Mains Voltage and Resistors.

I personally am too scared to mess with mains voltages, i always buy a power supply to fit the job always.

But, in theory, is it safe to use resistors (in series with a fuse and diode rectifiers and a cap to help filter it) with 120/240 applications? if so...

Two highish valued resistors (voltage divided) with just enough current (at roughly 4.5volts) to pass to power a microprocessor? -

my calculator says… 300VDC (after being rectified from 240vac)

R1 = 13k
R2 = 680 ohms

Voltage out 4.5volts (from 300vdc) using a voltage divider, the resistive load is 280ohms, what ever that equates to in amps, 4.5/280 = 0.0160 amps

/ 0.0723 watts.

How’s that sound? - but again, i still would not do it even if you told me it was safe :slight_smile:

To calculate the peak value of a sine wave, multiply the RMS value by square root of two. This will be the voltage that the filter capacitor will charge up to under no-load conditions. 240V * ?2 = 339V.

Resistive voltage dividers hardly ever make good power supplies.

A primary risk in powering anything from mains is the shock hazard that can result if a person comes in contact with the circuit while standing on a grounded surface, etc. The transformer used in typical step-down AC-to-DC supplies provides isolation from the mains.

The so-called All American Five tube radios were notorious in that they had no transformer isolation, and if the plug was not polarized, then the chassis could be connected to the hot side of the mains.

Aye, that's all well and good (and thank's for the sinewave formula, might come in handy one day) ....

but... go to the middle of a quarry in the desert with no signs of life anywhere near for 300 miles! - run a 240vac generator, run a 100 feet cable to the middle of nowhere, create that circuit on a breadboard, walk back 100 feet to the van/car start the generator and then quickly stand behind a 6ft high shield.......

Seriously now.... Would you're Arduino Board work using 2 voltage divders a Cap and a Rectifier diode?

Yes or no? (Standing 100feet away behind a blast shield) ...

cjdelphi: my calculator says.... 300VDC (after being rectified from 240vac)

R1 = 13k R2 = 680 ohms

Voltage out 4.5volts (from 300vdc) using a voltage divider, the resistive load is 280ohms, what ever that equates to in amps, 4.5/280 = 0.0160 amps

/ 0.0723 watts.

How's that sound? - but again, i still would not do it even if you told me it was safe :)

Firstly that 13k resistor will be dissipating 7W, so it better be rated for that power.

Secondly the load will not be a fixed 280 ohms if its a microprocessor, you will need regulate, so a 7805 or something would be needed. You need to check the highest current needed won't drop the input voltage to the regulator too low, and that the input voltage can't go too high either.

If you've got 240V ac anyway, a wall-wart is a simple safe reliable solution to the problem surely?

It depends. A resistive voltage divider only provides a constant voltage if the load that it is connected to draws a constant current. It has to be designed with a specific load in mind. If whatever the Arduino is doing results in it using a relatively constant current then it could work. But it might not work well for another Arduino project that has different current requirements.

It's a poor design for a power supply. So poor that it can hardly be called a power supply. Even a Zener diode would be preferable. Attempting such stunts will result in your EE friends laughing at you.

Curiosity, Not Reality....