Here is a design for an overhead light fixture (see the Creative Commons license notice below).

I’m not really sure I’m going to save any money with this.  I’m spending a lot for a very nice lamp, but economically, I’m not sure the energy and light bulb savings will be worth it.  But then again, that’s not the only reason to do anything.

This lamp will replace the aging standard fixture in the ceiling of my study in a house built in 1965.  This existing fixture is made for two Edison base light bulbs, and switched at the wall near the entry door.  Bo-rrring.

The idea is to stop the 120V wiring at the switch.  There I’ll put in a small DC power supply and a circuit inside the wall box with a special wall plate for the switches mounted on the PC board.  A three wire cable (ground, 24 volts DC, and a 0 – 5 volt square wave) will connect to the overhead fixture.  The lamp will be made of 6 pentagonal printed circuit boards connected together on the edges like the lower half of a dodecahedron (12 sided solid).  Each board will have 5 Cree XLamp MX-6 high-brightness LEDs, 6500 Kelvin (cool white color), for a total of 30 LEDs.

I selected the Cree XLamp MX-6’s for a couple of reasons… they have similar or better light output at 300mA as other LEDs that use a lot more amperage.   They are touted by Cree to have a nice long life with good reliability, retaining 70% of their luminosity after high-temperature age testing, which Cree seems to be taking very seriously even if there aren’t any international standards for it yet.  And… they are generally a dollar cheaper per LED.  That’s obviously not my biggest concern, but it’s a factor.  In reality, this board should support almost any LED with a minor change to the SMD pads.  I tried to limit costs where possible, but I didn’t skimp on the quality.

Here is the schematic for the dimmer board.  It features a triangle wave generator for the PWM signal, and uses a “Dallastat” digital potentiometer for 64-step up/down dimming.  There’s an adjustable hysteresis so that “all the way down” is really 0% and “all the way up” is really 100%.  A MAX6816 debouncer chip and a ‘D’ flip-flop are used to create an on/off signal, and these are  ‘and’ed with the PWM and buffered with an op-amp for the lamp on/off control output.

I used Paul Falstad’s circuit simulator to work out the component values for the triangle wave generator.  Here is the data to ‘import’ (in extremely small text so as not to bore everyone else) if you’re interested:

$ 1 2.0E-6 7.619785657297057 64 3.3 50
R 112 144 112 96 0 0 40.0 5.0 0.0 0.0 0.5
r 112 144 112 208 0 6800.0
r 112 208 112 272 0 6800.0
g 112 272 112 304 0
a 144 192 224 192 1 5.0 0.0 1000000.0
w 112 208 144 208 0
r 224 192 288 192 0 6800.0
r 224 192 224 128 0 6800.0
a 320 176 400 176 1 5.0 0.0 1000000.0
a 480 224 560 224 1 5.0 0.0 1000000.0
R 448 128 448 96 0 0 40.0 5.0 0.0 0.0 0.5
w 400 128 400 176 0
c 384 240 352 240 0 1.0E-7 0.5849248563899665
w 320 192 304 192 0
w 304 192 288 192 0
w 400 176 400 240 0
w 400 240 384 240 0
g 448 320 448 336 0
174 448 208 480 272 0 22000.0 0.2921 Duty
w 480 208 480 176 0
w 480 176 400 176 0
w 448 272 448 320 0
w 224 128 144 128 0
w 144 128 144 176 0
w 560 224 560 256 0
w 448 208 448 128 0
x 328 64 489 68 0 16 PWM Signal Generator
r 224 128 288 128 0 5900.0
w 288 128 336 128 0
w 368 96 400 96 0
w 400 96 400 128 0
174 336 128 400 112 0 1000.0 0.797 Hysteresis
w 368 96 368 112 0
w 352 240 320 240 0
w 320 240 320 192 0
w 320 160 304 160 0
w 304 160 304 240 0
w 304 240 144 240 0
w 144 240 144 208 0
x 259 381 420 384 1 12 http://pyroflatulence.tv/?p=466
w 400 240 400 288 0
w 400 288 192 336 0
w 192 336 144 400 0
w 560 256 480 400 0
o 41 64 0 43 5.0 9.765625E-5 0 -1
o 43 64 0 35 10.0 9.765625E-5 1 -1

