All About Circuits Forum Ac led
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

#1
06-25-2009, 11:13 AM
 dude521 Junior Member Join Date: Nov 2008 Posts: 37
Ac led

Hi guys,

I have an LED indicator light which is supposed to let me know if power is getting to a certain part of my circuit. That part of the circuit is getting ~173 VAC. I have only a 110 VAC LED indicator. I figure I need to put a resistor in series with the LED, but I'm really not sure what value, or how to do the analysis.

The LED indicator I have is designed for 110 VAC, but I'm not sure how. Its in a small case so it may have some circuitry like a diode and/or capacitor. Could you let me know if an LED in series is the best way to drop the voltage for the LED (I'm really limited here since I can't change the circuit or the LED directly)? Also how would one do the analysis be hand to determine how much voltage will be dropped across the resistor (assuming I know how much current the LED takes, which I don't).

Thanks.
#2
06-25-2009, 11:23 AM
 gotumal Member Join Date: Mar 2008 Location: München, Germany Posts: 95

Use a capacitor to have a voltage drop. Choose the value as per Xc=1/2*pi*f*C depending on the voltage drop. Don't forget to check the capacitor voltage aroud 1.5 times the AC voltage
__________________
- Gotumal

"I always win, except when I loose. But then I just don't count"
#3
06-25-2009, 11:35 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,044 Blog Entries: 5

Generally a LED using high voltage (ie 110VAC) MUST have a diode across it, as they have a really low PIV. After that condition is met a low value capacitor and resistor is needed. Truth to tell, I've never heard of a 110V LED indicators, I suspect it is something else (there are lots of gadgets that can do this better than an LED, and I'm not thinking of a neon).
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#4
06-25-2009, 02:45 PM
 ELECTRONERD Senior Member Join Date: May 2009 Location: Outer Space Posts: 1,145

Any LED will light up with DC and AC, here is what you do:

Now since your doing AC you'll need to find the rms voltage: 0.707 X 173 = about 123V. So imagine if you had a lightbulb (LED would be fried, just example) with 100V DC and 100VAC P-P. Will the lightbulb be more brilliant with the DC or AC? The DC! Because DC is a steady voltage while AC varies. So the rms is to take that AC and basically see how bright the bulb would light compared to DC.

So we have 123 rms V. Now for the LED: You could actually use any LED but you need to know the Forward voltage drop along with which current this happens at. Most LED's typically have a forward voltage drop of 3.2V with 20mA. So 123-3.2 = 128.8 or 129V. So R = E/I - 129V/20mA = 6,450Ω resistor. Now make sure you have a higher power resistor than a 1/4 of a watt because your using high power! Your LED is different so check your specs for the forward voltage drop and the current at which that happens.
#5
06-25-2009, 03:02 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

The peak voltage of 173VAC is 245V.
#6
06-25-2009, 06:35 PM
 ELECTRONERD Senior Member Join Date: May 2009 Location: Outer Space Posts: 1,145

Quote:
 Originally Posted by Audioguru The peak voltage of 173VAC is 245V.

Whoops, I meant 173VAC Peak, not Peak to Peak.

 Tags led

 Related Site Pages Section Title Worksheet Bipolar junction transistors in active mode Worksheet Clipper and clamper circuits Worksheet Basic AC-DC power supplies Worksheet AC network analysis Worksheet Autotransformers Worksheet Resonance Worksheet Series DC circuits Textbook Series R, L, and C : Reactance And Impedance -- R, L, And C Textbook Series resistor-inductor circuits : Reactance And Impedance -- Inductive Textbook Analysis technique : Series-parallel Combination Circuits

 Similar Threads Thread Thread Starter Forum Replies Last Post ridge84 Embedded Systems and Microcontrollers 4 11-29-2009 09:35 PM Bill_Marsden Feedback and Suggestions 7 03-21-2009 11:45 AM fish4fun The Projects Forum 3 03-03-2009 04:04 AM pfhorge The Projects Forum 6 02-27-2007 07:36 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 06:15 AM.