Perusing a LED-based gel nail polish UV lamp

This engineer doesn’t use nail polish, but his wife does. And he deals with plenty of PCBs. What do these things have in common? Read on.

Speaking of LEDs that lose their original intensity over time and use…

In the fall of 2020, after accepting that due to the COVID pandemic she wasn’t going to be getting back inside a nail salon any time soon, my wife invested in a UV lamp so she could do her own gel polish-based nails at home. While the terminology I just used in the prior sentence, not to mention the writeup title that preceded it, might be “old news” to at least some of you, others (like me, at first) might be confused. Here goes:

Gel details

First off, what is gel nail polish, both in an absolute sense and relative to its traditional counterpart? Here’s manufacturer OPI’s take:

A gel manicure is a coat of colored gel that looks deceptively similar to nail polish. It’s a thin brush-on formula, designed for high performance and a glossier finish than regular nail polish…An OPI GelColor manicure [also] lasts for up to 3 weeks…The primary difference between gel nails and a regular manicure is curing. Between each coat, you cure the color and set the gel nail polish by putting your nails under a special light.

That “special light” is a UV lamp. Initially, they were constructed using fluorescent tubes. But nowadays, mirroring the broader trend, they increasingly use LEDs instead. The one my wife first bought is Bolasen’s SunX Plus, a “Professional True 80W Salon Grade LED Nail Dryer for Gel Polish.” The link to it on Amazon’s main site now auto-forwards to a more recent battery-operated model (this one’s AC-powered), but I found a still-live product page copy on Amazon’s South Africa site (believe it or not). Here’s the associated stock artwork:

I’m not sure I want to know what the “no black hands” phrase references…

The black base shown in the stock images is missing in action; my wife found that foregoing the bottom plate expanded the lamp’s extremity-insertion gap spacing, thereby easing use. More generally, she’s now replaced this initial UV lamp with a newer successor; the original device’s intensity apparently faded over time, eventually taking excessively long to work its drying magic.

Polymer processing

Speaking of drying (or if you prefer, curing), what’s so special about UV light? Over to a blog post at the Manicurist website for an explanation:

Whether LED or UV, these lamps emit ultraviolet (UV) rays that trigger a chemical reaction called “polymerization”. Under UV exposure, the molecules in the polish bond together to form a solid and durable film known as a “polymer network”.

UV curing is more broadly used in a variety of applications and industries, as Wikipedia notes:

UV curing (ultraviolet curing) is the process by which ultraviolet light initiates a photochemical reaction that generates a crosslinked network of polymers through radical polymerization or cationic polymerization. UV curing is adaptable to printing, coating, decorating, stereolithography, and in the assembly of a variety of products and materials. UV curing is a low-temperature, high speed, and solventless process as curing occurs via polymerization. Originally introduced in the 1960s, this technology has streamlined and increased automation in many industries in the manufacturing sector.

More generally, electrical engineers out there will likely be particularly interested, for example, in UV light’s key role in the photolithography process used to make printed circuit boards!

So why, if this is a UV lamp that’s supposedly emitting light beyond the visible spectrum, is its output discernible by the human visual system (along with my smartphone’s camera)?

(cool photo, eh?)

Part of the answer may be that the LEDs in the design aren’t true UV at all, but instead leverage the lower-cost alternative referred to as near-UV. Part of it may be that the output spectral plot is sufficiently broad to still-noticeably “leak” into the violet portion of the visible range. And part of it may be that, to reassure users that the device is “on” (thereby preventing lengthy periods of peering at “pure” UV light, with likely retinal-damage consequences), the LED manufacturer added a phosphor layer to additionally generate a visible light output. Hold that thought.