The dimmer printed circuit board is 1.25 x 2.5 inches, small enough to fit inside the single-gang electrical wall switch box.  The pushbutton on/off switch and the up/down dimmer rocker switch mount on the board, but the pushbutton switch is a whole centimeter shorter, and so it rides on a short riser board held up with standoffs.  The four square ground pads at the corners will use 2 cm standoffs to mount to a specially cut wall plate.  There’s also an interface for digital control of up/down/on/off through a home automation mechanism.  Here is a gerber-generated drawing of the dimmer board:

Anybody who’s spent a few days in Michigan in February knows I’ll be popping big static sparks off this switch, so there’s plenty of ESD diodes and varistors.

The uncredited protagonist of the lamp printed circuit board is a constant current driver called the CAT4101.  It’s very easy to use, takes the higher voltage needed to drive a string of LEDs, and features a PWM/Enable input.

This sucker is gonna generate some heat.  As many of the parts as I could get are operational up to 125 degrees centigrade… however all of the fun components start to derate over 70 degrees.  The back side of the board has been cleared for room for a TO-3 transistor style heat sink, with mounting holes.  I specifically selected a heat sink with prongs on it that I could bend, as all the heat sinks pointing into the center of a polyhedron might poke each other… and I admit that I didn’t want to actually do the solid modeling or math involved in worrying about that.  The CAT4101 LED driver will need to handle about 5 watts, so it has it’s own tin-plated surface mount ‘D2PAK’ heat sink, which I really hope will be enough.  The lamp boards can be daisy-chained together by two connectors on the back of each, up to the amperage of the power supply.  The PWM signal from the dimmer board is buffered again by a receiver op-amp, and this need only be on the first board of a chain, the others can shunt the op-amp with a solder jumper.  Another solder jumper permits the board to be used ‘always on’ without the dimmer.   There’s a big honking mounting hole in the center of the board, and again, there’s lots of ESD protection.

The 6500K white LEDs generate 100 lumens at 300 milliamps, the suggested and my projected load (Cree MX-6’s are available in warmer colors, but with less light output).  This should be 3000 lumens total, which is pretty much enough for surgery.   But… the digital potentiometer and the response of the LED driver should be near flat-line linear at 400 Hz, so I don’t have to run it full blast all the time.  It will be nice to be able to turn the lights UP when I really need them (like during teach-yourself brain surgery).

February 15th, 2010: I worked all weekend and a holiday, and I’m convinced that any further soliloquizins’ will just screw me up, so I pushed the buttons.  Everything is ordered.  The Gerber files went into Sierra ProtoExpress’s “No Touch” automatic verification system on the first try.  That’s scary.  I have no idea if this is even going to work, but it a be a good circuit board dere bubba.  I doubled up the dimmer board and ordered 2, so I’ll get 4 of them for about $100.  The lamp boards were 6 for $200.  The parts were $300.

February 18th, 2010: Most of the parts arrived, and they look pretty good.  Digi-Key did a terrific job again. The power supply is really pretty, and I can adjust it down to 22 volts.  The 5 Cree MX-6’s on each board are projected to drop 3.3V at 300mA, and the CAT4101 drivers can’t take more than 25V, so between 17 and 24 volts should be okay.  From the data sheets, the 24V ESD devices will work just fine with less than ‘typical’ voltage.  The heat sinks look like they’ll do the job, putting on the surface mount D2PAK heat sinks looks daunting, I believe I may invest in an electric griddle and try my hand at pancake-style reflow soldering.

February 19th, 2010: I have ordered a Reflow Soldering Controller kit from SparkFun Electronics.  This kit will help me use an infrared toaster oven as a reflow oven.  The LEDs and the D2PAK drivers and heat sinks will just never work out right without this, and I’ll be needing it in the future for the much finer components I’ll likely need to use for the wireless home automation.  I’ll be heading over to Target for the toaster oven.  I wonder how the wife will react to me baking semi-toxic fumes.  I guess I better clean the garage while I’m waiting for the Fedex and UPS deliveries.

