Model train HO sound decoder, want to use terminals output of 14vdc reduce to 1.5vdc to operate led lights. What size resistor do I need?
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Assuming "HO sound decoder" means a DC supply, not AC....
If the LED has a Vf, for example, of 2 volts, and a current of 20 mA, then R = (14–2) / 20 mA = 600 ohms (you could use 680 ohms). IF you have more than one, do NOT put them in parallel. You can put some in series. For example, if you have 4 LEDs, then R = (14–2•4) / 20 mA = 300 ohms. Do not connect LEDs in parallel unless each one or each string has it's own resistor.
Assuming "HO sound decoder" means a DC supply, not AC....
If the LED has a Vf, for example, of 2 volts, and a current of 20 mA, then R = (14–2) / 20 mA = 600 ohms (you could use 680 ohms). IF you have more than one, do NOT put them in parallel. You can put some in series. For example, if you have 4 LEDs, then R = (14–2•4) / 20 mA = 300 ohms. Do not connect LEDs in parallel unless each one or each string has it's own resistor.
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I'll assume you want 20 mA through the LED use a resistor of (14-1.5)/.02=625 ohms.