Powering multiple led lights with single power source

[vecnar]

Hi,

Hope someone could help with below situation or recommend a good forum to ask about or find information on as I read a lot on the forums and watched many videos but it just got very confusing.

Objective

• We acquired 7 sets of led sugar canes, each set has 40 Red leds and is powered by 2 AA batteries. I would like to be able to power them all at once with 2 dc adapters and need to know what would be the safe voltage and amperage for 3-4 sets of 40 red bulbs.

Information gathered, parts on hand

• I measured the current between second battery positive terminal and wire that led to led lights 0.008A = 8ma
• Voltage when off was 2.95 volt, voltage when on was 2.85 volts.
• There is no constant current or constant voltage regulator, just connection to 2 AA batteries
• I also have 36 volt 0.235A, 5 volt 700ma and 12 volt 2.5A dc adapters available if it is better to connect all the products in series.

Questions

1. I have ordered 2x 3 volt dc adapters with fake batteries but not sure what amperage and thought to wire 3-4 sets (each 40 leds) to each in parallel circuit as they are all the same model/product led lights. Is it achievable?
2. What resistors do i need to add if any to prevent leds drawing extra current available from dc adapter and burning themselves, i am not able to check if existing leds have resistors somewhere due to their locations inside plastic sugar cane?
3. Providing leds have resistor can i use below formula to calculate resistor to add to parallel circuit that will connect 4 sets depending on dc adapter? (adapter voltage - sum of led forward voltage for each set)/sum of currents e.g. (12 - 8)/(4*0.01)=100 ohm

Thank you in advance

 

[vecnar]

Hi,

Hope someone could help with below situation or recommend a good forum to ask about or find information on as I read a lot on the forums and watched many videos but it just got very confusing.

Objective

• We acquired 7 sets of led sugar canes, each set has 40 Red leds (4 x 10 leds in parallel)and is powered by 2 AA batteries. I would like to be able to power them all at once with 2 dc adapters and need to know what would be the safe voltage and amperage for 3-4 sets of 40 (4 x 10 leds in parallel) red bulbs.

Information gathered, parts on hand

• I measured the current between second battery positive terminal and wire that led to led lights 0.008A = 8ma
• Voltage when off was 2.95 volt, voltage when on was 2.85 volts.
• There is no constant current or constant voltage regulator, just connection to 2 AA batteries
• I also have 36 volt 0.235A, 5 volt 700ma and 12 volt 2.5A dc adapters available if it is better to connect all the products in series.

Questions

1. I have ordered 2x 3 volt dc adapters with fake batteries but not sure what amperage and thought to wire 3-4 sets (each 40 leds, 4 x 10 leds in parallel) to each in parallel circuit as they are all the same model/product led lights. Is it achievable?
2. What resistors do i need to add if any to prevent leds drawing extra current available from dc adapter and burning themselves, i am not able to check if existing leds have resistors somewhere due to their locations inside plastic sugar cane?
3. Providing leds do have resistor can i use below formula to calculate resistor to add to parallel circuit that will connect 4 sets depending on dc adapter? (adapter voltage - sum of led forward voltage)/sum of currents e.g. (12 - 2)/(4*0.01)=250 ohm

Thank you in advance

I did some investigation on how 40 led's are wired and noticed errors in my question so i updated the above to reflect my findings.

 

[Simon47]

Red LEDs run at a forward voltage of around 2.2V - so there will be a current control device of some sort in the system. It may be a constant current circuit, or it's more likely just a resistor in series with each individual LED. If there are 40 LEDs in a cane, and between them they take 8mA, then that's only 0.2mA per LED.
Running at 3V would increase that to 0.25mA per LED, and I doubt they've found one that can't handle that.

And you could connect any number in parallel from one supply - just add up the currents.

BTW - how did you measure the current ? Many meters would be quite inaccurate at that end of the scale.

 

[vecnar]

Red LEDs run at a forward voltage of around 2.2V - so there will be a current control device of some sort in the system. It may be a constant current circuit, or it's more likely just a resistor in series with each individual LED. If there are 40 LEDs in a cane, and between them they take 8mA, then that's only 0.2mA per LED.
Running at 3V would increase that to 0.25mA per LED, and I doubt they've found one that can't handle that.

