"120W" UV-torch, advanced redesign - NO!

[ulf]

In another topic I lately tried to improve an UV-torch marketed to have a 120W input, and failed due to the original electronics design.

https://www.ultravioletphotography.com/content/index.php?/topic/5122-alonefire-h45-45w-dissection-and-analysis/page/2/

 

At the end I mentioned that I might do a major redesign adding proper constant current driver-circuits.

I have now decided it is not at all worth the efforts and costs.

 

I searched for suitable ICs to use and the only type I found also need a complete redesign of the metal board holding the LEDs.

To make it work the LEDs the would have to be de-soldered and re-transplanted onto the new LED board.

There would be a need for three DC-DC converter circuits that just might be fitted inside the torch too.

 

The risks and total cost of such an adventure are way too high for a short period of almost constant intensity, before it would have to be automatically switched off to save the LEDs form dying. The drive power could never be much more than 20-30W, giving maximum 7 - 10 W optical power.

 

I have to accept that the torch I bought is a real dud! 

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I have another UV-illumination project that is more interesting to restart. It has been dormant a few years.

 

It is based on a Copper based LED array that I already have.

The array has 15 pcs of 10W 365nm LEDs mounted on a proper copper-based PCB.

With proper cooling and constant current drivers it will be able to push out ca 50W OPTICAL power for a 150W input!

 

I have already assembled the array, heatsink, fan and UV-pass filter holder:

    

The heatsink and fan combination is suitable for cooling off the heat from the LEDs. There will be around 100W to take care of.

The filter will be quite hot as the transmission of 2mm ZWB2 at 360-370nm is less than 100%.

Just now I do not remember the exact numbers, but it is low enough to heat  up the glass.

---

I will do this project in two main steps.

 

First the driving would come from a Lab power supply and I will focus on the mechanical and thermal design. 

That will be a nice flexible light source for indoors UVIVF.

 

Later I might make this lamp portable, for outdoor usage, with some belt or shoulder-bag based battery + drive circuit. 

The weight of a driver circuit and batteries will be way too high to be comfortable being integrated together by the LED head with heatsink and fan.

The driver will instead be connected to the LED Head with a flexible cable.

The LED-head will be handled like an old fashioned search light.

 

I think the battery packs and connection mechanics will be salvaged from some 18V hand tool system.

There will be at least two battery packs driving the CC-conversion modules that will feed the LED-head cable. 

 

 

  

[dabateman]

Sounds fun and like what Colin was attempting years ago.

[Doug A]

@ulf I always find your research educational and fascinating. Appreciate your testing and analysis of these torches. Wish more were top notch designs . Looking forward to your next project. 

Thanks,

Doug A

 

[ulf]

Thanks Doug,

 

I wasn't sure I had an audience for all my technical writing here. 

Electronic design is quite far from photography and  I was afraid I went too deep being too technical.

When you are working all day with things like this it is difficult to know.

 

If something I write seams unclear or you want to know more about some detail, please do not hesitate, just ask.

 

@dabateman Yes I vaguely remember Colin's project, but was't that aiming at a stationary usage only?

He has been experimenting with LED arrays for many different wavelengths over the years.

[ulf]

Updated the first post with pictures.

[Andrea B.]

Even if the audience for deep tech writing is small, I think it is still important to document it. 

Someone, somewhere will eventually read it some day.

We do have a lot of visits to UVP by non-members which we never know about.

Make sure you have some searchable phrases and keywords in such a write-up.

 

There is a torch/flashlight building site. Let me go find it and put the link here. LINKIE

[Stefano]

My 10 W UV LED (very similar to one of those in Ulf's matrix, in 15 copies), was very powerful. I still have it, although it is damaged, and I have a brand new one I bought I think in 2019 which I haven't even opened, waiting to get a proper heatsink for it.

 

My LED was so intense it could burn paper, not because the LED was hot, but because of the intensity of the light. 15 of them must be very bright. They will noticeably heat objects within a few centimeters.

 

The filter in front will get warm as stated before. If it absorbs a reasonable 20% of the light, it's 10 W of heat. For comparison, it's approximately the power in a 10 cm diameter circle of sunlight. Leaving the UV-pass filter under the Sun (considering almost 100% absorption of overall sunlight between 300 and 1000+ nm), will heat it up about as much. It's a very rough calculation anyway.

