blueeighties Posted January 16, 2012 Share Posted January 16, 2012 Evening all. I am currently constructing a control panel, as per the photograph. I would like to be able to have an indication of which way the points are set, using led's set into the track diagram. I am going to trigger these using a microswitch linked to the point motor bar. My query is, can anyone tell me what resistors I will need to use for the individual led's? I am purchasing these, http://www.jprelec.c...m-Standard-LEDs in both red and green. I have seen the formulae using Ohms law, but its a long time since I was at School and don't remember what it all means! I would appreciate any help as I would like to place my order for a whole pile of electrical components this evening. I should mention that the power supply will be either 12vdc or 16vac, again not sure what will suit best. Many thanks in anticipation, Lee. Link to post Share on other sites More sharing options...
Fay Singpoint Posted January 16, 2012 Share Posted January 16, 2012 Hello, Try here http://www.merg.org.uk/merg_resources/led.php HTH Link to post Share on other sites More sharing options...
Baby Deltic Posted January 16, 2012 Share Posted January 16, 2012 If you are using 16V AC, the LED will not be strong enough to reverse block the negative current, so you will need a second diode, such as a IN4001 either in series with the LED or connected in reverse bias across it to channel the reverse current in the negative cycle away from the LED. Link to post Share on other sites More sharing options...
Brian Posted January 17, 2012 Share Posted January 17, 2012 Hi If your supply volts are 16 volts ac I personally would wire a bridge rectifier across the 16 volt ac supply (Fitted inside the panel?) and then use the dc output from the bridge rect. to feed all the LEDs etc. About £0.69p from Maplin e.g. 2.0Amp - Part Nos. KBPC1005 or KBPC104 would both be ok. http://www.maplin.co.uk/bridge-rectifiers-19088 Then use a 1K0 (1000 OHM) 1/4watt series resistor wired to each lit LED. Mimic panel LED indications are discussed on my web page if it helps? http://www.brian-lambert.co.uk/Electrical-2.htm#Mimic%20Panel%20Indications. Link to post Share on other sites More sharing options...
blueeighties Posted January 17, 2012 Author Share Posted January 17, 2012 Thank you everyone for the useful replies. I have ordered some appropriate resistors to use with the l.e.d's. Brian, I have looked at your site before, thanks for the link, I will have a look at your mimic section tonight after work. I plan to have a green indication for the straight ahead route, and A red one for the diverging. Triggered by microswitches from the point motor. Regards, Lee. Link to post Share on other sites More sharing options...
meil Posted January 17, 2012 Share Posted January 17, 2012 If you're using a 16v ac supply through a bridge rectifier you are going to gat about 23vdc out. Personaly I would use a 2k resistor with those LEDs. Link to post Share on other sites More sharing options...
cpman46 Posted January 17, 2012 Share Posted January 17, 2012 A 16v ac supply via a bridge rectifier without capacitor smoothing on the output will give 16 x 0.9 Vdc = roughly 14.4V. With a 1K led current limiting resistor this will equate to approximately 12mA through the LED which is well within the spec of the proposed LEDs. HTH Mike Link to post Share on other sites More sharing options...
Brian Posted January 17, 2012 Share Posted January 17, 2012 Hi 16 volts ac (RMS) fed into a bridge rectifier will give less volts on the dc side than the ac input. Add an electrolytic style capacitor to the dc side and the dc volts will rise to the Peak ac of 1.4 times the RMS. Link to post Share on other sites More sharing options...
Crosland Posted January 18, 2012 Share Posted January 18, 2012 Hi 16 volts ac (RMS) fed into a bridge rectifier will give less volts on the dc side than the ac input. The peak voltage will still be 1.4 times the RMS, less two diode drops. The RMS value of the output will be the same, less the diode drops. The current limiting resistor ideally needs to be chosen appropriately for the peak voltage, although you will get away with not doing so unless you are driving the LEDs hard. The full wave rectified waveform will flicker at 100Hz, imperceptible to most people. Andrew Crosland Link to post Share on other sites More sharing options...
Brian Posted January 18, 2012 Share Posted January 18, 2012 The peak voltage will still be 1.4 times the RMS, less two diode drops. The RMS value of the output will be the same, less the diode drops. <Snip> Andrew Crosland Errr.... thats exatly what I stated!!!! Feed 16 volts ac into a bridge rectifer and the dc output volts will be less, reduced by 1.4volts (2 x 0.7volts per diode). Add a capacitor onto the dc output of a bridge rectifier and the dc volts will increase to 1.4 times the ac RMS input volts. Which is the Peak ac voltage!!! Link to post Share on other sites More sharing options...
Crosland Posted January 19, 2012 Share Posted January 19, 2012 Errr.... thats exatly what I stated!!!! Feed 16 volts ac into a bridge rectifer and the dc output volts will be less, reduced by 1.4volts (2 x 0.7volts per diode). Add a capacitor onto the dc output of a bridge rectifier and the dc volts will increase to 1.4 times the ac RMS input volts. Which is the Peak ac voltage!!! Not quite. You said "16 volts ac (RMS) fed into a bridge rectifier will give less volts on the dc side than the ac input. " Which, to me, reads as the DC side being less than 16V. I tried to clarify this by stating that *peak* voltage will be 1.4 times 16 less the diode drops, about 21V, which may influence your choice of current limit resistor for the LED. The *average* voltage will be a bit less than the 16V RMS due to the diode drops. This is what you are saying. Andrew Link to post Share on other sites More sharing options...
Dukedog Posted January 19, 2012 Share Posted January 19, 2012 Post edited to prevent causing any further argument! I still have to chuckle though at some of the statements and counter statements here! Reminds me very much of one of the reasons I posted THIS in my blog! Cheers! Frank. Link to post Share on other sites More sharing options...
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