And you could connect any number in parallel from one supply - just add up the currents.

BTW - how did you measure the current ? Many meters would be quite inaccurate at that end of the scale.

Thank you for your reply Simon.
I found one video that explains very well led circuits and putting it here in case it will help someone to understand it.

I think all 40 leds are run in parallel circuit as i doubt 3 volts would be able to power many in series.

I used 12 volt 1.5 A adapter and 1k resistor to power 1 set of lights. I used below formula to calculate resistor
input voltage-led forward voltage/sum of currents in parallel circuit, in my case that would be 12.54-2/0.01=1054 ohm.
Voltage before resistor was 12.54, voltage after resistor 2.59, didn't have 54 ohm resistor to add.

I read one datasheet of some red led and it is showing recommended voltage 2 v and maximum 2.6. I understood that brightness is caused by how much current is going through led but i also read somewhere that higher voltage causes led to pass more current and will die sooner.

Do you think adding 54ohm resistor will lower the voltage to 2volts?
Should i used voltage divider circuit with resistors, to me it just looks that 1 faulty resistor/wire can short the circuit and damage both lights and dc adapter?

Thank you

 

[Simon47]

Both your circuit design and calculation are wrong.

Firstly, use a separate resistor for each LED set. Otherwise, if you disconnect a set of LEDs, more current will flow through the others - and if you disconnect enough of them, you'll blow what's left.

Secondly, your sets are already designed to run off around 3V - it's probably safe to assume there's a small resistor in series with each LED.

So you need to drop from 12V to 3V (or 2.85V if you want to match your battery) at a current of 8mA. Apply Ohms law (V=IR) and we have (12-2.85)=0.008R, which we can re-arrange to R=(12-2.85)/0.008, which works out at 1144 Ohms - so I'd use a 1.2k resistor as the nearest E12 series value. The resistor power (P=IV) will be less than 1/10W - so pretty well any small resistor (0.125W is a common size) will do.
So put a 1.2k resistor between your 12V supply and each of the LED canes, and they'll operate independently - disconnecting one won't make the others change brightness.

Now, as to how to drive an LED ... (I haven't had time to watch through that video)
LEDs are current operated - it is the movement of electrons that causes the excitation of electrons into higher orbits, and the subsequent drop back results in the emission of light. The voltage is merely a result of the physics of the arrangement the current is flowing through. Furthermore, if you find a curve of voltage vs current for an LED, you'll see that it has a very sharp variation of current with voltage once it reaches the turn-on voltage - so a tiny voltage change would cause a massive current change if we applied a constant voltage. What's more, the curve moves with variations in temperature. But since the light output is directly related to current, it's much much easier to control the current - and although the voltage will vary with both the current and temperature, that doesn't really matter.
The simplest way to control the current is to use a constant voltage and a resistor. Since the variation of LED voltage is fairly small, the current will be fairly constant and this is enough for many applications. But especially when you get to higher powers, it's more normal to use circuits which directly control the current.
And for many applications, a pulsed current is used. Rather than trying to accurately control a specific current, pulses of a higher current are used and the pulse width is varied to keep the average current constant. Partly it's because it's easier - and hence cheaper - to do. But also, when dimming is involved, LED colour (especially for white LEDs) changes with current - so keeping the pulses the same current, but narrower, keeps the light colour the same; while reducing a smoothed current results in the colour changing slightly when dimmed. But pulsed LEDs create really horrible strobe effects, and cause headaches and similar problems for some particularly sensitive people.

 

[vecnar]

Both your circuit design and calculation are wrong.

Thank you for pointing that out and explaining the below.

Firstly, use a separate resistor for each LED set. Otherwise, if you disconnect a set of LEDs, more current will flow through the others - and if you disconnect enough of them, you'll blow what's left.

Great to know that one set will not bring down all the rest, that was my biggest concern. I have purchased and received a resistor set so shouldn't be a problem to match the ones i need, providing i calculate resistor power and do not go above limits.

Secondly, your sets are already designed to run off around 3V - it's probably safe to assume there's a small resistor in series with each LED.

