Electrical noise-free LED workbench lighting

[cosmicray]

Just don't use switchers.

Linear is OK, but a simple solution would be capacitive reactive dropper.

Yes, a linear regulator (e.g. LM317 as a two-terminal current regulator) does not generate the EMI that some regulators do. But you need a (relatively clean) DC source to feed it from. I'm currently looking at using a USB-C power source with a PPS controller to achieve the voltage/current I need (~18v).

As to dimming ... Dimming can be achieved by building a board with multiple interleaved (or dithered) arrays of LEDs. Simply turn on/off as many of those as needed to achieve the desired level of light. All of those LEDs will be run down the curve so as to control heat dissipation and current loading. It may even be possible to tweak the adjust voltage resistor (on the LDO) value in real time, to achieve different amounts of current/light thru the LEDs, but that becomes fiddly with heat dissipation values (primarily on the LEDs, but possibly also the adjust resistor). With the low current values (to each branch of the dithered LEDs) I might even toss in a few ferrite beads to see of they suppress any additional circuit noise.

[Edison]

The problem is that my LED strip is not just diodes and resistors, it has blocks of twelve LEDs and each block has its own linear current source, dimming is not possible due to this design.

Could you post more details on this?  That sounds pretty... unusual.

The LED strip from which I made the work table lighting was not bought in a normal store, but from a company that produces industrial lighting. I have been alerted to the fact that due to this LED connection, the strip cannot be dimmed, but has a completely constant brightness. I didn't open the light all the way, it's hard to get into, but here you have at least a piece as proof that the tape isn't just balance resistors.

[5U4GB]

Interesting, here's my one, also not from a generic supplier, but with the standard resistor-only configuration.



Edited to add: This was lit with a flash to get light into the alu channel which was otherwise partly shaded, thus the uneven colour appearance of the LEDs.

[Edison]

Interesting, here's my one, also not from a generic supplier, but with the standard resistor-only configuration.

I don't know what chips you have installed on your LED strip, but the photo clearly shows the degradation of the phosphor and thus overheating of the chips.
The color of the phosphor determines the temperature of the color of the emitted light, the brighter the yellow, the colder the light, some LED strips have mixed chips, but mostly they alternate regularly. In your photo, you can see a series of bright yellows (according to the color, this would correspond to 5500 K and above - both the camera and the monitor can be deceiving) but then towards the right the color of the phosphor changes and it seems to gradually darken - this would indicate the use of an unsuitable source - thanks due to deviations in resistance values, the chips may be overloaded in individual sections, or due to uneven cooling - although it looks like the tape is stuck in an aluminum strip, but the chips themselves are mounted in width and because they are cooled to the wider side, in the place where there are resistors which also emit heat, they overheat and thus degrade the phosphor and the chip itself.
On my strip, you can notice the longitudinal assembly of the chips and the uniform color of the phosphor - the strip has been used every day for several hours for about 10 years, because the LED chips are cooled to the sides, there is no mutual transfer of temperatures between the components, and thanks to the approx. 5 mm thick cooler, will cool down.

[Randy222]

PWM is one way to have "dimmable" light. But that's very digital and can introduce EMI / noise.
One other way to dim is to keep it analog. As example, lower the diode amps (lower voltage), and you can do that w/o wasting power. There are those addressable volt regulators, many made with i2c interface. You can essentially dim the led by lowering voltage by way of i2c signal to the regulator. An MC that has i2c works best for this. Yep, more complicated, but less wasted power and "all" linear.

I guess one could also make a linear reg setup with a dial pot for adjustment.

[Edison]

A source with current regulation, not voltage regulation, must be used to power the LED. When using a voltage-regulated source, the lifetime of the LED can be greatly reduced

[Randy222]

How do you "regulate" current if voltage is fixed? Even PWM is not constant current, and PWM slams the led junction open and closed at whatever frequency is used. Good thing with PWM is you can actually drive the led a bit harder at lower duty cycle.
If you mean like fixed 12v (as example) and then add resistance between source and device, that's still dropping voltage on the device (also wasted power).

[cosmicray]

How do you "regulate" current if voltage is fixed? Even PWM is not constant current, and PWM slams the led junction open and closed at whatever frequency is used. Good thing with PWM is you can actually drive the led a bit harder at lower duty cycle.
If you mean like fixed 12v (as example) and then add resistance between source and device, that's still dropping voltage on the device (also wasted power).

The short answer is you are going to drop voltage somewhere (and produce some heat as a by-product).

The longer answer is that you design so that the input voltage is ~2v above what your calculations say the LED string (series or parallel) will need, and then you run with it. Dropping volts on an LDO (assuming it's not too many volts) is likely preferable to heating up a bunch of LEDs. In a Cree thermal management guide, it even says to plan for (up to) 70% of the LED current being dissipated as heat. You really don't want that if you can design around/away from it. In some specific use cases, you have to accept all that heat, and plan accordingly.

[Edison]

How do you "regulate" current if voltage is fixed?

