Are LED power supplies magic?

[MrBond]

Can anyone please explain how led drivers work? The old transformer says 35 to 60V DC output, how does that work? The unit is not variable, nor is the cabinet it came from. Spoke to a guy selling those and he claimed thay the way it works is basically like the current works when voltage meets resistance and it determines itself out of the range it got. This is not the 1st time i have this issue.

[Whales]

It's a constant current power supply.  Presumably one of the numbers along the middle of the label is supposed to have a tick in its box to tell you what the current level is (probably in mA).

If its not outputting enough current then it will gradually increase its output voltage, up to a maximum up 60V, until it does.

If it's outputting too much current then it will gradually decrease its output voltage, presumably down a min of 35V but I'm not sure what it will do when it hits that (possibly still deliver too much current, possibly shut off).

[MrBond]

So how does it know when it outpuys enough current? Is it based on the rating on the transformer? E.g if the transformer says it will output one amp it just jiggles around with the voltage until that one amp target is matched?

Also, why don't leds take up as much current as they need at a constant voltage, like every other circuit?

Thank you.

[IanB]

So how does it know when it outpuys enough current? Is it based on the rating on the transformer? E.g if the transformer says it will output one amp it just jiggles around with the voltage until that one amp target is matched?

So consider what happens instead if you have a 12 V power supply, for example. It jiggles around with the current until the 12 V is matched. If a constant voltage supply can jiggle around with the current until the voltage is matched, is it so hard to conceive of a constant current supply jiggling around with the voltage until the current is matched?

Also, why don't leds take up as much current as they need at a constant voltage, like every other circuit?

That's a complicated question involving physics. However, it's mainly that the required voltage for a given current is variable and is very sensitive to conditions.

[tooki]

So how does it know when it outpuys enough current? Is it based on the rating on the transformer? E.g if the transformer says it will output one amp it just jiggles around with the voltage until that one amp target is matched?

The 700mA box has a (lazy) tick mark in it. So the power supply (which isn’t a transformer (though it contains one), regardless of what lighting electricians may call them) is going to regulate to 700mA by “jiggling the voltage” within the voltage range listed (the maximum is called the “compliance voltage”).

[Bud]

So how does it know when it outpuys enough current?

It measures the output current with a sensing circuit and controls the power supply via a feedback loop

Is it based on the rating on the transformer?

There is not only a transformer inside, there is a board with regulators. The transformer just defines the max current that the device as a whole can supply.

[magic]

Correction: since it's described as SELV there should be a transformer inside

(The simplest LED drivers are unisolated bucks.)

[TimFox]

So how does it know when it outpuys enough current? Is it based on the rating on the transformer? E.g if the transformer says it will output one amp it just jiggles around with the voltage until that one amp target is matched?

Also, why don't leds take up as much current as they need at a constant voltage, like every other circuit?

Thank you.

A resistor will obey Ohm's Law, where the voltage and current are related by V/I = R.
If you apply a voltage (e.g., from a battery, where the terminal voltage is almost independent of the current), the current through the resistor will be given by that equation.
Semiconductor diodes (including LEDs) are not linear:  i.e., the voltage is a monotonic function of the current (and vice-versa), and the current is almost an exponential function of the voltage.
Therefore, it is very dangerous to apply a voltage from a "stiff" source to a diode or LED, since the current could be very high and destroy the device.
Therefore, LED drivers use the above-described feedback from a current sensing resistor to control the current through the load by automatically adjusting the voltage to obtain the desired current through the current sensing circuit.

[Whales]

Also, why don't leds take up as much current as they need at a constant voltage, like every other circuit?

Indeed constant current (and constant power) power supplies are a lot less common than constant voltage ones, but they do have their niches.  LEDs are a good example.

LEDs behave abysmally if you provide them a constant voltage.  Put 3V across a 25degC white LED and it might draw 20mA, but as it gets hotter the current draw can skyrocket to perhaps 100mA or more, which causes more heating, which causes more current draw.  This is known as thermal runaway.  The LED goes dim as it heats up (efficiency drops at higher temps) and then finally pops.

The lazy way to fix this is to put a resistor in series with the LED, because resistors are well behaved under constant voltage conditions.  For low power circuits (indicator lights) this is fine, but for bulk room lighting it's incredible inefficient if you are wasting all of your power in resistors.

Constant current power supplies are much better suited to operating LEDs and especially strings of LEDs.  As an LED heats up it may actually draw less power overall (its voltage drops a little bit), so you don't have thermal runaway.  There are also other advantages, like being able to use series strings of LEDs of different lengths and having a single LED burn out into a short without causing other issues.