130 Watt Cree-Based 98-CRI Led Lamp for Artist’s Studio — Solving the Inrush Current Problem

I’d assumed that my previous LED lamp project was it for a while, but I needed an artist studio lamp with very strong light and the highest color rendition index available. So, when I stumbled on these new Cree Chip-on-Board LED units, I went “wow” and went for it. These LED units have a CRI rating of 98 — meaning that 98 percent of the visible spectrum is illuminated, making these LEDs suitable for the most demanding photographers or oil painters or other artists.

With their rating of 36 volts nominal, four in series is perfect for a nominal 144V. In assembling the unit, the individual LEDs were mounted first on a small extruded heatsink. Thanks to their ceramic board mounting the LEDs had no electrical connection to the heatsink. The heatsink was then screwed into a 12 inch stainless kitchen pan, resulting in four units like this (apologies for the poor photo — it’s shown mounted to the ceiling):

In building the control box, a very high value to the filter capacitor was desired, for best performance, so two 470 uF in parallel capacitors were used. After building the circuit, this change resulted in repeated damage to the AC switch due to excessive turn-on current, during the time that the capacitors were charging. This is referred to as “inrush current”. And the problem did not go away by leaving the switch on and just plugging the unit in already on — there was quite a large arc and pop at the wall socket. Too scary!

The solution was a new change in the driver circuit, to cause the initial charge of the filter circuit to proceed so slowly that no large current inrush occurs.

In the new design, transistor Q2 goes slowly from an off state to an on state after power is applied, causing the charge entering C1 and C5 to be 5 amps or less, preserving the switch SW1A from blowing out. Note the resistor R14. This resistor is required to avoid blowout of the Q2 mosfet – the RC time constant of the 2.2M resistor and the input capacitance of Q2 results in a delay which prevents damage to the gate. Learning this cost me 8 MosFETs! Below is shown the PCB just before final assembly.

The increased power requires installation of a fan. In this case, the power FET was mounted to the heatsink, mounted directly to the fan, which is set to expel air. Intake holes were drilled a few inches away from the fan.

Operation of the light is wonderful for an artist’s studio. The natural light from this high power LED lamp reaches an intensity which feels very close to midday sun, and colors pop with a richness that is perfect for photography.

I’ll close with the LTSpice listing, in case the reader is interested in playing with the circuit operation.

