Roy
Malc,
Most leds run quite happily on 3 volts with or without a resistor, but if using any more power yes a resistor is necessary.....
You are perfectly correct in this statement, except that 'power' relates to Wattage (Amps x Volts) not Voltage alone, but as others have intimated, how many models actually use 3 Volts? We manufacture a range of working lights for model boats, and sell all of them complete with matching resistors to ensure the compatibility of our lights with a range of voltages from (in extremis) 3 Volts up to 20 volts. We state 6 Volts to 12 Volts on the packaging.
The searchlights have the resistors built into them. Other lights are supplied with loose resistors, (for installation purposes), and we supply a connection diagram with each pack which includes a warning NOT to connect a supply across the LED's without the resistor in circuit.
We sometimes get modellers on the phone telling us that our LED's don't work. This is strange as we test each one before they're packed (only had one faulty one in the past two years). The reason that the LED's don't work is because the modeller hasn't read the instructions and connected them across Heaven only knows what voltage without a resistor in circuit? Sometimes I even suspect that they've had them across mains power supplies - these guys are really out there and are dangerous.
I've tried to replicate the sort of damage that modellers do, and with a calibrated, stabilised digital DC power supply, can often get an LED to work up to 18 volts without a resistor in circuit before it blows. Try it yourself.
As far as I am concerned, and knowing from bitter experience how little modellers understand electronics, I have always previously, and will continue to tell them ad nauseum, That they should NOT connect an LED across a power supply without the supplied resistor in circuit. It saves us a fortune in returns!
I hope this offers some background to my previous statement?
Regards, Malc
http://www.readebusiness.com