30 watt LED Bench Light

Several months ago I bought a set of 10W LED modules like this:

They have a multi-chip configuration, with an array of LEDs embedded in the center phosphor part. These units come in various voltages; the ones I got were ~30V. I designed a quick constant current circuit that runs off of a ~35VDC supply (open circuit, ~30VDC under load):

The constant current circuit is a simple N-channel MosFET with sense resistor feedback (using the 2N3904 to control the ‘FET). Knowing that there will be an ~0.6V drop between the NPN transistor’s base and emitter, we can calculate V=IR, or V/R = I, which gives us 0.6/3.3 = ~0.181A or 180mA through the LED module.

Here is a photo of the built circuit (there are 3 of them, one per LED module):

The PCB that I used was actually the first PCB that I ever designed, and it had an error so i never ended up using it. I modified the circuit on the PCB to accommodate the LED driver circuit above (reuse and recycle!).

And from a different angle, showing the aluminum bar with the heat sinks under:

Originally I thought that it would be possible to thermal-paste and screw the main aluminum bar to the enclosure, but that didn’t work out too well in terms of height and clearances, so I added two big heat sinks (no idea what the specs are, they are “cheapie” ones from EBay). They do a good enough job, and the whole thing stays at a reasonable temperature as long as it is not left on for more than a few hours. Of note is that the heat sinks don’t make full contact with the aluminium bar; this will be fixed in the future, and I will be added a fan to the enclosure with some venting.

Here is a photo of the power input rectification and filtering board I put together:

Note the 56 kΩ resistor soldered to the filtering capacitor in the foreground. I ran into a small problem that there was enough residual voltage across the capacitor after the lamp was turned off that caused some of the LEDs with lower forward voltage drops to stay lit. The resistor helps drain the capacitor quickly.

Here you can see the fully assembled unit on the electronics bench, plus a shot of it lit up during thermal testing:

And here is the final light mounted above my work bench in the garage:

You can see in the back to the right the power bar with the AC adapter (right side of the power bar) that I used to power the light. It is a 24VAC unit (~27VAC without a load).

Measurements of the light output and power consumption will be performed and reported on in another post.

Frozen Privacy Screen Build

Last Christmas I decided to make my daughter her present. She had asked for a privacy screen that appears in the Frozen Fever animated short (only for a few seconds). I decided to go the “easy” route and build it out of pine wood and paint it.

The screen can be seen to the left of this scene:

For the design, I decided to go with a total height of about 4 feet (my daughter was 4.5 years old at this point, and about 3 feet tall). I wanted to get the domed tops of the panels looking similar, so I sketched something that looked about right on graphing paper:

Here you  can see a half arc (the top of the dome is on the right and the bottom on the left) which would then be mirrored to get the full dome. It looked about right so I proceeded to scale it up to full dimension and create a template:

Scaling the template involved measuring the distance from the center line of the half-dome to the curve of the dome. I then scaled both the distance from the mid-line out as well as the vertical position of the line to the full 12″ wood panel. The result of this was a fairly close enlargement of the dome’s curve onto a template sized for have the panel width, or 6″. After this was done I traced the template onto the 6″x4″x1/2″ pine boards I had bought (4 of them):

Cutting the boards was done using a jig-saw, but a band-saw (if available) would have yielded nicer results. As is, I did a fair amount of sanding to get the panels looking even and matched. The next step was to notch out the spaces for hinges (so the panels could fold up flat onto themselves for easy storage).



The tools used for this were a hammer and chisel.

Mark the panels that mate to each other right after cutting them or you will have a fun time trying to figure out how to get them to fit as cut later on!

The perfect hinge pocket is going to be a bit deeper than the hinge fastening sides, and this is because of the hinge joint, which is a bit taller than the two metal flanges. Also test and fit, since you will want to get adjust the depth a bit, and cut the second pockets a bit shallow and then keep testing as you deepen the second pockets to get a good matching fit that swings easily and without torquing the hinge at all.

Some assembly, paint and a really awful job at detailing (I need to work on that!) and voila, a Frozen inspired privacy screen!