Power spec uncertainty

Last (but definitely not least), before diving in, what’s with that “80W” output claim? The device actually supports two different power output modes, 80W and “low-heat” 55W, user-selectable via one of the four topside switches. When I initially plugged the lamp in without the LEDs illuminated, my Kill A Watt electricity usage meter measured 1W of power consumption:

Switch the LEDs on, in low-output mode, and I got 12W:

And in “high” mode? 23W:

12W ≠ 55W. And 23W ≠ 80W. So, what gives? At first, I wasn’t confident that my Kill A Watt was measuring power consumption correctly. But then I looked at the “wall part” that powers the lamp (in the first image of the sequence that follows, as with subsequent images in this post, accompanied by a 0.75″, i.e., 19.1 mm diameter U.S. penny for size comparison purposes):

Let’s zoom in on that last one:

Unless my math’s totally whacked, 24V times 1.5A equals 36W, not 80W, far from higher than that (to account for consumption inefficiency). So again, what gives? Was Bolasen’s marketing team being flat-out deceiving? Maybe: my cynical side certainly resonates with that conclusion.

But at the end of the day, I’ve decided to give the company the benefit of the doubt and conclude that, just as LED light bulb manufacturers do in spec’ing their devices vs incandescent precursors, Bolasen is using fluorescent UV tube intensity equivalents in rating its LED-based UV device. Online references I’ve found equate the lumens brightness rating of a 20-plus watt LED to that of an 80W fluorescent tube. Granted, that’s for visible light, but perhaps the comparison holds in the ultraviolet band as well…regardless, let me know your thoughts in the comments!

Diving inside

My background-info pontification now concluded (thank goodness, right?), let’s get to tearing down, shall we? Here’s our patient:

Raise the transport handle!

FCC-certified? Really? Call me cynical (again):

The specs say 42 total “beads” (LEDs). That matches my count in the photo that follows:

Look closely and you’ll also see, among other things, five screw heads, which I’m wagering are our pathway inside, along with an array of passive ventilation holes. Here’s the 12-LED cluster at the top (when the device is in its normal operating orientation, that is):

and the cluster-of-six at the back:

Along each side are four more clusters-of-three, such as this one:

IR enhancements

The ones at either end, i.e., straddling the lamp’s opening in its normal orientation, are special. The one at the right side (again, in normal orientation) also includes an IR (infrared) transmitter:

while the other additionally incorporates an IR receiver:

This, dear readers, is how the lamp implements the following function (quoting the original broken English on the Amazon product page):

Use the auto-sensor, it would turn on or off automatically when you put hand/foot in or out.

Insert your appendage (hand or foot, to be precise), and its presence breaks the infrared beam that normally traverses the transmitter-to-receiver gap in an uninterrupted fashion. Voila!

OK, let’s get those five screws outta there:

and with only a bit of remaining prying to do:

we’re in!

Although this original lamp may now be too slow to operate for my wife to tolerate, it still works. I’d therefore prefer to put it back together still fully functional and then donate it for someone else to use…or maybe I’ll keep it and use it to cure resin or…hey…make my own PCBs! Regardless, I’m keeping the internal wiring intact. Don’t worry, we’ll still be able to see its guts a’plenty. Let’s start with the inside of the top half of the chassis:

The power connector pops right out of its usual location:

Now for that PCB in the center:

At left is the two-wire connection that powers the LED array. At right is the power input. And the four-wire harness coming out the bottom feeds (and is fed by) the IR transmitter and receiver. The 14-lead IC PCB-labeled U1 in the upper right corner is unmarked, alas, but is presumably the system “brains”. And at lower left is a P60NF03 n-channel MOSFET likely employed for both LED power switching and variable voltage generation (for both “80W” and “55W” modes) purposes.

Flip the PCB over:

and what now emerges into view is the multi-digit eight-segment display along with the four-switch control cluster.

Beating the heat

Now for the inside of the other (lower) half. Wow, look at all those thermal-dissipating metal plates (operating in conjunction with the earlier-mentioned passive ventilation array)!

