds, and the "Capacitor" typically 0.33uF/250VAC paralleled itself to a 0.5Mohm. Works great, all times.

What considerations are to be taken in account to replace the leds with two counterparalleled 12VDC 'muffin' fans as those from PC power supplies - 12V - 0.1A ?

Larger capacity, like >4uF/250VAC to supply the needed current, fans fed in series trough ~1A 1N4004 diodes; and... ?

[haklesup]

Well, the LED is a rectifier in and of itself. Even though the current reverses 60 times per second, thats fast enough to not see any flicket.

A DC motor will just try to move one way then immedietly after try to move the other way. Even if you do drop enough voltage and limit the current so the fans don't burn up, they will still just sit there vibrating or humming but not spinning. Series or parallel, you are still going to need at least one diode but if you want good torque and airflow, you will probably want 4 diodes to make a full wave rectifier. YOu can drop AC voltage before the recitfier with the caps.

Notmally one would just get an AC fan

[jollyrgr]

Be safe and do this with a wall wart! Putting LEDs or incandescent bulbs in series "Christmas Light" style is okay for them. Don't risk doing this with a muffin fan. Since we are talking muffin fans you don't need a regulated supply but it won't hurt. If these fans will in any way come in contact with people you should NOT try this as the risk of electric shock is too great.

If you REALLY need to do this without a transformer (even switching power supplies have an isolation transformer) you need to look at a "transformerless power supply". (Google these terms.)

Typically you have on the input side:

AC line HOT ---- Fuse------resistor------- ~ bridge rectifier ~ ------non polarized capacitor ------------ AC Line Neutral

On the output of the bridge rectifier you have a 12V zener diode (band toward + of bridge) and an capacitor.

DO NOT use this circuit such that the "DC" side is available to people! This should only be used if this supply is sealed in a case and no fingers can come in contact with the DC buss or any of the fans or devices being powered.

Using a transformerless supply is very dangerous and wall warts are VERY cheap (likely you can scavenge one cheaper than the bridge itself).