A few things to note:

  • The screen panels are actually rather heavy, so this is not a great design for a little kid that is likely to bash stuff around or crash into it,
  • Detailing shapes on four separate panels is /HARD/, so practice before committing!
  • The bottom is never going to be perfectly flush, so the best method would be to assemble the panels as shown above before paint (before or after sanding) and cut them flush with a table saw or something (a circular saw would be fine as well, just use a guide!),
  • Height is a big consideration; ideally for a girl the bottom ends of the top dome shape should be about shoulder length, perhaps a bit higher.

PCBs in the Wild!

I have been working on a couple of prototyping boards for general “hey I need a PCB for this oh I have one here” type of projects. Since I’m working more with Surface Mount Technology (SMT) these days and have more of those part in my “stock”, I designed some prototyping boards with this in mind:



I have sent a few of these to some people that I chat with on the #eevblog IRC channel, and one of them was kind enough to send me an “action shot” of a little LED-based project he used my PCB for!


And another one from c4757p:



Power Designs Inc 6050C Display Mod

Here’s a modification that I have been meaning to do for a while. It involved replacing the PCB in one of my power supplies with a modified version designed by me that upgraded the size of the 7-segment LED display.


I ordered the board from OSHPark. My experience with them has been positive; the turn around time was about 2 weeks and the gold finish is very nice. I’m fine with the colour of the solder mask, however note that the mask is a matte finish rather than the typical “pearl” or “glossy” finish that I am accustomed to seeing on PCBs. Not a big deal, but something to think about. Also, the traces are a bit difficult to see through the solder mask.

IMG_5737 IMG_5739 IMG_5741

I of course did not fully check my notes when throwing together the schematic for this project which resulted in the boards I received having the ‘a’ and ‘g’ segments reversed. A few cut traces and a jumper wires later and all was working as expected.

IMG_5745 IMG_5744  IMG_5742IMG_5743

I originally tried to think of a better way of re-attaching the new display board to the existing display measurement/logic board, but in the end the simplest solution won out and  I just re-soldered the new display board back. The header pitch is 3.81mm and was a total pain to find (yay eBay!).


The end result is a bright, clear and LARGER display. Here I have contrasted it with the model right after the 6050C, the 6050D which has a larger digital display (and also does not display the measurement mode as the 6050C does, E or I).IMG_5748If I decide to pick up any more of these supplies, I think I’d make the same display modification to them as well. I have been looking at the 6050A models (which can usually be had for cheaper): these might also be good for a “digital makeover” involving removing the analog meter and designing a new digital display PCB.


Tektronix DMM 916 Back-light Mod

In keeping with the theme of back-light mods, I have another one here for you all. Recently I was able to get my hands on a well-loved (read: had the piss kicked out of it) Tektronix DMM 916. The specs are nice:

  • 4.75 digits
  • 40,000 count
  • Basic DC accuracy of 0.06%+1 count

The only problem which I didn’t know until I got the meter in my hands was that the back light was horrible:


Wait, where is that back-light?IMG_5671

Still can’t see it? Turn off the lights!IMG_5670

I’m not sure if this is “factory standard” or just a sign of the age of the unit, but either way it needed some change. The first thing I did was to open up the meter and check out the display:


There is a small slot on one side of the display assembly where the lamp bulb pokes into the light pipe. At first I thought I might use a standard through-hole LED, but realized that I wouldn’t be able to mount it without either cutting the trace (for the limiting resistor) or cutting the display. I didn’t want to mod the board, in case I or someone else wanted to restore it back to a incandescent bulb. So I choose to use a SMD chip LED and resistor, and build it “tee-pee” style on the top of the display PCB, so that the LED and resistor would stick up vertically into the display light-pipe recess:


The LED is blue, Digikey part number 475-2816-1-ND with a 270ohm current limit resistor. The bulb sank about 20mA while the LED uses ~18mA, so a bit more efficient. I’m still not sure about the blue, but I figured it matches the theme of the case, so why not:


And as is evident, it is much brighter even with the lab lights on. Curiously, it is not much more legible in the dark in terms of the digits on the screen as I would have thought.

Board Update

Just as a quick followup, I have received my first circuit board in the mail today, and I must say, I am kinda impressed at the quality of the work done in producing the boards.  I give much thanks and props to the people at iTead Studio:


And a little comparison with our long friend (this will date this post if anything will):


There are couple of little issues with the board, but none of them are due to the manufacturer, just due to my in-experience 🙂  So ~20 boards for $63 shipped and delivered in about 2 weeks.  Pretty good 🙂