Version 4
SHEET 1 7116 1492
WIRE 912 -208 224 -208
WIRE 1008 -208 912 -208
WIRE 1376 -208 1008 -208
WIRE 2304 -208 1376 -208
WIRE 2544 -208 2304 -208
WIRE 2544 -144 2544 -208
WIRE 2304 -112 2304 -208
WIRE 2544 -32 2544 -80
WIRE 224 64 224 -208
WIRE 1280 96 848 96
WIRE 1728 96 1280 96
WIRE 1856 96 1728 96
WIRE 2128 96 1856 96
WIRE 2304 96 2304 -32
WIRE 2304 96 2128 96
WIRE 2544 96 2544 32
WIRE 1600 176 1472 176
WIRE 1968 176 1600 176
WIRE 1472 240 1472 176
WIRE 1968 240 1968 176
WIRE 2128 240 2128 96
WIRE 1008 272 1008 -208
WIRE 1968 384 1968 320
WIRE 2544 416 2544 176
WIRE 1376 448 1376 -208
WIRE 1008 464 1008 352
WIRE 1344 464 1008 464
WIRE 2304 464 2304 96
WIRE 1472 480 1472 320
WIRE 1472 480 1408 480
WIRE 2128 480 2128 320
WIRE 2272 480 2128 480
WIRE 1280 496 1280 96
WIRE 1344 496 1280 496
WIRE 2496 496 2336 496
WIRE 1968 512 1968 464
WIRE 2272 512 1968 512
WIRE 1600 544 1600 176
WIRE 1728 544 1728 96
WIRE 1968 544 1968 512
WIRE 2128 624 2128 480
WIRE 2544 624 2544 512
WIRE 2544 624 2128 624
WIRE 1008 640 1008 464
WIRE 1120 640 1008 640
WIRE 1856 672 1856 96
WIRE 2544 672 2544 624
WIRE 1120 688 1120 640
WIRE 912 704 912 -208
WIRE 1008 704 1008 640
WIRE 912 816 912 768
WIRE 1008 816 1008 768
WIRE 1008 816 912 816
WIRE 1120 816 1120 768
WIRE 1120 816 1008 816
WIRE 1200 816 1120 816
WIRE 1376 816 1376 512
WIRE 1376 816 1200 816
WIRE 1600 816 1600 608
WIRE 1600 816 1376 816
WIRE 1728 816 1728 608
WIRE 1728 816 1600 816
WIRE 1856 816 1856 736
WIRE 1856 816 1728 816
WIRE 1968 816 1968 624
WIRE 1968 816 1856 816
WIRE 2304 816 2304 528
WIRE 2304 816 1968 816
WIRE 2544 816 2544 752
WIRE 2544 816 2304 816
WIRE 848 912 848 96
WIRE 848 912 576 912
WIRE 576 976 576 912
WIRE 848 1056 848 912
WIRE 848 1184 848 1136
WIRE 1008 1184 848 1184
WIRE 1136 1184 1008 1184
WIRE 576 1216 576 1040
WIRE 848 1216 848 1184
WIRE 848 1216 576 1216
WIRE 1200 1232 1200 816
WIRE 848 1264 848 1216
WIRE 1008 1312 1008 1184
WIRE 1136 1312 1136 1184
WIRE 1152 1312 1136 1312
WIRE 224 1392 224 144
WIRE 848 1392 848 1328
WIRE 848 1392 224 1392
WIRE 1008 1392 1008 1376
WIRE 1008 1392 848 1392
WIRE 1200 1392 1200 1328
WIRE 1200 1392 1008 1392
WIRE 1568 1392 1200 1392
WIRE 1664 1392 1568 1392
WIRE 1568 1472 1568 1392
FLAG 1568 1472 0
DATAFLAG 1472 816 “”
DATAFLAG 1200 1120 “”
DATAFLAG 960 1184 “”
DATAFLAG 1152 1392 “”
SYMBOL voltage 224 48 R0
WINDOW 123 24 124 Left 2
WINDOW 39 -5 -286 Left 2
WINDOW 3 -7 -321 Left 2
SYMATTR SpiceLine Rser=.03
SYMATTR Value PULSE(0 155 .1 .016 .016 1 2 158)
SYMATTR InstName V1
SYMBOL res 1104 672 R0
SYMATTR InstName R9
SYMATTR Value 121000
SYMBOL cap 992 704 R0
SYMATTR InstName C1
SYMATTR Value 1e-6
SYMBOL nmos 2496 416 R0
SYMATTR InstName M1
SYMATTR Value IXFX90N30
SYMBOL res 2528 656 R0
SYMATTR InstName R6
SYMATTR Value 5.6
SYMBOL res 2288 -128 R0
SYMATTR InstName R7
SYMATTR Value 49900
SYMBOL cap 1712 544 R0
SYMATTR InstName C2
SYMATTR Value 10e-6
SYMBOL res 1952 528 R0
SYMATTR InstName R10
SYMATTR Value 9999
SYMBOL res 1952 368 R0
SYMATTR InstName R11
SYMATTR Value 1
SYMBOL zener 1872 736 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D151
SYMATTR Value TDZ6_2B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 896 704 R0
SYMATTR InstName C3
SYMATTR Value 940e-6
SYMBOL OpAmps\UniversalOpamp2 2304 496 R0
SYMATTR InstName U1
SYMBOL res 1952 224 R0
SYMATTR InstName R1
SYMATTR Value 10000
SYMBOL res 2112 224 R0
SYMATTR InstName R2
SYMATTR Value 5600
SYMBOL res 992 256 R0
SYMATTR InstName R3
SYMATTR Value 3e6
SYMBOL OpAmps\UniversalOpamp2 1376 480 R0
SYMATTR InstName U2
SYMBOL cap 1584 544 R0
SYMATTR InstName C4
SYMATTR Value 10e-6
SYMBOL res 1456 224 R0
SYMATTR InstName R4
SYMATTR Value 49.9
SYMBOL zener 2560 -80 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D152
SYMATTR Value UMZ24K
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL zener 2560 32 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D153
SYMATTR Value UMZ27K
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 2528 80 R0
SYMATTR InstName R5
SYMATTR Value 215
SYMBOL nmos 1152 1232 R0
SYMATTR InstName M2
SYMATTR Value IXFX90N30
SYMBOL cap 992 1312 R0
SYMATTR InstName C5
SYMATTR Value 10e-6
SYMBOL zener 864 1328 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value TDZ18B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 832 1040 R0
SYMATTR InstName R12
SYMATTR Value 1e6
SYMBOL diode 592 1040 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value MURS320
TEXT 728 -328 Left 2 !.tran 0 5 0 .01 startup
TEXT 2616 720 Left 2 ;2 watts
TEXT 2336 88 Left 2 ;1/4 W
TEXT 2256 112 Left 2 ;1/8 W
TEXT 1984 368 Left 2 ;10K pot
TEXT 2024 464 Left 2 ;Grn
TEXT 1912 368 Left 2 ;Blu
TEXT 2552 376 Left 2 ;Blk
TEXT 2312 -256 Left 2 ;Red
TEXT 744 -360 Left 2 !.include opamp.sub
TEXT 2592 -8 Left 2 ;Modeling the\nLED array with\nZener diodes

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