February 28th, 2010: The dimmer works perfectly.  All the dimensions are good, all the parts fit.  The voltage regulator isn’t even getting warm.  I can adjust the hysteresis potentiometer so that one click up from the bottom starts the PWM square wave.  One click from the top, it’s almost completely on, and then at the top it’s fully on.  The switch risers and the standoffs measure up well, the LED in the pushbutton switch works.  If I hold the dimmer one way or the other, the Dallastat digital potentiometer clicks one notch, pauses, and then starts ticking along.   When power is lost and restored, it always comes up with the lamp off, and the Dallastat remembers it’s last setting.  I didn’t do any breadboarding to test any of this, so I’m pretty impressed with it.
Here is a scope picture of the PWM output, 1 volt / division vertical (center mark is zero), 1 millisecond / division horizontal.  It puts out a very clean 5V square wave at ~330 Hz.  This is 32 clicks (of 63 top to bottom) from the top of the Dallastat, so right around a 50% duty cycle.

Here is the built dimmer circuit attached to the power supply.  The pushbutton is lit (it has it’s own white LED inside), the black thing is the up/down rocker dimmer switch.  The four stand-offs at the corners will connect to the back of the wall switch plate.

The Reflow Toaster controller is not working out very well.  To write any kind of meaningful control program, I had to buy a compiler.  It also won’t work well with the goofy boot loader that’s on the chip, so I have to get a programmer.  This is not what I expected, and very disappointing.

So instead, I’m going to try hand-baking one of the boards.  The Cree lamps are what I’m worried about the most, but the tin heat sink for the driver transistor would be very difficult to solder by hand, and I’d likely destroy the transistor in the process.  Here’s my manual stopwatch profile:

1. cook at 175-200F for 5 minutes (pre-pre-heat)
2. bump to 275F, cook for 2 minutes (pre-heat)
3. bump to 410F.  As soon as it melts, count to 10, turn off heat and crack open the door.
4. Open the door a little wider after a minute.  Cool down shouldn’t happen faster than 2 – 3 minutes, so walk away and have a smoke.

I worry too much.  It all worked out great, and I’ve cooked up one pentagon.  And the result…

It works.  It would work better if I remembered that LEDs are polarized.  I got too excited and got two of them backwards on the first pentagon.  However, I soldered the bypasses on the two and calibrated the amperage to 300mA, so the working three are fully loaded.

It’s using a lot more wattage than I suspected.  The three working LEDs are using 7 watts according to my Kill-A-Watt meter, which measures at the plug.  It’s bright, but it’s also getting hot.  I’ll have to risk it and let it cook for a while and see if it cuts out.  The driver transistor has a thermal shut-off if it gets too warm, and I suspect it will.  I may only be able to use two or three LEDs per pentagon with this design.

On the second pentagon, I discovered a design flaw.  I’ve made the driver pins too close to the ground plane, and pins other than 5V shorted through the solder mask.  I fried the driver transistor.  When investigating this on the first pentagon, I found a solder ball behind the pins, which shorted the LED return to ground putting full amperage to the LEDs, and fried one of the LEDs.  Hard lesson learned.  The first now has two working LEDs (at least I didn’t fry the driver on that one) and the second is toast.  It’ll be difficult to rework them.  I repaired all the boards with a dremel.

I made a third pentagon, and that one is working great.  With 5 LEDs, it’s drawing only 6 watts at full amperage and full on PWM, which is much closer to what I was expecting.  It’s hot, both the back heat sink and the driver surface mount heatsink are hot, but not so hot that I can’t hold them and keep my fingers on them.  This suggests that they are working within acceptable limits, and I can turn down the amperage a little and be pretty comfortable that this circuit will last a long time.  I’ll get a temperature sensor soon and find out for sure exactly how hot they’re getting.  The amperage adjustment trimmer starts at 0.20A fully clockwise, and turned up to 0.25A is plenty bright and allows some comfortable lee way, Cree MX-6 nominal amperage is 0.30A, absolute maximum is 0.35A.