Current regulation by resistance only is possible only with signaling LEDs with a relatively low current consumption of approx. up to 20mA, this can be applied up to approximately 20V differential voltage, then the losses are already too big.
First of all, we have to realize how the LED behaves with the increase in temperature, after reaching a certain temperature point of the chip itself, there will be a sharp change in the internal resistance and a logarithmic increase in the current - if the current maximum is not set on the source below this breaking point, degradation and possible destruction of the chip will occur very quickly .
The general rule says - with a really well-designed switching power source, we can load the LED to almost 100% of its nominal value, if we use a linear current source, the load should not exceed the limit of 75%

Industrial current-regulated sources are produced directly for specific lamps with a fixed I/U ratio or universal ones for which the ratio curve can be set, for example, using DIP or using an IR controller (my 200W LED source). The source primarily regulates to the maximum set current and the voltage follows the regulation curve.
A simple example is an ordinary laboratory power supply in CC mode.
Probably the simplest current source is a current mirror made of two transistors and a couple of resistors to set the operating points - this circuit controlled the interior lighting of my previous car for 12 years without any problem.
Thanks to the expansion of LED lights, specially manufactured integrated circuits for these purposes can be found today.
However, heat dissipation must always be ensured, preferably according to the specifications of the LED manufacturer

[beanflying]

A little late to the party but there is a some thoughts and issues here  in a thread of mine from a few years ago. Several years later and the Linear supplies over the bench are still doing what I need.

https://www.eevblog.com/forum/metrology/metrology-grade-lighting-i-am-in-the-dark/msg2225853/#msg2225853

[5U4GB]

I don't know what chips you have installed on your LED strip, but the photo clearly shows the degradation of the phosphor and thus overheating of the chips.

Actually it shows that a flash was used to get light into the alu channel :-).  That strip has been lit for maybe a minute in total, and never even got to the point of being noticeably above ambient temperature.

[5U4GB]

If you mean like fixed 12v (as example) and then add resistance between source and device, that's still dropping voltage on the device (also wasted power).

From the very basic test I ran there's no need to externally limit or otherwise try to control current, it varies based on voltage, or at least decreases with voltage drop.  This isn't meant to be any high-tech measurement so don't rely on it, I just wanted to find out what level to undervolt at to get the amount of light I wanted.

Oh, and one of the photos shows it illuminated rather than externally lit via flash, so you can see the colour is consistent in that one.

[Edison]

Actually it shows that a flash was used to get light into the alu channel :-).  That strip has been lit for maybe a minute in total, and never even got to the point of being noticeably above ambient temperature.

I took a photo with a flash and the luminophore on my LED strip is the same color everywhere, but as I wrote - both the camera and the monitor can be deceiving

[Terry Bites]

I'd use an enclosed smps and be careful about the wiring and filtering.
Your digitial scope has an smps inside it so its not really the issue of psu type.
Use star grounds and star supply wiring. Use CM and DM filters in the dc line.
I had to build a low noise lighing scheme for a metrology lab. I used a a Traco (nice) enclosed switching supply and added post regulation of the LED current with LM317HV's as the sources. Twisted pair everywhere I could. It worked out really well.
At the end of the day modern smps are fine if you take care to keep radiating loops to a minimum.

I have found that linear supplies can be harder to tame. Even noiser than a decent smps.
The charging current pulses in the rectifier circuit can create naasty EMI problems.
If you got down the linear path make sure to use snubbing caps the fullwave bridge.

[Xena E]

I have found that linear supplies can be harder to tame. Even noiser than a decent smps.
The charging current pulses in the rectifier circuit can create naasty EMI problems.
If you got down the linear path make sure to use snubbing caps the fullwave bridge.

This.
The biggest problem is this instance on the use of slow rectifiers, many, notables are the 1N 400x/1N540x, they are hardly fast enough for 50/60 Hz.

Not being able to switch fast enough, reverse current builds until the junction effectively "snaps" shut causing the RFI.

Made worse if the current gulps are overly narrow by use of gross over capacity in the smoothing. The old RoT here is 1microfarad per milliamp, (50Hz supplies).

A side project for us was the LED lighting in a development lab that had to be EMI quiet. It was found there was still a need to tame the 100Hz switching noise in the chosen linear supplies: it was just a case of swapping to UF series rectifiers in the PSUs in each panel.

Saved face for the guy who specified the linear supplies in the first place, his not apparently knowing they had their own vices.

It did ultimately prove to be the best choice to use linear with current regulation.

Though not the most efficient.

[5U4GB]

The biggest problem is this instance on the use of slow rectifiers, many, notables are the 1N 400x/1N540x, they are hardly fast enough for 50/60 Hz.

Not being able to switch fast enough, reverse current builds until the junction effectively "snaps" shut causing the RFI.

Ah, that would explain the HER 60x's in a power supply I looked at recently.  I didn't realise you needed a fast-recovery diode at 50/60Hz.

[Edison]

It did ultimately prove to be the best choice to use linear with current regulation.

Though not the most efficient.

Yes, this is what I mentioned - the maximum current load of the LED is 75% (because if we exceed this limit, there will be rapid degradation due to overheating even with sufficient cooling) and greater losses on the source - the reward is completely silent operation