Calculation for resistors

Hi Guys,

Can anyone please remind me of the calculation required for choosing a resistor?

In this instance, I want to power the 'on / off' LED on my pultec from the 240v mains rail. The LED is 24v.

Also, is this safe (assuming I insulate it properly)? I could take a 15v rail from the secondary of one of the torroids, but this would mean messier wiring, and a dim light. The 240v rail is already running to the front of the case for the main power switch.

Thanks
Jake

Hi,

don't mess with mains! I would never power a light in front of the case with it, it's dangerous! Use 15 volts for this, there are calculations one the net for all kind of voltages and leds.
And try to think about this little things before you start a project, this way you can buy appropriate material.

Miha

I thought that might get said.

Just so I know... why is it so dangerous? Is it because a resistor isnt reliable for this kind of large voltage step down?

Jake

rotation said:

Hi,

don't mess with mains! I would never power a light in front of the case with it, it's dangerous! Use 15 volts for this, there are calculations one the net for all kind of voltages and leds.
And try to think about this little things before you start a project, this way you can buy appropriate material.

Miha

Click to expand...

YES , IT'S DANGEROUS !!!

Most LED's don't need anywhere near 24 V and they use DC. So maybe this is some sort of lamp or lighting system.  So if this LED does really need 24 V AC,  I would use a voltage step-down transformer.

snipsnip said:

I thought that might get said.

Just so I know... why is it so dangerous? Is it because a resistor isnt reliable for this kind of large voltage step down?

Jake

Click to expand...

Don't use mains voltage for anything other than going right into a power transformer.

230v to 1,5 (or 24V ???) volt from a led is a huge stepdown and will require a huge series resistor. That's pure waste of heat (electricity)

here's a calculator

http://led.linear1.org/1led.wiz

Cheers Guys,

After actually thinking about this, I realised I was being pretty stupid. Always good to have it confirmed though! 

Thanks a lot
Jake