LED Wiring

billingw

Oct 10, 2012
13
I bought four 20-bulb LED holiday light strings, each powered by 3 AA batteries (thus 4.5V DC). I use them to light up two very small spruce trees, two strings per tree. In addition to a switch, there is a 20 ohm resistor inside the battery box for each string. I have to manually turn on/off each of the four strings, which is a pain. I want to wire all four strings to, say, a 4.5V DC 500mA AC adapter, plugged into a timer. Can I just connect the four strings (with resistor still in place) in parallel to the adapter? If so, what should I do about the resistors? Four 20 ohm resistors in parallel means 5 ohms total and that would make the current larger... In conclusion: does the 4.5V 500mA adapter look like the correct power source, can I connect the strings in parallel to the adapter and what resistance set-up should I include? Thanks!

Rayregula

Dec 20, 2016
84
Four 20 ohm resistors in parallel means 5 ohms total and that would make the current larger
5 ohms? Four 20 ohm resistors make 80 ohms total.
Resistors make less not more current.
how much current does the led string draw. I have a 12V 33 led string that draws 250ma.
Times your current by four and make sure your adaptor exceeds that and it should be fine. And leave in the resistors.

billingw

Oct 10, 2012
13
Four 20 ohms in series makes 80 ohms but in parallel they add up to 5 ohms. Resistors in parallel make more current - that's why too many appliances plugged into a circuit make the current so big that the breaker trips...

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The LEDs are all in parallel in each string.

For the health of the LEDs retain the 20 ohm resistor for each string.

This does mean you can't connect them end to end though.

chopnhack

Apr 28, 2014
1,573
Can you remove one battery and put a multimeter set to read current in series with the battery pack and removed battery? That should answer your question regarding what current is being drawn.
I would leave the resistors that are built in and simply feed the power supply to each LED 'unit'. The power supply (adapter) will be in parallel to the 4 units, they will each receive 4.5v. Find out what your current requirements are for each 'unit' and let us know how you get on.

Audioguru

Sep 24, 2016
3,640
If each LED uses 3.2V then the resistor has 4.5V - 3.2V= 1.3V across it and the total current is 1.3V/20 ohms= 65mA. with 16 LEDs in parallel then each LED uses 65mA/16= 4.1mA.
If the LEDs are 1.9V red LEDs then the resistor has 4.5V - 1.9V= 2.6V across it, the total current is 2.6V/20 ohms= 130mA and each LED draws 130mA/16= 8.1mA.

You cannot accurately measure the current because your current meter has some resistance that will reduce the voltage and current. Simply measure the voltage across the 20 ohms resistor and use Ohms Law to calculate the current.

Your 4.5VDC power supply is the same as a 4.5V battery but it can power many strings of these LEDs with their resistor.

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
There are some important differences between batteries and a 4.5V power supply.

Firstly, the voltage on the batteries will change from maybe 4.8V when new down to a little over 3V when the batteries are to flat to produce any noticable light from the LEDs.

The 4.5V power supply, if regulated, will give you 4.5V all the time.

If the 4.5V power supply is not regulated, it may give a voltage substantially higher than 4.5V under a light load. It may be sufficient to damage the LEDs.

I recommend that you measure the output voltage of the power supply with nothing connected to it. If it is 4.8V or less, you will be safe. Any higher and you should proceed with caution.

chopnhack

Apr 28, 2014
1,573
I recommend that you measure the output voltage of the power supply with nothing connected to it
Great point to raise Steve - I have a 12v adapter (transformer inside, not switching) and with no load it registers 28vdc initially and settles at 21vdc on my multimeter. Under very light load (less than 30mA) its about 14 volts!

billingw

Oct 10, 2012
13
The LEDs are all in parallel in each string.

For the health of the LEDs retain the 20 ohm resistor for each string.

This does mean you can't connect them end to end though.

billingw

Oct 10, 2012
13
The LEDs are all in parallel in each string.

For the health of the LEDs retain the 20 ohm resistor for each string.

This does mean you can't connect them end to end though.

billingw

Oct 10, 2012
13
The LEDs are all in parallel in each string.

For the health of the LEDs retain the 20 ohm resistor for each string.

This does mean you can't connect them end to end though.

billingw

Oct 10, 2012
13
In other wprds, connect all four sets in parallel to the power supply?

billingw

Oct 10, 2012
13
If each LED uses 3.2V then the resistor has 4.5V - 3.2V= 1.3V across it and the total current is 1.3V/20 ohms= 65mA. with 16 LEDs in parallel then each LED uses 65mA/16= 4.1mA.
If the LEDs are 1.9V red LEDs then the resistor has 4.5V - 1.9V= 2.6V across it, the total current is 2.6V/20 ohms= 130mA and each LED draws 130mA/16= 8.1mA.

You cannot accurately measure the current because your current meter has some resistance that will reduce the voltage and current. Simply measure the voltage across the 20 ohms resistor and use Ohms Law to calculate the current.

Your 4.5VDC power supply is the same as a 4.5V battery but it can power many strings of these LEDs with their resistor.

You are correct as I found out: inserting the ammeter into the circuit yielded a current of about 9 micro amps... The voltage across the R was 1.5V, so i = V/R = 1.5V/20 ohms = 0.075A = 75 mA. So, since I'm connecting four of these strings to the power supply, I presume the power supply should be 300 mA minimum?

billingw

Oct 10, 2012
13
There are some important differences between batteries and a 4.5V power supply.

Firstly, the voltage on the batteries will change from maybe 4.8V when new down to a little over 3V when the batteries are to flat to produce any noticable light from the LEDs.

The 4.5V power supply, if regulated, will give you 4.5V all the time.

If the 4.5V power supply is not regulated, it may give a voltage substantially higher than 4.5V under a light load. It may be sufficient to damage the LEDs.

I recommend that you measure the output voltage of the power supply with nothing connected to it. If it is 4.8V or less, you will be safe. Any higher and you should proceed with caution.
The power supply voltage was 5.3V, which I think is close enough to the batteries' 4.7V?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
In other wprds, connect all four sets in parallel to the power supply?

Yes.

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The power supply voltage was 5.3V, which I think is close enough to the batteries' 4.7V?

Not for LEDs without series resistors.

Try connecting one up for an instant. If it's not greatly brighter than usual, connect it again and measure the voltage with it connected.

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