resistor values

[SimonCD]

i called to an elderly hoover softwave dryer today, it has a row of leds along the front to show time remaining, it is a really simple pcb containing a resistor, diode and leds. the resistor has failed and the middle band has discoloured. it is either brown, grey, red (1.8k) or brown, grey, orange(18k). it has been years since college and i cant remember the correct sum. With 240vac into resistor, which value will give me approx 6v for leds? (3v after diode rectifies) . sorry for the long winded post, i hope i have made it clear!
thanks, simon

[smartin]

Re: resistor values

follow this link simon and it clculates values for you
http://www.dannyg.com/examples/res2/resistor.htm

[SimonCD]

Re: resistor values

thanks but i know the values, i need to know which will drop 240v to approx 6v when running a couple of led’s

[neptune]

Re: resistor values

First try a 18k resistor, and if leds do not light, try the 1.8k. To do it by calculation, You need to know the resistance of the two leds when lit. Why not light them with a 6volt battery, and measure the surrent flow. Then use ohms law, R=V devided byI to give the resistance. Assuming the mains is 220 volts, the resistor in question needs to be about 35 times the resistance of the leds because it needs to drop 35 times the voltage.[35 x6=210 so actually 36 is nearer the mark] hope this helps. Ther are cleverer blokes out there than me, but that is what I would do.

[roly16]

Re: resistor values

Actually, I’d phone QER and ask them if they’ve got one of those boards in stock, and if so, would they kindly have a look and tell me the value of the resistor.

[electrofix]

Re: resistor values

this is more current than voltage based you need a current of approx 20ma through the leds if you take 240v and 20ma R=V/I this gives you 12000 allowing for voltage increase when rectified i would go with 18k cant go wrong with that if it goes across the mains it pulls 13 ma and emits 3.2 watts so make sure you use a big enogh one

Dave

[maltheviking]

Re: resistor values

roly16 wrote:Actually, I’d phone QER and ask them if they’ve got one of those boards in stock, and if so, would they kindly have a look and tell me the value of the resistor.

You can rule this one out Roly, QER are very reluctant to give out info, they want to repair things themselves, rightly so, but they could be a bit more flexible.
They got my back up one day when they refused to give advice on a burnt resistor on a board, backfired on them really cos I don’t use them as often now 😥

[roly16]

Re: resistor values

I didn’t used to have that problem years ago when Eddie was there all the time. I’d phone up occasionally for advice and he’d give it me.

[SimonCD]

Re: resistor values

Thanks for all the advice guys, 18k 4watts it is…

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