# Sjöström Audio DIY forum

## Projects => Power supplies => Topic started by: westers on January 11, 2016, 09:24:30 PM

Title: SST01 R1 and LED Questions
Post by: westers on January 11, 2016, 09:24:30 PM
Hi

What is the purpose of R1 - is it for limiting current inrush?

I have an insulated LED that is rated for 1.7VDC, and a forward current of 20 ma; I am confused on what values to use to work out the resistor value of R2 to allow the LED to operate. (I know V=IR, but I don't know what value of V to use (1.7VDC, or 230 VAC), or what value of I to use.

Thanks
Title: Re: SST01 R1 and LED Questions
Post by: peranders on January 11, 2016, 09:40:16 PM
R1, yes

You can estimate the resistor to 1.7V/20 mA = 85 ohm, 82 or 100 ohm maybe-
Title: Re: SST01 R1 and LED Questions
Post by: westers on January 12, 2016, 10:40:48 PM
Thanks.

Can you explain the theory behind that - why those values are chosen instead of the supply voltage of the circuit and the 47ma current that is on those rails (from a previous thread).  Electronics is not my speciality,  however I like to learn,  and google doesn't provide answers (it says to use (Vs - Vd) /I,  where Vs = supply voltage,  Vd= voltage drop,  and I = current required.  That equation is obviously no use in this context)

Thanks