Here’s the third pentagon working

Here’s a shot of my desk

March 7th, 2010: I’ve been doing some thermal testing.  The programmer for the PIC chip came, and the toaster controller works.  SparkFun also sent the wrong PIC chips I ordered, big surprise… they got something else wrong.  I’m not sure I want to pursue it.  But the controller has a working thermocouple on it, and taped to the bottom of one LED, I ran the light at full blast under a porcelain food bowl.  I ran it long enough to stabilize the temperature, and it does get hot.  At full amperage, 300mA, it goes up to 78C.  At 250mA, it goes to 71C.  This is just over the limit, at 70C most of the active and critical parts start to derate rapidly.  With the bowl removed, it stabilizes at around 57C.  This is pretty good.  I can’t truly seal the thing inside a glass fixture, but given any amount of air flow, it should be just fine at nominal 300mA amperage.

Cree does “High Temperature Output Life” testing on their stock, they cook LEDs at 85C for 1008 hours and ensure that light output is not diminished by more than 15%.  Running at less than 60C, I’m guessing that I’ll have plenty of light from these for a long time.  My goal is about 250K hours, or 25 years.

I’ve cooked up the remaining boards.  I’ve also successfully repaired my hot air pencil, and removed the parts I messed up from the first two boards.  Replacements are on the way, I just hope I can successfully reflow them again.  A tip o’ the hat to my friend John who pointed me to Silver Circuits ( http://custompcb.com/ ) who I’ll be trying next time.

It’s continuing to use more wattage than I expected.  The third board with 5 LEDs at full dwell reports 12 watts on my Kill-A-Watt.  The first pentagon was doing 7 watts with three working LEDs, so this makes sense.  I’ve ordered the next size power supply.   Sometime I can make another lamp with only 3 or 4 pentagons and use the other one. I measured this one at 6 watts before? It’s possible that this one is goofy, I’ll have to finish the last 3 pentagons and see what they do.

Wednesday, March 10th, 2010: I am a knucklehead.  I didn’t need the new power supply (which is here and very pretty) it’s only using 7-8 watts per pentagon.  I had the Kill-A-Watt set to volt-amps (VA), instead of watts.  It’s pulling 12-13 VA, and 7-8 watts.  I don’t even know what a volt-amp is.  Well, I’ve got a big honking power supply for the next project and this sucker is working per specs I was expecting, woo hoo!  Replacement parts have arrived, and the Fluke 62 Mini infrared temperature sensor gun is here, it confirms the temperatures I was seeing with the thermocouple.  Nothing in the way of final assembly now, just need the time and energy.  The weather has really warmed up, so I may even get a chance to ‘splore the attic a bit and see what’s going to have to happen there for the installation.

Monday, March 15th, 2010: Driver transistors are dropping like flies.  I’m not completely sure what I’m doing to them, but when everything else looks like it’s working perfectly but there’s just no current, I’m replacing the transistor.  I’m pretty sure it has to do with me testing the 5V supplies without the filter capacitor in place.  I’m actually getting pretty good at using the hot air rework to remove the transistor and the surface-mount heat sink, I just wish they weren’t electrically so fragile.  I’ll be adding more protection in the final version of this circuit.

I’ve wired the reflow toaster with the controller, it works.  It will need some calibration, it heats up 1 degree Celcius per second, but doesn’t really cool off very fast.  I don’t have a serial port on any of my working portables, so I’m waiting on a USB-to-serial cable so I can capture and graph the temperatures.

More spare parts, more hurry up and wait.  This is what I get for going straight to prototype without component level testing.  But hey, the dimmer worked right the first time, so it could be worse.

Thursday, March 18th, 2010: More transistors are here, and USB to serial port is here.  I plugged it into the old laptop and it recognized and started the serial port driver instantly, I love Kubuntu Karmic Koala.

I’ve also purchased an Atmel EVK1101 AT32UC30256 evaluation / demonstration circuit board, and an Atmel “Dragon” programmer/debugger.  These are for experimentation with using the One-Net protocol for wireless home automation… but this is for another posting.

So once again… barring any more stupid errors on my part, I may be able to do the final assembly this weekend.  Big maybe, baby, I’ve got the next project all lined up and ready, but there’s one other problem.  I’m thinking I can resist it long enough to get some work done.

Sunday, March 21st, 2010: It works pretty well.  I abandoned the power connectors, they were just too much trouble.  Copper foil tape also didn’t work well connecting the pentagons, I used a piece of wire in between the boards and just leaded the hell out of them.  It’s messy, but it’s pretty solid.

The lamp, all 6 pentagons assembled

This is the lamp with the LEDs at full brightness, and using a camera flash.

