# How does the led driver figure it out..?

#### thedoc8

Joined Nov 28, 2012
159
Say I have a led driver with a dc output of 25 to 90 volts and 300ma. How does this driver know which output voltage to use on a given string of leds.

#### ericgibbs

Joined Jan 29, 2010
15,342
Hi doc,
probably a constant current source of 300mA.
E

#### ElectricSpidey

Joined Dec 2, 2017
2,143
It raises the output voltage until it senses the output is at 300mA.

There is probably a million ways to do that...well maybe less.

#### Ian0

Joined Aug 7, 2020
4,838
It switches the output on. The current ramps up (because it is going through an inductor). When it reaches 400mA it switches off and the current ramps down. When it reaches 200mA it switches on again.
It doesn't even know what the voltage is.

#### thedoc8

Joined Nov 28, 2012
159
It raises the output voltage until it senses the output is at 300mA.

There is probably a million ways to do that...well maybe less.
That don't work out, push 300ma into 18 led's, now switch to 9 leds and the driver gets the correct voltage at half the ma. I thought the same, run the voltage up until 300ma was pulled and quit. I got to do some more simple test, this is to easy.

#### thedoc8

Joined Nov 28, 2012
159
Hi doc,
probably a constant current source of 300mA.
E
Yes on the cc of 300ma, but that changes with different strings, how is it figuring out the correct amp draw.

#### Ian0

Joined Aug 7, 2020
4,838
That don't work out, push 300ma into 18 led's, now switch to 9 leds and the driver gets the correct voltage at half the ma. I thought the same, run the voltage up until 300ma was pulled and quit. I got to do some more simple test, this is to easy.
18 LEDs is about 54V, 9 LEDs is 27V. The correct voltage is between 25V and 90V so they are both the "correct" voltage.
Switching from 18 LEDs to 9 LEDs just halves the voltage and the current remains at 300mA

#### thedoc8

Joined Nov 28, 2012
159
I have a universal led driver, no mater what string of led's I connect to, it sets itself to the correct voltage and ma draw. Just trying to figure out how

jjw

#### Ian0

Joined Aug 7, 2020
4,838
I have a universal led driver, no mater what string of led's I connect to, it sets itself to the correct voltage and ma draw. Just trying to figure out how
As stated in post #4

#### AnalogKid

Joined Aug 1, 2013
9,941
A "standard" power supply maintains a constant voltage no matter (within its ratings) how much or how little current is drawn from it by the load. If a supply is rated for 12 V at 2 A, it will keep its output at 12 V if the load is 0.1 A, 0.5 A, 1 A, 1.999 A ... This is a "constant voltage" supply. Internally, it has a feedback loop from the output to the control electronics. The electronics adjust the drive signals to the output stage, turning them up if the output sags below 12 V, and turning them down if the output is above 12 V. This is what most people are familiar with. It is why a USB charger can charge anything from a little headset to a tablet pc. 5 V is 5 V.

A "constant current" supply works the exact same way, only the regulated property is the output current, not its voltage. It senses the output current, and adjusts the output voltage (with an almost identical feedback loop) to maintain it. So as the number of LEDs increases, so does the output voltage; whatever it takes to maintain 300 mA. Your supply has a maximum compliance of 90 V. If the load requires more than 90 V to have 300 mA through it, the supply output will max out at 90 V and then the current will begin to decrease.

The core circuit concept in both cases is an amplifier with negative feedback. In effect it looks what it is being told to produce, looks at the output to see if in fact that is happening, and if not, adjust itself to bring things into agreement.

https://en.wikipedia.org/wiki/Negative-feedback_amplifier

ak

#### Yaakov

Joined Jan 27, 2019
5,495
I think the TS question is: How can the driver provide the correct current for 1, 10, and 100 LEDs without prior knowledge of the number of LEDs connected? He is saying that his driver works no matter how many LEDs are connected.

#### Yaakov

Joined Jan 27, 2019
5,495
@thedoc8 what is the make and model of the driver you are using?

#### crutschow

Joined Mar 14, 2008
29,484
I have a universal led driver, no mater what string of led's I connect to, it sets itself to the correct voltage and ma draw.
That's not really possible.
How do you know it does that?

#### ronsimpson

Joined Oct 7, 2019
2,004
That's not really possible.
As long as the LED string has a operating voltage in the range of "25 to 90 volts" it will work.
I have built drivers that will produce "300mA" down to 0V.

#### Yaakov

Joined Jan 27, 2019
5,495
As long as the LED string has a operating voltage in the range of "25 to 90 volts" it will work.
I have built drivers that will produce "300mA" down to 0V.
The problem is, if the LEDs need 30mA each, then 10 will work properly and 1 will let out the magic smoke.

#### BobTPH

Joined Jun 5, 2013
4,749
I have a universal led driver, no mater what string of led's I connect to, it sets itself to the correct voltage and ma draw. Just trying to figure out how
Test you assertion. Hook it up to a single 20mA LED. See how long it lasts.

Bob

#### ElectricSpidey

Joined Dec 2, 2017
2,143
The problem is, if the LEDs need 30mA each, then 10 will work properly and 1 will let out the magic smoke.
Then that's not a "string" is it?

But it is impossible to get 300mA at 0 volts.

#### Yaakov

Joined Jan 27, 2019
5,495
Then that's not a "string" is it?

But it is impossible to get 300mA at 0 volts.
For very large values of 0, it is possible.

#### crutschow

Joined Mar 14, 2008
29,484
As long as the LED string has a operating voltage in the range of "25 to 90 volts" it will work.
He stated it adjusted both voltage and current, which is not feasible in my book.

#### Papabravo

Joined Feb 24, 2006
18,086
For very large values of 0, it is possible.
Would that be like a normally distributed random variable with a mean 0 and a large variance?