First off, here are the connections to the IR transmitter:

and IR receiver:

Now for the LED power distribution network. The two-wire harness coming from the PCB first routes to the three-LED plate that’s in the lower left, just below the six-LED plate, in the earlier overview photo:

From there it splits in two directions. The “upper” (for lack of a better word) span first routes to the aforementioned six-LED plate:

Then to a series of three mid-span three-LED plates:

And finally, to a three-LED plate at the end in proximity to the IR receiver:

The “lower” span also then cycles through its own set of three three-LED plates, the last of them alongside the IR transmitter, and terminates at the 12-LED cluster:

Dual-frequency LEDs

These one more aspect to this design that I want to make sure I highlight, which keen-eyed readers may have already noticed. Check out this closeup of one of the LED “beads”:

The yellow tint is reflective of the thin phosphor layer applied to the inside of the “bead” dome to assist in generating augmented visible light for user-operation-stupidity-prevention purposes. But peer underneath it…are there two die in there? Indeed, there are. I’d originally thought I was instead just looking at the LED’s leadframe structure:

but, in the comments to a teardown video of a different UV lamp by “Big Clive” (whose always-excellent work I’ve showcased before):

was an enlightening insight from “restorer19”:

I have the 6-led UV panel you did a video on years ago, from the same brand, and it likely uses the same LEDs – I’ve sacrificed once of the LED chips and an additional one of the phosphor domes/blobs. It appears to have two LED dies on each chip, one bonded with two wires to each end of the module, and one bonded directly downward with only one bond wire leading to it. The 2-wire die (presumably 405nm) lights a visible purple at a lower voltage (just under 3V), and the 1-wire die takes greater voltage to light up. The 1-wire die looks identical to the large one in a 365-nm LED flashlight I recently bought – the surface of the die itself seems to phosphoresce in white, and any color from the semiconductor itself is invisible. Looking at an individual LED module under magnification while powered at about 3.2V makes the two different dies obvious without being too bright to look at.

“Big Clive” had done an earlier teardown of a more elementary UV lamp containing these same dual-die LEDs (this video is, I believe, the same one that “restorer19” was referring to):

wherein he’d conjectured (at least as I interpreted his comments) that the white color, i.e. full-visible-spectrum-when-illuminated die inside might purely be for “powered on” visual user-reassurance purposes. However, a Google search using the phrase “dual die UV LED” produced an interesting (at least to me) AI Overview response:

A dual die UV LED refers to a UV-LED light source, often in nail lamps or curing devices, that combines two different LED chips (dies), usually at wavelengths like 365nm and 395nm, to effectively cure a wider range of UV-sensitive gels, including both traditional UV gels and newer LED-only gels, offering faster, more complete curing than single-wavelength lamps. These lamps are popular in nail salons for their versatility, providing professional results by ensuring all gel types, from base coats to builder gels, are fully hardened.

Key Features & Benefits

• Dual Wavelength: Uses two distinct UV wavelengths (e.g., 365nm for deeper penetration, 395nm for surface cure) for comprehensive curing.
• Broad Compatibility: Cures all gel types (UV, LED, builder, hard gels).
• Faster Curing: Significantly reduces curing time compared to older UV-only lamps.
• User-Friendly: Often includes auto-sensors, timers (15, 30, 60, 90s), and removable bases for pedicures.
• Professional Quality: Common in salons for consistent, high-quality results.

How it Works

Instead of a single type of UV emitter, a dual die lamp integrates two different LED chips within the same unit, each emitting at a specific UV wavelength, ensuring that various photoinitiators in different gels react and harden the product effectively.

In Summary: A “dual die” UV LED lamp is a modern, efficient solution for curing gel nails, combining multiple LED technologies for faster, more reliable results across the spectrum of gel products.

And, in finalizing this write-up just now prior to submitting it to Aalyia, I revisited the previously mentioned Amazon product page and noticed the following (bolded emphasis is mine):

Specifications:

• Timer: 10s/30s/60s/99s low heat mode
• Wattage: 80w(Max)
• Display: Digital Time Display
• Lamp beads: 42pcs Dual Dual Light Source
• Spectrum: 365nm+405nm
• Lifespan: 50,000H
• Voltage: 100V-240V 50Hz/60Hz
• Output: DC12V
• Lamp Size:235*223*102mm
• Ideal for: All nail gels

So, I’m guessing we now have our answer! In retrospect, I also realized that one of the earlier “stock” graphics referenced a “dual light source” and included an LED close-up revealing the dual die internal structure. That said, I’ll wrap up for now and await your thoughts in the comments!

—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.

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