Here’s the whole setup: power supply, dimmer, lamp and tape measure for scale

Here’s a shot in the box with no camera flash and the lamp turned down to one click above off… 1/64th, or ~ 1.5% of maximum brightness.

Saturday, April 3rd, 2010: I had to turn down the amperage.  At nominal 0.3 amps and full brightness (100% PWM dwell), it gets too hot.  A few places were creeping above 75 degrees C, and I really have to draw the line there.  Turned down to 0.25A (individually controllable per pentagon), I haven’t seen any scanned spot go over 65 degrees C.  Turning down the amperage will cost 15% of light output, but that’s why it was over engineered in the first place.

I’ve been thinking about another design using a snap-together linear railed model rather than this polyhedral design, but again, that will need it’s own post someday, and if I ever build it.

It’s time for me to get practical on this, I have to buy a hard box to put the power supply in, and mount the sucker in the ceiling.  The weather in improving rapidly, I think I may have to get on this for this Easter weekend.


Here is the bill of materials that I used:

Digi-Key Part Number Manufacturer Manufacturer Part Number Quantity Description Notes
MAX6816EUS+TCT-ND
MAXIM INTEGRATED PRODUCTS (VA)
MAX6816EUS+T
1
IC DEBOUNCER SWITCH SGL SOT143-4
debounce IC for on/off pushbutton switch on dimmer board
DS1809Z-010+-ND
MAXIM INTEGRATED PRODUCTS
DS1809Z-010+
1
IC DALLASTAT 10K 8-SOIC
64-step non-volatile dimmer digital potentiometer, configured to save value to EEPROM when power fails
MMBT3904FSCT-ND
FAIRCHILD SEMICONDUCTOR (VA)
MMBT3904
1
TRANSISTOR GP NPN AMP SOT-23
drives the LED on the on/off switch on the dimmer board
445-1604-1-ND
TDK CORPORATION (VA)
C1608X7R1C105K
10
CAP CER 1.0UF 16V X7R 10% 0603
used on the dimmer board  to delay the ‘reset’ logic pull-up on the ‘D’ flip flop… optional, but if used may ensure that the lamp always is OFF when power is applied
EG4661-ND
E-SWITCH
LP4OA1PBBTW
1
SWITCH PB ILL DPDT MOM WHI LED
momentary pushbutton switch with white 20mA LED, for dimmer board
CG0603MLC-05LECT-ND
BOURNS INC (VA)
CG0603MLC-05LE
20
SUPPRESSOR ESD 5VDC 0603 SMD
5V varistors for ESD protection, used on all boards
641-1086-1-ND
COMCHIP TECHNOLOGY (VA)
CEBS065V0-G
1
TVS ESD BIDIRECT 5V SOT23-6
5V zener gang, 4 bidirectional ESD protection circuits, used on dimmer board
ST32ETB102CT-ND
COPAL ELECTRONICS INC (VA)
ST32ETB102
1
POT 1.0K OHM 3MM CERM SQ TOP SMD
PWM hysteresis adjustment trimmer potentiometer on dimmer board
CAT4101TV-T75CT-ND
ON SEMICONDUCTOR (VA)
CAT4101TV-T75
6
IC LED DRVR HP CONST CURR D2PAK
LED drivers with PWM input, for lamp boards
RR08P6.8KDCT-ND
SUSUMU CO LTD (VA)
RR0816P-682-D
10
RES 6.8K OHM 1/16W .5% 0603 SMD
precision resistors for the triangle wave generator on dimmer board
RR08P5.9KDCT-ND
SUSUMU CO LTD (VA)
RR0816P-5901-D-75H
10
RES 5.9K OHM 1/16W .5% 0603 SMD
precision resistor for the triangle wave generator on dimmer board
RMCF1/1610KFRCT-ND
STACKPOLE ELECTRONICS INC (VA)
RMCF 1/16 10K 1% R
10
RES 10K OHM 1/10W 1% 0603 SMD
5V pull-ups for logic high on dimmer board
338-1821-1-ND
CORNELL DUBILIER ELECTRONICS (CDE) (VA)
AVRF108M06F24T-F
10
CAP ALUM 1000UF 6.3V ELECT SMD
5V voltage tanks, buffers 5V power circuits, for all boards
568-4053-1-ND
NXP SEMICONDUCTORS (VA)
PESD5V0S1BA,115
10
DIODE BIDIR ESD PROTECT SOD323
5V bidirectional zener diodes for ESD protection
360-2263-ND
NKK SWITCHES OF AMERICA INC
M2018TZW13-JA-RO
1
SW ROCKER SPDT BK SILV .250″ PC
dimmer rocker switch, SPDT momentary with center off
338-1841-1-ND
CORNELL DUBILIER ELECTRONICS (CDE) (VA)
AVEK107M35G24T-F
10
CAP ALUM 100UF 35V ELECT SMD
LED voltage tanks, helps buffer the voltage going into the LED drivers on the lamp boards