So you need to drop from 12V to 3V (or 2.85V if you want to match your battery) at a current of 8mA. Apply Ohms law (V=IR) and we have (12-2.85)=0.008R, which we can re-arrange to R=(12-2.85)/0.008, which works out at 1144 Ohms - so I'd use a 1.2k resistor as the nearest E12 series value. The resistor power (P=IV) will be less than 1/10W - so pretty well any small resistor (0.125W is a common size) will do.

I can see now where i did an error, I was subtracting led forward voltage instead of whole circuit voltage and i was trying to treat each circuit as a separate led in parallel led circuit.
I have tried 1k and 220R in series and lights were not bright enough compared to battery, it has reduced voltage of the led set to 2.57, i didn't see any current measurement on multi-meter. I switched back to 1k and voltage at 2.6, i still do not see current measurement when trying to power one set of 40 led lights using multi-meter, as you mentioned previously many are not accurate at low levels so it could be the case.

So put a 1.2k resistor between your 12V supply and each of the LED canes, and they'll operate independently - disconnecting one won't make the others change brightness.

I tried to power 3 sets of lights off single 12 v 1.5A adapter with 1k resistor for each set of 40 leds, 3 x 1k resistors in total. I had positive dc cable -> 3 cables to each positive led set with 1k resistor in them and negative dc cable -> 3 negative cables from led sets. Lights light up, voltage is slightly higher on 1 of the sets 2.61 where 2.6 is on the other 2. I tried to measure current with multi-meter but it showed 0.000, i tried my socket meter where i plugged in dc adapter and it was showing 0.01A for 3 sets but i haven't used it before so not sure how accurate it is.

Now, as to how to drive an LED ... (I haven't had time to watch through that video)
LEDs are current operated - it is the movement of electrons that causes the excitation of electrons into higher orbits, and the subsequent drop back results in the emission of light. The voltage is merely a result of the physics of the arrangement the current is flowing through. Furthermore, if you find a curve of voltage vs current for an LED, you'll see that it has a very sharp variation of current with voltage once it reaches the turn-on voltage - so a tiny voltage change would cause a massive current change if we applied a constant voltage. What's more, the curve moves with variations in temperature. But since the light output is directly related to current, it's much much easier to control the current - and although the voltage will vary with both the current and temperature, that doesn't really matter.
The simplest way to control the current is to use a constant voltage and a resistor. Since the variation of LED voltage is fairly small, the current will be fairly constant and this is enough for many applications. But especially when you get to higher powers, it's more normal to use circuits which directly control the current.

Thank you for explaining that led is current driven and that the best way to control it using constant voltage and resistor.

And for many applications, a pulsed current is used. Rather than trying to accurately control a specific current, pulses of a higher current are used and the pulse width is varied to keep the average current constant. Partly it's because it's easier - and hence cheaper - to do. But also, when dimming is involved, LED colour (especially for white LEDs) changes with current - so keeping the pulses the same current, but narrower, keeps the light colour the same; while reducing a smoothed current results in the colour changing slightly when dimmed. But pulsed LEDs create really horrible strobe effects, and cause headaches and similar problems for some particularly sensitive people.

It is great that somebody tries to find a way to accomplish some functionality for cheaper but not at the cost of some bad side effects. I hope it is advertised to buyers before selling it to them.

Could you please confirm that below calculations are correct if you can. I use current of 10mA as on batteries it was different between 0.2mA to 14mA and brightness level is sufficient at 10mA.

Scenario 1:
dc adapter 5v 2A, connected to 2 sets of lights each 0.01A
R= (5v - 2.85v) / 0.01 = 215 Ohm
Resistor to use in each set 220R

Scenarion 2:
dc adapter 3v 0.5A, 2 sets of lights each 0.01A.
R= (3v - 2.85v) / 0.01 = 15 Ohm
Resistor to use in each set 15R

Scenarion 3:
12v 1.5A, 3 sets of lights each 0.01A
R= (12v - 2.85v) / 0.01 = 915 Ohm
Resistor to use in each set 820R and 100R in series

Thank you for all the help with this Simon, attached picture off the first 3 sets (1 cane is not in picture) powered by dc adapter, 7 more sets to go divided into 3 adapters.