 

I have a curiosity about the thermal bonding between the LEDs and the heatsink: of course, to properly mount them, you need to apply a thin layer of thermal paste in between, and tighten the screws at the corners. The thermal paste should fill any gaps between the LED copper base and the heatsink.

 

This is easily done with small LEDs, and it's how I mounted mine (10 W each, the 9-chips type). But with large PCBs, if they aren't perfectly flat, there might be gaps too big for the thermal paste to fill. If the PCB is slightly concave, then pressing it onto the heatsink will flatten it, and it will make good contact. If it is slightly convex instead, no matter how much you tighten the screws, it will not make good contact in the middle. I always wondered about this.

 

Maybe this problem doesn't exist, but I would want to be sure there's good contact.

[ulf]

11 hours ago, Stefano said:

The filter in front will get warm as stated before. If it absorbs a reasonable 20% of the light, it's 10 W of heat. For comparison, it's approximately the power in a 10 cm diameter circle of sunlight. Leaving the UV-pass filter under the Sun (considering almost 100% absorption of overall sunlight between 300 and 1000+ nm), will heat it up about as much. It's a very rough calculation anyway.

But the diameter of the exposed filter glass is less than 5cm and the peak of the guessed 80%  transmission is not the same as the peak of the LED's.

My guess is that the filter glass will be more than 100°C hot.

---

LED arrays for higher power are often made with thicker copper PCBs to improve the heat spread from the individual LED components and to keep the PCB's bottom surface as flat as possible. The LED components also makes the array more stiff.

It is almost as modern computer CPUs and their need for cooling.

 

As you can see in the pictures above my 150W array consist of 15 LED-components each containing four LED-chips. 

That metal PCB is not yet mounted with any thermal paste. It is just there for mechanical fitting.

[Stefano]

Thanks Ulf.

[Doug A]

Thanks for adding the photos @ulf. It makes your new project easier to follow. Any idea what the approximate weight of this UV "spotlight" will be?

 

Never realized glass filters can become so hot. Besides a fan, are there other ways to cool glass? Electrical components often use heatsinks. Does that work with glass?

 

Agree with @Andrea B. that you should keep posting technical items. Perhaps some future designers will incorporate your tips to make better UV torches. 

Thanks for sharing,

Doug A

[ulf]

5 hours ago, Doug A said:

Thanks for adding the photos @ulf. It makes your new project easier to follow. Any idea what the approximate weight of this UV "spotlight" will be?

 

Never realized glass filters can become so hot. Besides a fan, are there other ways to cool glass? Electrical components often use heatsinks. Does that work with glass?

 

Agree with @Andrea B. that you should keep posting technical items. Perhaps some future designers will incorporate your tips to make better UV torches. 

Thanks for sharing,

Doug A

The unit on the pictures has a weight of 430g.

The silly "120W" with batteries weighs 490g.

 

Then comes the weight of a connecting cable, a very small box for connectors, switch and overheating protection.

On the intended bottom side you can see a short standard mounting rail, where the light head can be mounted either on a tripod or a handle making it look like a spotlight.

 

The weight of the power source with driver and battery packs are naturally not included but will be carried in a bag or possibly clipped onto the belt. TBD

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Yes filters can be hot if they are exposed to enough optical power. Everything that is absorbed will be converted to heat. That is one reason why I NEVER would use a Baader U as a light source cleanup filter on a more powerful light source.

 

If I remember correctly Colin had that problem when combining power LED arrays with longer UV-wavelengths with a big filter, possibly a ZWB2.

Then most of the optical power was absorbed and converted into heat in the filter and he needed a fan for cooling them.

 

Fortunately glass materials can withstand rather high temperatures and if evenly distributed there is not that high risk of cracking. However the optical transmission characteristics will change over time and then a cheap replaceable ZWB2 filter is  a good idea.

 

Cooling by airflow is more efficient when there are a big temperature difference between the air and the hotter glass surface, so I am not worried, except that O must remember not to touch the filter after usage.