ST32ETB202CT-ND
COPAL ELECTRONICS INC (VA)
ST32ETB202
6
POT 2.0K OHM 3MM CERM SQ TOP SMD
LED amperage adjustment trimmer potentiometer on dimmer board
MX6AWT-A1-R250-000C51CT-ND
CREE INC (VA)
MX6AWT-A1-R250-000C51
30
LED XLAMP COOL WHITE 6500K SMD
Payload!
A31297-ND
TYCO ELECTRONICS AMP
3-640426-3
25
CONN RECEPT 3POS 18AWG MTA156
power plugs, used on all ground/24V/PWM connections
HS406-ND
AAVID THERMALLOY
7109DG
6
BOARD LEVEL HEATSINK .45″ D-PAK
LED driver surface mount heat sinks for lamp board
LM2931AD-5.0R2GOSCT-ND
ON SEMICONDUCTOR (VA)
LM2931AD-5.0R2G
7
IC REG LDO 100MA 5V 8SOIC
5V regulators, one on each dimmer and lamp board
A26230-ND
TYCO ELECTRONICS AMP
382811-9
10
SHUNT, ECON, PHBR 15AU, RED
amperage test jumper on dimmer board, this can be removed when testing the amperage to the LEDs during adjustment
WM8108-ND
MOLEX CONNECTOR CORPORATION
90121-0762
10
CONN HEADER 2POS .100″ R/A GOLD
amperage test 2-pin header on dimmer board
HS264-ND
AAVID THERMALLOY
500403B00000G
6
HEATSINK TO-3 12W H=1.25″ BLK
aluminum TO-3 heat sinks for the back of the lamp boards
495-2285-1-ND
KEMET (VA)
B45196H6106M309
10
CAP TANTALUM 10UF 35V 20% SMD
dimmer capacitors, power buffering and digital potentiometer non-volatile EEPROM support
490-1519-1-ND
MURATA ELECTRONICS (VA)
GRM188R71H104KA93D
20
CAP CER .1UF 50V 10% X7R 0603
ripple rejection power supply capacitors, used on all boards
568-4034-1-ND
NXP SEMICONDUCTORS (VA)
PESD24VL1BA,115
7
DIODE ESD PROTECTION SOD323
24V bidirectional zener diodes for ESD protection, used on all boards
CG0603MLU-24ECT-ND
BOURNS INC (VA)
CG0603MLU-24E
10
SUPPRESSOR ESD 24VDC 0603 SMD
24V varistors for ESD protection, used on all boards
24393K-ND
KEYSTONE ELECTRONICS
24393
4
STANDOFF METRIC HEX M3 THD 10MM
brass standoffs for pushbutton switch riser on dimmer board
641-1282-1-ND
COMCHIP TECHNOLOGY (VA)
CDBU0130L
1
DIODE SCHOTTKY 100MA 30V 0603
schottky diode for dimmer digital potentiometer non-volatile EEPROM support
S2A-FDICT-ND
DIODES INC (VA)
S2A-13-F
10
RECTIFIER GPP 50V 1.5A SMB
safety diodes, prevent reverse voltage damage, used on all boards
MF-SM075CT-ND
BOURNS INC (VA)
MF-SM075-2
6
FUSE RESETTABLE .75A 30V SMD
LED PTCs, high enough value to offset their low thermal derating, but still some safety in case of runaway current on lamp boards
M74VHC1GT125DT1GOSCT-ND
ON SEMICONDUCTOR (VA)
M74VHC1GT125DT1G
1
IC BUFF CMOS LVL/SFTR N-I SOT235
PWM receiver buffer for lamp circuit (not needed on each lamp board)
MCP6L94T-E/SLCT-ND
MICROCHIP TECHNOLOGY
MCP6L94T-E/SL
1
IC OPAMP 10MHZ 2.4V 14-SOIC
PWM sender quad op-amp for dimmer board triangle wave generator
MC74VHC1G08DTT1GOSCT-ND
ON SEMICONDUCTOR (VA)
MC74VHC1G08DTT1G
1
IC GATE AND SGL CMOS 2IN SOT23-5
AND gate, combines PWM and on/off signal on dimmer board
568-4494-1-ND
NXP SEMICONDUCTORS (VA)
74LVC1G74DC,125
1
IC SNGL D FF POS-EDG TRIG 8VSSOP
D flip-flop with set/reset, used for single pushbutton on/off signal on dimmer board
A24165-ND
TYCO ELECTRONICS AMP
644752-3
25
CONN HEADER VERT 3POS .156 TIN
male pin power jacks, used on all boards for Ground/24V/PWM connections
102-1937-ND
CUI INC
VGS-50-24
1
POWER SUPPLY 52.8W 24V 2.2A METAL
power supply, 100-240VAC input, 24VDC 50 watt output
RMCF1/16300FRCT-ND
STACKPOLE ELECTRONICS INC (VA)
RMCF 1/16 300 1% R
10
RES 300 OHM 1/10W 1% 0603 SMD
resistor for LED in the on/off switch on dimmer board
311-680DCT-ND
YAGEO (VA)
RT0603DRD07680RL
10
RES 680 OHM 1/10W .5% 0603 SMD
LED amperage adjustment constant, ensures LED driver shouldn’t be adjusted for more than 800 mA, used on lamp boards
24437K-ND
KEYSTONE ELECTRONICS
24437
4
STANDOFF HEX M3 THR ALUM 20MM
standoffs for dimmer mounting to wall plate

