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I have scoured the net for a direct answer but people always end up talking resistor because they don't have constant current.

I have a constant current power supply. LEDs are 700 mA and vary between 2.0 - 2.2 forward voltage to 3.2 - 3.4 Vf.

My question is:

If I run one 2.2 Vf LED in series with a 3.4 Vf led and have my power set at 700 mA then I would need to also supply 5.6 volts. Does running the different forward voltage matter? As long as only 700 mA is supplied I believe I could supply 5.6 to infinity volts without causing damage..?.?

Basically the inverse of conventional circuits with constant voltage and unlimited-ish current.

Thanks

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2 Answers 2

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Yes this is fine. LED's are more current than voltage sensitive (a switching current controller prevents thermal runaway without dissipating a large amount of power through an inline resistor) and placing elements in series ensures that the same current runs through them, as there is nowhere else for the current to go. As long as you don't exceed the current rating for a significant period of time and cooling is adequate, the LEDs won't be damaged by rated current.

Your current controller will provide whatever voltage is necessary to the load to cause the set current to flow, within it's limits. If it is a buck(step down) only controller, it can only provide voltage up to it's input voltage, and if it's a boost(step up) converter, it will provide the necessary output voltage up to the point where its input current hits max, so there will be a minimum input voltage that must be provided for a given load. Paralleling LEDs with different Vf however requires individual control of each LED.

As far as a resistor goes, think of it as a cheap high-safety, poor quality current controller. Use them when input voltage is relatively steady and load current is very small compared to available power. Switching controllers at higher current levels, for variable source voltage, or for variable control of the LEDs.

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  • \$\begingroup\$ Ok thanks for re affirming this. So if i am using a buck controller what would happen if i had the above scenario but with 2 parralel line off the same controller? I believed i just had to maintian 5.6volts but up the amperage to 1.4amps..? \$\endgroup\$
    – Jon
    Commented Dec 8, 2018 at 0:24
  • \$\begingroup\$ @Jon If this answer satisfies your question mark it as the accepted answer by clicking the button to the left of it. If you appreciated it but it doesn't fully answer, then just upvote(if you believe it has merit) and adjust your question so the next person to try knows where this answer was lacking. \$\endgroup\$
    – K H
    Commented Dec 8, 2018 at 0:26
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    \$\begingroup\$ CC's use any series connections where the load voltage is within limits but adding shunt connections must be matched voltage or W/A to share the same CC supply. The amount of mismatch in the 5.6V string vs xxx mA results in mismatched sharing by its incremental series resistance and threshold voltage \$\endgroup\$
    – D.A.S.
    Commented Dec 8, 2018 at 1:03
  • \$\begingroup\$ @TonyEErocketscientist Thanks. Good point. In more layman's terms for OP, if part of the series string is parallel, both parallel legs (Shunt and main path) must be voltage balanced with each other so each receives the correct current. If thermals are an issue this may also necessitate a series resistor on each parallel path to prevent thermal runaway. \$\endgroup\$
    – K H
    Commented Dec 8, 2018 at 1:10
  • \$\begingroup\$ Awsome. This is what i was looking for! \$\endgroup\$
    – Jon
    Commented Dec 8, 2018 at 2:50
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A buck LED driver will have a very specific voltage range. With a max and min Vf. LED CC drivers are sold by their wattage and max voltage.

For example this is a Mean Well 120 Watt driver available in various voltages. Each voltage model has a range of Vf supported.

enter image description here

None of which will work for you becasue your Vf is too low. The 12V min is 6V.

A higher Vf is usually more efficient. It also helps to use a Vin not too far over the Vf.

What would be your Vf if you wired all the LEDs in series?

I limit the string voltages to less than 48V. typicality 16 in series for white, blue and green, and 21 in series for red, orange, and yellow.

I NEVER power two parallel multiple LED strings with a CC driver. I have tried many times with various string and never did the current balance between the two strings. Closest was a 45/55 split.

I am using a buck and a cheapo power supply

When all the LEDs have the same Vf and the desired current is constant, a CV supply and resistors can be an excellent solution. With the correct constant voltage you can get better than 98% efficiency from the resistors. It depends upon how closely you need the radiance or irradiance of each LED to match. Even with the same current flowing there will be a variance in output intensity between each LED.

enter image description here

This would probably work better if you had 8 sets of 5.6V pairs all wired in series for a total string Vf of 44.8. You then use a 48V CV supply and a 4.3 Ω 4W resistor for a minimum efficiency of 94%. With a good CV source you can adjust the output voltage and resistor value for even better efficiency.

If you were to use a 5.7 V CV source and 5.6 Vf. This would require measuring the Vf of the LEDs at 700 mA after they reach thermal equilibrium to calculate the resistor. And use quality LEDs from a manufacturer that specifies the min and max Vf where the range is not too large.

Running all LEDs in a single string with a singe CC source, is generally the best way to go when you want the same brightness from each LED.

In some projects I use an inexpensive 48V CC driver made with a TI LM3414HV tweaked to 97% efficiency (switching freq. and inductance) powered with a stable high efficiency CV source like a Mean Well HEP at 96% efficiency. Each driver powers a sting of 16 white or blue or 21 red for horticulture projects. I need current adjustment as the plants grow so resistors will not work well in most projects.

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