Trainsrgr8 Posted November 2, 2009 Share Posted November 2, 2009 Hi guys! Admin, not sure if this was the right place to ask my question so move it to another section if needs be! Sorry! Right, just some help I would like from someone in the know with electrics, this is a stupid question I suppose but I am planning to build some signals, powering them off a 15 volt AC power supply and would like to know what kind of resistor I need???? I am buying all the parts I need from maplin hoefully so if anyone could include a URL that would be much appreatiated!! Many thanks! Danny Link to post Share on other sites More sharing options...
RMweb Gold Dagworth Posted November 2, 2009 RMweb Gold Share Posted November 2, 2009 Are your signals going to be using bulbs or LEDs? Andi Link to post Share on other sites More sharing options...
RMweb Premium zarniwhoop Posted November 3, 2009 RMweb Premium Share Posted November 3, 2009 I am buying all the parts I need from maplin hoefully so if anyone could include a URL that would be much appreatiated!! No idea what sizes you need (in part, it depends on how bright you want the lights), but last time I went to Maplin they were only selling packs of resistors (and with a limited range of steps, something like 10,22,47 per decade instead of the old E12 range they stocked a few years ago). So, I'd suggest you get a pack of resistors, for LEDs do the "nominal" calculation (it was on the Maplin website last time I looked), then experiment with *larger* resistors (2x, 4x) - put them in series if necessary - until you have a feel for the brightness you want. ??en Link to post Share on other sites More sharing options...
DavidB-AU Posted November 3, 2009 Share Posted November 3, 2009 What current is the AC power supply? LEDs will handle 12-15 volts but they only need a low current. High current will burn them out. I have run LEDs off 16V AC, full wave rectified to DC, with a 1K resistor in series. Cheers David Link to post Share on other sites More sharing options...
cutting42 Posted November 3, 2009 Share Posted November 3, 2009 Hi guys! Admin, not sure if this was the right place to ask my question so move it to another section if needs be! Sorry! Right, just some help I would like from someone in the know with electrics, this is a stupid question I suppose but I am planning to build some signals, powering them off a 15 volt AC power supply and would like to know what kind of resistor I need???? I am buying all the parts I need from maplin hoefully so if anyone could include a URL that would be much appreatiated!! Many thanks! Danny Hiya Try this link, very useful: LED Resistor Calculator Link to post Share on other sites More sharing options...
Trainsrgr8 Posted November 3, 2009 Author Share Posted November 3, 2009 Are your signals going to be using bulbs or LEDs? Andi I want to use 3mm LED's. One sircuit will have yellow/red/green and another will have 2 white LED's for a shunt signal. What current is the AC power supply? LEDs will handle 12-15 volts but they only need a low current. High current will burn them out. I have run LEDs off 16V AC, full wave rectified to DC, with a 1K resistor in series. Cheers David It is 16V AC! I think i might just go with the general idea and buy a pack of Resistors and soem spare LED's and just experiment! Danny Link to post Share on other sites More sharing options...
RMweb Gold beast66606 Posted November 3, 2009 RMweb Gold Share Posted November 3, 2009 Danny, use a battery to have a play, very difficult to blow things up then. Link to post Share on other sites More sharing options...
Trainsrgr8 Posted November 3, 2009 Author Share Posted November 3, 2009 I want to use 3mm LED's. One sircuit will have yellow/red/green and another will have 2 white LED's for a shunt signal. It is 16V AC! I think i might just go with the general idea and buy a pack of Resistors and soem spare LED's and just experiment! Danny Cheers for the Advice Dave! Much Appretiated! Danny Link to post Share on other sites More sharing options...
Edwin_m Posted November 3, 2009 Share Posted November 3, 2009 You need a bridge rectifier or some other more complicated arrangement if you are running LEDs off 15Vac. Even if you put resistors or ordinary diodes in series, the reverse voltage across the LED may be more than it can tolerate. Link to post Share on other sites More sharing options...
RMweb Gold Dagworth Posted November 3, 2009 RMweb Gold Share Posted November 3, 2009 You need a bridge rectifier or some other more complicated arrangement if you are running LEDs off 15Vac. Even if you put resistors or ordinary diodes in series, the reverse voltage across the LED may be more than it can tolerate. It needn't be a bridge, a simple diode in the supply circuit would leave you with half wave rectified DC and prevent any problems with reverse bias on the LEDs. They would flicker at 50Hz though. Adding a capacitor across the DC output from the diode would stop the flicker. Something in the region of 1000mf at 25v should be fine. Andi Link to post Share on other sites More sharing options...
Edwin_m Posted November 4, 2009 Share Posted November 4, 2009 It needn't be a bridge, a simple diode in the supply circuit would leave you with half wave rectified DC and prevent any problems with reverse bias on the LEDs. They would flicker at 50Hz though. Adding a capacitor across the DC output from the diode would stop the flicker. Something in the region of 1000mf at 25v should be fine. Andi But with the diode in series, when reverse voltage is applied almost no current flows so the resistor is dropping almost no volts and the entire peak voltage is across the diode plus the LED. Exactly how it is distributed between the two depends on their leakage characteristics, but it is a fair bet that the voltage across the LED is greater than its specified maximum reverse voltage. This may be OK but may cause deterioration over time. Link to post Share on other sites More sharing options...
Guest Digital Posted November 4, 2009 Share Posted November 4, 2009 Express Models have a good selection of regulated power supplies. Converts 16V AC to smother ( regulated) 12V DC. http://www.expressmodels.co.uk/acatalog/Regulated_Power_Supplies.html They also have a good range of LED's with resistors and can advise you on what you need Give them a ring they will be only to happy to help you. John Link to post Share on other sites More sharing options...
RMweb Gold Dagworth Posted November 7, 2009 RMweb Gold Share Posted November 7, 2009 But with the diode in series, when reverse voltage is applied almost no current flows so the resistor is dropping almost no volts and the entire peak voltage is across the diode plus the LED. Exactly how it is distributed between the two depends on their leakage characteristics, but it is a fair bet that the voltage across the LED is greater than its specified maximum reverse voltage. This may be OK but may cause deterioration over time. Adding the cap, as I suggested, would keep a forward bias across the LED all the time, as what flows becomes DC. OK so it's not as clean a waveform DC as it would be with a bridge but the limited current draw through the LEDs will not be enough to discharge the cap completely before the next charge pulse arrives. Andi Link to post Share on other sites More sharing options...
Guest Digital Posted November 7, 2009 Share Posted November 7, 2009 Danny There is a very good article on LED's and resistors on Building "O" Gauge online that may be of help to you. http://www.7mmlocomo...id=38&Itemid=45 John Link to post Share on other sites More sharing options...
RMweb Gold 57xx Posted November 7, 2009 RMweb Gold Share Posted November 7, 2009 Something in the region of 1000mf at 25v should be fine.I assume you mean uF (mirco-Farad) not mF (milli-Farad)? 1000mF is 1 farad, a bit large just for some simple smoothing! Link to post Share on other sites More sharing options...
RMweb Gold Dagworth Posted November 7, 2009 RMweb Gold Share Posted November 7, 2009 I assume you mean uF (mirco-Farad) not mF (milli-Farad)? 1000mF is 1 farad, a bit large just for some simple smoothing! Doh! yes, microfarad is what I meant :icon_redface: Andi Link to post Share on other sites More sharing options...
Edwin_m Posted November 7, 2009 Share Posted November 7, 2009 Adding the cap, as I suggested, would keep a forward bias across the LED all the time, as what flows becomes DC. Agreed. Link to post Share on other sites More sharing options...
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