Iv a set of fairy lights, Rated Voltage 3v, 15mA, I would like to connect four sets together in series (12v 60ma).I can get uk ac/dc 12V 1A Power.
Question1 : Do i need to connect a resistor if so what one.
Question2 :Is 12 volts enough.
Any advice would be appreciated
Electronic – Connecting led fairy lights together powered by ac mains
ac-dcledresistors
Related Solutions
I think the 'very quickly' rules out using mechanical relays. MOSFETs would give the most energy efficient switching (always useful for battery powered equipment.)
A high side P MOSFET switch.
T1 is any jelly bean NPN BJT. Q2 is a digital P channel MOSFET (i.e. low gate turn on). You can substitute any suitable digital P MOSFET here.
It would also be possible to substitute Q2 for a suitable PNP BJT (change R2, 2k2 for 2 x 1k0 in series with base of PNP to midpoint.)
The grounds for the battery (0V) and the Pi must be commoned.
- The LEDs in the linked picture are white and probably have a forward voltage drop of about 3 to 3.6 V.
- Without any specifications we'll assume they can handle 20 mA.
- On a 5 V supply we need to drop 1.5 to 2 V across the resistor.
- From Ohm's law we can calculate a suitable resistance: \$ R = \frac {V}{I} = \frac {1.75}{0.02} = 87 \; \Omega \$ (choosing mid-range on the voltage). 80 Ω will be good enough for now.
- Connect four of your 20 Ω resistors in series with each other and the LED. You would need to disconnect one led from the string for this.
- Connect up to your 5 V supply and measure the voltage drop across the LED. You could also measure the current through the line. (Be careful not to measure voltage on current range.)
- Go back to step 1 and recalculate with the new values.
The image below may help.
Figure 1 shows that a green LED at 20 mA will have a forward voltage drop of about 2.2 V. If the supply voltage is 5 V then the resistor has to drop 5–2.2=2.8V. The required value is \$ R=\frac {V}{I}= \frac {2.80}{02}=140\;Ω\$. The nearest standard value of 150 Ω will do fine. Source: LEDnique.
In your case you will use the 'W' curve for white LEDs.
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Best Answer
Four 3 Volt LEDs connected in series will work diretly from your 12 Volt supply, with no extra components. One such group will draw 15 mA.
You can connect up to 66 such sets in parallel to the same 12 Volt 1 Amp power supply.
Correction: I must have been thinking of incandescent lamps when I wrote the first sentence.
LEDs must always be used with a current-limiting resistor, so you want a group of three LEDs, plus a 200 Ohm resistor to operate from 12 Volts. One group will draw 15 mA. (second sentence is OK)