What I’ve missed so far:

  • explore option to bypass the 5V regulator on daisy-chained successor lamp boards,  100mA should be enough for 6 boards or more
  • move the pushbutton switch ESD components to the riser (maybe not)
  • add lots more ground plane vias around the LEDs
  • reverse the dimmer rocker switch!  The pushbutton switch should be above the rocker when mounted, and then the rocker should have ‘up’ make the lights brighter.  The switch is currently (electrically) upside down I must have gotten this backwards twice, because it’s working correctly.

Creative Commons License

Copyright (c) 2010, Roger Cody Venable (MDVE.NET).  Some rights reserved.

This design is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License .

2 Responses to “Bright Light”
  1. 03 October 2010

    Hello Stomping, I had a chance to look at your site and a few of the projects including the pie recipes. Since I have not sampled the pie I must admit my personal favorite is your bright light project.

    The documentation you have provided makes it easier to understand how you are going about this project and as with anything that is worthwhile there will always be tweaking necessary to get things just right.

    You had mentioned that you were thinking about a snap-together model …..

    So at any rate you put all this effort into describing the project. …. Keep in mind that I am no where near as skilled at circuit design as you are, but, still had a few questions and comments.

    First, According to the data sheet of the CAT 4101, the thing runs most efficiently when you get the voltage drop of the LEDs close to the supply voltage rails. What’s nice about your design is that once one board is finely tuned it’s simply a matter of combining them.

    The question I had was about the drive current you are running at and the forward voltage of the LEDs which is about 3.3 volts? Is that correct? So if this is correct then the voltage drop across the five LEDs is about 16.5 volts? And the supply is 24.

    Also wanted to know about the circuit board, looks like everything (leds and the driver) is on the same board…. The proposed snap together model may be a way to further develop your concept and get the temperature down a bit more. There are 7 watt heat sinks available for this TO-263 5 Driver package. Could the driver be relocated to an adjacent riser plug in and the LED board be made of aluminum to help with the thermal management of the junction temperature at the five locations?.

    There are a few new connectors on the market introduced this year for your application. Molex and Tyco make some decent ones. They may work well with your snap together concept.

    Also had a few questions about the dimming. The driver documentation describes a dimming granularity of control down to 1 percent at dimming frequencies between 100 and 500 Hz. Does your digital potentiometer really get you this kind of dimming control?

