At 11:35 PM 3/31/98 +0100, you wrote: >-----Original Message----- >From: Buell Smelt >To: PICLIST@MITVMA.MIT.EDU >Date: 31 March 1998 22:13 >Subject: Beginner Input and Output Questions > > >Hi Nathan, > >Lets take a couple of your questions in a step by step fashion:- > > >> >>I have a 5V lamp, and I do not know how much current it needs. > >You say lamp! Do you mean LED Or LAMP that is lamp as in filament lamp. > > Is there >>anyway to figure this out with a multimeter? Let's say it uses 50 >>milliamps. > >Yes, measure the resistance of the filament, now apply Ohms Law. Here once >again we assume its a filament lamp were are talking about, cos if its an >LED this don t work, and anyway an LED will only draw something in the order >of 10 to 25 milliamps. > >Once you have the resistance of the lamp, divide the voltage the lamp will >be running at by the resistance and you have the current. Careful here - the resistance of a cold filament is on the order of 25 Ohms and will go up to 300 ohms (40W lamp) when it incandesces. This behavior was often used as a feedback element in many amplifier and oscillator designs. If a 15W lamp operates at 110V then R = 110 * 110 / 15 = 806 Ohms, but only when operated at 110V. The current I = 15 / 110 = 136 mA (when operated at 110V). Rich BTW: This inrush of current is not being nice to the filament and is the reason that many household switches turn on at the zero crossing of the voltage waveform. ========================================= = Abolish the Income Tax! Fire the IRS! = = http://www.nrst.org/ = ========================================= ========================================= = Here's a site that wants your views = = http://www.not4irs.org/ = =========================================