    It would appear that by using a different dimming topology granularity could be increased. How difficult would it be to utilize a variable frequency square wave to control dimming.?

    Nice Job….

    Also had a few thoughts about a housing for your device. Excellent work

    I can be reached by email at

    Best Mark

  2. Hi Mark, thanks for the very cool comment.

    I chose to use a 24 volt supply because it’s a common voltage. The V-Infinity VGS-50-24 I’m using is about $27. It withstands dead shorts, has a very long MTBF rating, and is remarkably well built. I can adjust it about +/- 2.5 volts, I’m running it at the bottom of the adjustable range at about 21.22V right now. The LEDs measure about 3.2V drop (16.0V drop across all 5 LEDs on the top pentagon), so this could run on a little less voltage. This *does* shave a few watts off the total load, and probably runs a little cooler. At 24V, the circuit uses 39 watts at the plug, turned down to 21.25V, it only reads 34 watts – with no discernible drop in light output. I’m using a rather simple wattmeter called a ‘Kill-a-watt’, so single digit significant figures are the best I can do there, but you are completely correct, adjusting the supply closer to the LED drop results in marked energy efficiency.

    On the thermal management, yes and yes. The CATs do get toasty and need extra help staying cool, the sinks I used are adequate, but not by much. As I designed the CAT4101 drivers and the SMT D2PAK heat sinks to be mounted on the LED side of the board, at that time I was thinking I needed them to be low profile so they didn’t adversely affect LED output (shadows). Knowing what I know now, I’m not sure a larger sink would be bad. As long as I can put a nice piece of frosted glass over this, and it’s not too fragile (the SMT sinks are VERY securely leaded to the PCB), I’d call it good.

    I’m only running at 0.25 amps, down from a nominal 0.3 (0.35 absolute max), so heat output when running at full brightness is the big problem on this design. Running at this lower amperage costs me 15% light output. At full bright, it takes 34 watts measured at the plug. I never run it that high, it puts out plenty of light at 10 – 20 watts. It’s on my desk still, and I’m using it as I write this.

    The prototyping shops I used didn’t offer thermal aluminum boards. I admit I should have used more thermal vias. The LEDs could be wire mounted on aluminum starboards or anything like that, but we’re talking a whole different PCB design here. I admit I cheaped out on the back side LED heat sinks, another design must improve on that, better ones are available at higher cost, but are clearly needed. Computer grade heat pipes might be fun if available. I’ve recently seen a grow light that uses Cree LEDs, a lot of metal, and a small computer fan to stay cool.

    Snappy connectors and a more modular design are completely possible. I’d never be able to sell this in Europe with all the solder lead on it. It works, and also demonstrates how lousy it can be built and still work.

    The PWM square wave is on/off when the bias voltage is less/more than the sawtooth wave (pick either word in both places, and if it’s backwards, the rocker switch might need to get flipped over). This feeds into the CAT driver and dims the LEDs. By changing the bias voltage, the frequency stays the same but the dwell of the square wave is changed.

    The DALLASTAT digital rheostat has 64 steps (so each step is about 100/64 = 1.5625 percent), each step increases the bias voltage from rail-to-rail (0 – 5 volts) into an op amp being fed a pretty clean sawtooth. The cleaner the sawtooth (nice sharp flat-sided triangles), and the higher fidelity of the op amp, the more linear the dimming over range. The sawtooth circuit is very efficient, so there aren’t really any two dimmer values that are close together. Any digital or analog potentiometer could be used to adjust the bias, I liked the way the DALLASTAT saves it’s value in EEPROM when power is lost. This enables (and I like) the rocker switch and no slidey or twisty thing for the dimming, you can adjust this lamp wearing hockey gloves. Modifying the circuit to use a regular rotary potentiometer instead, the dimming should be impressively smooth up and down the range.

    The PWM frequency is around 330 Hz, so when dimming is mid range, a certain amount of stroboscopic effect is mildly noticeable, and fast moving objects like fans occasionally reach a harmonic and can appear stopped. That made me jump the first time I saw it, I thought one of my coolant fans stopped and my computer was going to melt.

    I really enjoyed your comment, I hope this helps.

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